acl: report correct ownership in some ovl use-cases

This cycle we added support for mounting overlayfs on top of idmapped mounts.
Recently I've started looking into potential corner cases when trying to add
additional tests and I noticed that reporting for POSIX ACLs is currently wrong
when using idmapped layers with overlayfs mounted on top of it.

I'm going to give a rather detailed explanation to both the origin of the
problem and the solution.

Let's assume the user creates the following directory layout and they have a
rootfs /var/lib/lxc/c1/rootfs. The files in this rootfs are owned as you would
expect files on your host system to be owned. For example, ~/.bashrc for your
regular user would be owned by 1000:1000 and /root/.bashrc would be owned by
0:0. IOW, this is just regular boring filesystem tree on an ext4 or xfs
filesystem.

The user chooses to set POSIX ACLs using the setfacl binary granting the user
with uid 4 read, write, and execute permissions for their .bashrc file:

        setfacl -m u:4:rwx /var/lib/lxc/c2/rootfs/home/ubuntu/.bashrc

Now they to expose the whole rootfs to a container using an idmapped mount. So
they first create:

        mkdir -pv /vol/contpool/{ctrover,merge,lowermap,overmap}
        mkdir -pv /vol/contpool/ctrover/{over,work}

The user now creates an idmapped mount for the rootfs:

        mount-idmapped/mount-idmapped --map-mount=b:0:10000000:65536 \
                                      /var/lib/lxc/c2/rootfs \
                                      /vol/contpool/lowermap

This for example makes it so that /var/lib/lxc/c2/rootfs/home/ubuntu/.bashrc
which is owned by uid and gid 1000 as being owned by uid and gid 10001000 at
/vol/contpool/lowermap/home/ubuntu/.bashrc.

Similarly, the user creates another idmapped mount using the same idmapping:

        mount-idmapped/mount-idmapped --map-mount=b:0:10000000:65536 \
                                      /vol/contpool/ctrover/ \
                                      /vol/contpool/overmap/

Assume the user wants to expose these idmapped mounts through an overlayfs
mount to a container.

       mount -t overlay overlay                      \
             -o lowerdir=/vol/contpool/lowermap,     \
                upperdir=/vol/contpool/overmap/over, \
                workdir=/vol/contpool/overmap/work   \
             /vol/contpool/merge

The user can do this in two ways:

(1) Mount overlayfs in the initial user namespace and expose it to the
    container.
(2) Mount overlayfs on top of the idmapped mounts inside of the container's
    user namespace.

Let's assume the user chooses the (1) option and mounts overlayfs on the host
and then changes into a container which uses the idmapping 0:10000000:65536
which is the same used for the two idmapped mounts.

Now the user tries to retrieve the POSIX ACLs using the getfacl command

        getfacl -n /vol/contpool/lowermap/home/ubuntu/.bashrc

and to their surprise they see:

        # file: vol/contpool/merge/home/ubuntu/.bashrc
        # owner: 1000
        # group: 1000
        user::rw-
        user:4294967295:rwx
        group::r--
        mask::rwx
        other::r--

indicating the the uid wasn't correctly translated according to the idmapped
mount. The problem is how we currently translate POSIX ACLs. Let's inspect the
callchain in this example:

        idmapped mount /vol/contpool/merge:      0:10000000:65536
        caller's idmapping:                      0:10000000:65536
        overlayfs idmapping (ofs->creator_cred): 0:0:4k /* initial idmapping */

        sys_getxattr()
        -> path_getxattr()
           -> getxattr()
              -> do_getxattr()
                  |> vfs_getxattr()
                  |  -> __vfs_getxattr()
                  |     -> handler->get == ovl_posix_acl_xattr_get()
                  |        -> ovl_xattr_get()
                  |           -> vfs_getxattr()
                  |              -> __vfs_getxattr()
                  |                 -> handler->get() /* lower filesystem callback */
                  |> posix_acl_fix_xattr_to_user()
                     {
                              4 = make_kuid(&init_user_ns, 4);
                              4 = mapped_kuid_fs(&init_user_ns /* no idmapped mount */, 4);
                              /* FAILURE */
                             -1 = from_kuid(0:10000000:65536 /* caller's idmapping */, 4);
                     }

If the user chooses to use option (2) and mounts overlayfs on top of idmapped
mounts inside the container things don't look that much better:

        idmapped mount /vol/contpool/merge:      0:10000000:65536
        caller's idmapping:                      0:10000000:65536
        overlayfs idmapping (ofs->creator_cred): 0:10000000:65536

        sys_getxattr()
        -> path_getxattr()
           -> getxattr()
              -> do_getxattr()
                  |> vfs_getxattr()
                  |  -> __vfs_getxattr()
                  |     -> handler->get == ovl_posix_acl_xattr_get()
                  |        -> ovl_xattr_get()
                  |           -> vfs_getxattr()
                  |              -> __vfs_getxattr()
                  |                 -> handler->get() /* lower filesystem callback */
                  |> posix_acl_fix_xattr_to_user()
                     {
                              4 = make_kuid(&init_user_ns, 4);
                              4 = mapped_kuid_fs(&init_user_ns, 4);
                              /* FAILURE */
                             -1 = from_kuid(0:10000000:65536 /* caller's idmapping */, 4);
                     }

As is easily seen the problem arises because the idmapping of the lower mount
isn't taken into account as all of this happens in do_gexattr(). But
do_getxattr() is always called on an overlayfs mount and inode and thus cannot
possible take the idmapping of the lower layers into account.

This problem is similar for fscaps but there the translation happens as part of
vfs_getxattr() already. Let's walk through an fscaps overlayfs callchain:

        setcap 'cap_net_raw+ep' /var/lib/lxc/c2/rootfs/home/ubuntu/.bashrc

The expected outcome here is that we'll receive the cap_net_raw capability as
we are able to map the uid associated with the fscap to 0 within our container.
IOW, we want to see 0 as the result of the idmapping translations.

If the user chooses option (1) we get the following callchain for fscaps:

        idmapped mount /vol/contpool/merge:      0:10000000:65536
        caller's idmapping:                      0:10000000:65536
        overlayfs idmapping (ofs->creator_cred): 0:0:4k /* initial idmapping */

        sys_getxattr()
        -> path_getxattr()
           -> getxattr()
              -> do_getxattr()
                   -> vfs_getxattr()
                      -> xattr_getsecurity()
                         -> security_inode_getsecurity()                                       ________________________________
                            -> cap_inode_getsecurity()                                         |                              |
                               {                                                               V                              |
                                        10000000 = make_kuid(0:0:4k /* overlayfs idmapping */, 10000000);                     |
                                        10000000 = mapped_kuid_fs(0:0:4k /* no idmapped mount */, 10000000);                  |
                                               /* Expected result is 0 and thus that we own the fscap. */                     |
                                               0 = from_kuid(0:10000000:65536 /* caller's idmapping */, 10000000);            |
                               }                                                                                              |
                               -> vfs_getxattr_alloc()                                                                        |
                                  -> handler->get == ovl_other_xattr_get()                                                    |
                                     -> vfs_getxattr()                                                                        |
                                        -> xattr_getsecurity()                                                                |
                                           -> security_inode_getsecurity()                                                    |
                                              -> cap_inode_getsecurity()                                                      |
                                                 {                                                                            |
                                                                0 = make_kuid(0:0:4k /* lower s_user_ns */, 0);               |
                                                         10000000 = mapped_kuid_fs(0:10000000:65536 /* idmapped mount */, 0); |
                                                         10000000 = from_kuid(0:0:4k /* overlayfs idmapping */, 10000000);    |
                                                         |____________________________________________________________________|
                                                 }
                                                 -> vfs_getxattr_alloc()
                                                    -> handler->get == /* lower filesystem callback */

And if the user chooses option (2) we get:

        idmapped mount /vol/contpool/merge:      0:10000000:65536
        caller's idmapping:                      0:10000000:65536
        overlayfs idmapping (ofs->creator_cred): 0:10000000:65536

        sys_getxattr()
        -> path_getxattr()
           -> getxattr()
              -> do_getxattr()
                   -> vfs_getxattr()
                      -> xattr_getsecurity()
                         -> security_inode_getsecurity()                                                _______________________________
                            -> cap_inode_getsecurity()                                                  |                             |
                               {                                                                        V                             |
                                       10000000 = make_kuid(0:10000000:65536 /* overlayfs idmapping */, 0);                           |
                                       10000000 = mapped_kuid_fs(0:0:4k /* no idmapped mount */, 10000000);                           |
                                               /* Expected result is 0 and thus that we own the fscap. */                             |
                                              0 = from_kuid(0:10000000:65536 /* caller's idmapping */, 10000000);                     |
                               }                                                                                                      |
                               -> vfs_getxattr_alloc()                                                                                |
                                  -> handler->get == ovl_other_xattr_get()                                                            |
                                    |-> vfs_getxattr()                                                                                |
                                        -> xattr_getsecurity()                                                                        |
                                           -> security_inode_getsecurity()                                                            |
                                              -> cap_inode_getsecurity()                                                              |
                                                 {                                                                                    |
                                                                 0 = make_kuid(0:0:4k /* lower s_user_ns */, 0);                      |
                                                          10000000 = mapped_kuid_fs(0:10000000:65536 /* idmapped mount */, 0);        |
                                                                 0 = from_kuid(0:10000000:65536 /* overlayfs idmapping */, 10000000); |
                                                                 |____________________________________________________________________|
                                                 }
                                                 -> vfs_getxattr_alloc()
                                                    -> handler->get == /* lower filesystem callback */

We can see how the translation happens correctly in those cases as the
conversion happens within the vfs_getxattr() helper.

For POSIX ACLs we need to do something similar. However, in contrast to fscaps
we cannot apply the fix directly to the kernel internal posix acl data
structure as this would alter the cached values and would also require a rework
of how we currently deal with POSIX ACLs in general which almost never take the
filesystem idmapping into account (the noteable exception being FUSE but even
there the implementation is special) and instead retrieve the raw values based
on the initial idmapping.

The correct values are then generated right before returning to userspace. The
fix for this is to move taking the mount's idmapping into account directly in
vfs_getxattr() instead of having it be part of posix_acl_fix_xattr_to_user().

To this end we split out two small and unexported helpers
posix_acl_getxattr_idmapped_mnt() and posix_acl_setxattr_idmapped_mnt(). The
former to be called in vfs_getxattr() and the latter to be called in
vfs_setxattr().

Let's go back to the original example. Assume the user chose option (1) and
mounted overlayfs on top of idmapped mounts on the host:

        idmapped mount /vol/contpool/merge:      0:10000000:65536
        caller's idmapping:                      0:10000000:65536
        overlayfs idmapping (ofs->creator_cred): 0:0:4k /* initial idmapping */

        sys_getxattr()
        -> path_getxattr()
           -> getxattr()
              -> do_getxattr()
                  |> vfs_getxattr()
                  |  |> __vfs_getxattr()
                  |  |  -> handler->get == ovl_posix_acl_xattr_get()
                  |  |     -> ovl_xattr_get()
                  |  |        -> vfs_getxattr()
                  |  |           |> __vfs_getxattr()
                  |  |           |  -> handler->get() /* lower filesystem callback */
                  |  |           |> posix_acl_getxattr_idmapped_mnt()
                  |  |              {
                  |  |                       /* This is a nop on non-idmapped mounts. */
                  |  |                              4 = make_kuid(&init_user_ns, 4);
                  |  |                       10000004 = mapped_kuid_fs(0:10000000:65536 /* lower idmapped mount */, 4);
                  |  |                       10000004 = from_kuid(&init_user_ns, 10000004);
                  |  |                       |___________________________________
                  |  |              }                                           |
                  |  |> posix_acl_getxattr_idmapped_mnt()                       |
                  |     {                                                       |
                  |             /* This is a nop on non-idmapped mounts. */     V
                  |             10000004 = make_kuid(&init_user_ns, 10000004);
                  |             10000004 = mapped_kuid_fs(&init_user_ns /* no idmapped mount */, 10000004);
                  |             10000004 = from_kuid(&init_user_ns, 10000004);
                  |     }       |_________________________________________________
                  |                                                              |
                  |> posix_acl_fix_xattr_to_user()                               |
                     {                                                           V
                                 10000004 = make_kuid(0:0:4k /* init_user_ns */, 10000004);
                                        /* SUCCESS */
                                        4 = from_kuid(0:10000000:65536 /* caller's idmapping */, 10000004);
                     }

And similarly if the user chooses option (1) and mounted overayfs on top of
idmapped mounts inside the container:

        idmapped mount /vol/contpool/merge:      0:10000000:65536
        caller's idmapping:                      0:10000000:65536
        overlayfs idmapping (ofs->creator_cred): 0:10000000:65536

        sys_getxattr()
        -> path_getxattr()
           -> getxattr()
              -> do_getxattr()
                  |> vfs_getxattr()
                  |  |> __vfs_getxattr()
                  |  |  -> handler->get == ovl_posix_acl_xattr_get()
                  |  |     -> ovl_xattr_get()
                  |  |        -> vfs_getxattr()
                  |  |           |> __vfs_getxattr()
                  |  |           |  -> handler->get() /* lower filesystem callback */
                  |  |           |> posix_acl_getxattr_idmapped_mnt()
                  |  |              {
                  |  |                       /* This is a nop on non-idmapped mounts. */
                  |  |                              4 = make_kuid(&init_user_ns, 4);
                  |  |                       10000004 = mapped_kuid_fs(0:10000000:65536 /* lower idmapped mount */, 4);
                  |  |                       10000004 = from_kuid(&init_user_ns, 10000004);
                  |  |                       |___________________________________
                  |  |              }                                           |
                  |  |> posix_acl_getxattr_idmapped_mnt()                       |
                  |     {                                                       |
                  |             /* This is a nop on non-idmapped mounts. */     V
                  |             10000004 = make_kuid(&init_user_ns, 10000004);
                  |             10000004 = mapped_kuid_fs(&init_user_ns /* no idmapped mount */, 10000004);
                  |             10000004 = from_kuid(0(&init_user_ns, 10000004);
                  |             |_________________________________________________
                  |     }                                                        |
                  |> posix_acl_fix_xattr_to_user()                               |
                     {                                                           V
                                 10000004 = make_kuid(0:0:4k /* init_user_ns */, 10000004);
                                        /* SUCCESS */
                                        4 = from_kuid(0:10000000:65536 /* caller's idmappings */, 10000004);
                     }

Link: brauner/mount-idmapped#9
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

fs/ksmbd/vfs.c

@@ -963,7 +963,7 @@ ssize_t ksmbd_vfs_getxattr(struct user_namespace *user_ns,
  */
 int ksmbd_vfs_setxattr(struct user_namespace *user_ns,
 		       struct dentry *dentry, const char *attr_name,
-		       const void *attr_value, size_t attr_size, int flags)
+		       void *attr_value, size_t attr_size, int flags)
 {
 	int err;
 

fs/ksmbd/vfs.h

@@ -109,7 +109,7 @@ ssize_t ksmbd_vfs_casexattr_len(struct user_namespace *user_ns,
 				int attr_name_len);
 int ksmbd_vfs_setxattr(struct user_namespace *user_ns,
 		       struct dentry *dentry, const char *attr_name,
-		       const void *attr_value, size_t attr_size, int flags);
+		       void *attr_value, size_t attr_size, int flags);
 int ksmbd_vfs_xattr_stream_name(char *stream_name, char **xattr_stream_name,
 				size_t *xattr_stream_name_size, int s_type);
 int ksmbd_vfs_remove_xattr(struct user_namespace *user_ns,

fs/overlayfs/overlayfs.h

@@ -256,7 +256,7 @@ static inline ssize_t ovl_path_getxattr(struct ovl_fs *ofs,
 }
 
 static inline int ovl_do_setxattr(struct ovl_fs *ofs, struct dentry *dentry,
-				  const char *name, const void *value,
+				  const char *name, void *value,
 				  size_t size, int flags)
 {
 	int err = vfs_setxattr(ovl_upper_mnt_userns(ofs), dentry, name, value, size, flags);

fs/posix_acl.c

@@ -710,46 +710,119 @@ EXPORT_SYMBOL(posix_acl_update_mode);
 /*
  * Fix up the uids and gids in posix acl extended attributes in place.
  */
-static void posix_acl_fix_xattr_userns(
-	struct user_namespace *to, struct user_namespace *from,
-	struct user_namespace *mnt_userns,
-	void *value, size_t size, bool from_user)
+static int posix_acl_fix_xattr_common(void *value, size_t size)
+{
+	struct posix_acl_xattr_header *header = value;
+	int count;
+
+	if (!header)
+		return -EINVAL;
+	if (size < sizeof(struct posix_acl_xattr_header))
+		return -EINVAL;
+	if (header->a_version != cpu_to_le32(POSIX_ACL_XATTR_VERSION))
+		return -EINVAL;
+
+	count = posix_acl_xattr_count(size);
+	if (count < 0)
+		return -EINVAL;
+	if (count == 0)
+		return -EINVAL;
+
+	return count;
+}
+
+void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
+				     const struct inode *inode,
+				     void *value, size_t size)
 {
 	struct posix_acl_xattr_header *header = value;
 	struct posix_acl_xattr_entry *entry = (void *)(header + 1), *end;
 	int count;
 	kuid_t uid;
 	kgid_t gid;
 
-	if (!value)
+	if (no_idmapping(mnt_userns, i_user_ns(inode)))
 		return;
-	if (size < sizeof(struct posix_acl_xattr_header))
+
+	count = posix_acl_fix_xattr_common(value, size);
+	if (count < 0)
 		return;
-	if (header->a_version != cpu_to_le32(POSIX_ACL_XATTR_VERSION))
+
+	for (end = entry + count; entry != end; entry++) {
+		switch (le16_to_cpu(entry->e_tag)) {
+		case ACL_USER:
+			uid = make_kuid(&init_user_ns, le32_to_cpu(entry->e_id));
+			uid = mapped_kuid_fs(mnt_userns, &init_user_ns, uid);
+			entry->e_id = cpu_to_le32(from_kuid(&init_user_ns, uid));
+			break;
+		case ACL_GROUP:
+			gid = make_kgid(&init_user_ns, le32_to_cpu(entry->e_id));
+			gid = mapped_kgid_fs(mnt_userns, &init_user_ns, gid);
+			entry->e_id = cpu_to_le32(from_kgid(&init_user_ns, gid));
+			break;
+		default:
+			break;
+		}
+	}
+}
+
+void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
+				     const struct inode *inode,
+				     void *value, size_t size)
+{
+	struct posix_acl_xattr_header *header = value;
+	struct posix_acl_xattr_entry *entry = (void *)(header + 1), *end;
+	int count;
+	kuid_t uid;
+	kgid_t gid;
+
+	if (no_idmapping(mnt_userns, i_user_ns(inode)))
 		return;
 
-	count = posix_acl_xattr_count(size);
+	count = posix_acl_fix_xattr_common(value, size);
 	if (count < 0)
 		return;
-	if (count == 0)
+
+	for (end = entry + count; entry != end; entry++) {
+		switch (le16_to_cpu(entry->e_tag)) {
+		case ACL_USER:
+			uid = make_kuid(&init_user_ns, le32_to_cpu(entry->e_id));
+			uid = mapped_kuid_user(mnt_userns, &init_user_ns, uid);
+			entry->e_id = cpu_to_le32(from_kuid(&init_user_ns, uid));
+			break;
+		case ACL_GROUP:
+			gid = make_kgid(&init_user_ns, le32_to_cpu(entry->e_id));
+			gid = mapped_kgid_user(mnt_userns, &init_user_ns, gid);
+			entry->e_id = cpu_to_le32(from_kgid(&init_user_ns, gid));
+			break;
+		default:
+			break;
+		}
+	}
+}
+
+static void posix_acl_fix_xattr_userns(
+	struct user_namespace *to, struct user_namespace *from,
+	void *value, size_t size)
+{
+	struct posix_acl_xattr_header *header = value;
+	struct posix_acl_xattr_entry *entry = (void *)(header + 1), *end;
+	int count;
+	kuid_t uid;
+	kgid_t gid;
+
+	count = posix_acl_fix_xattr_common(value, size);
+	if (count < 0)
 		return;
 
 	for (end = entry + count; entry != end; entry++) {
 		switch(le16_to_cpu(entry->e_tag)) {
 		case ACL_USER:
 			uid = make_kuid(from, le32_to_cpu(entry->e_id));
-			if (from_user)
-				uid = mapped_kuid_user(mnt_userns, &init_user_ns, uid);
-			else
-				uid = mapped_kuid_fs(mnt_userns, &init_user_ns, uid);
 			entry->e_id = cpu_to_le32(from_kuid(to, uid));
 			break;
 		case ACL_GROUP:
 			gid = make_kgid(from, le32_to_cpu(entry->e_id));
-			if (from_user)
-				gid = mapped_kgid_user(mnt_userns, &init_user_ns, gid);
-			else
-				gid = mapped_kgid_fs(mnt_userns, &init_user_ns, gid);
 			entry->e_id = cpu_to_le32(from_kgid(to, gid));
 			break;
 		default:
@@ -758,34 +831,20 @@ static void posix_acl_fix_xattr_userns(
 	}
 }
 
-void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
-				   struct inode *inode,
-				   void *value, size_t size)
+void posix_acl_fix_xattr_from_user(void *value, size_t size)
 {
 	struct user_namespace *user_ns = current_user_ns();
-
-	/* Leave ids untouched on non-idmapped mounts. */
-	if (no_idmapping(mnt_userns, i_user_ns(inode)))
-		mnt_userns = &init_user_ns;
-	if ((user_ns == &init_user_ns) && (mnt_userns == &init_user_ns))
+	if (user_ns == &init_user_ns)
 		return;
-	posix_acl_fix_xattr_userns(&init_user_ns, user_ns, mnt_userns, value,
-				   size, true);
+	posix_acl_fix_xattr_userns(&init_user_ns, user_ns, value, size);
 }
 
-void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
-				 struct inode *inode,
-				 void *value, size_t size)
+void posix_acl_fix_xattr_to_user(void *value, size_t size)
 {
 	struct user_namespace *user_ns = current_user_ns();
-
-	/* Leave ids untouched on non-idmapped mounts. */
-	if (no_idmapping(mnt_userns, i_user_ns(inode)))
-		mnt_userns = &init_user_ns;
-	if ((user_ns == &init_user_ns) && (mnt_userns == &init_user_ns))
+	if (user_ns == &init_user_ns)
 		return;
-	posix_acl_fix_xattr_userns(user_ns, &init_user_ns, mnt_userns, value,
-				   size, false);
+	posix_acl_fix_xattr_userns(user_ns, &init_user_ns, value, size);
 }
 
 /*

fs/xattr.c

@@ -282,13 +282,19 @@ __vfs_setxattr_locked(struct user_namespace *mnt_userns, struct dentry *dentry,
 }
 EXPORT_SYMBOL_GPL(__vfs_setxattr_locked);
 
+static inline bool is_posix_acl_xattr(const char *name)
+{
+	return (strcmp(name, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
+	       (strcmp(name, XATTR_NAME_POSIX_ACL_DEFAULT) == 0);
+}
+
 int
 vfs_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
-	     const char *name, const void *value, size_t size, int flags)
+	     const char *name, void *value, size_t size, int flags)
 {
 	struct inode *inode = dentry->d_inode;
 	struct inode *delegated_inode = NULL;
-	const void  *orig_value = value;
+	void  *orig_value = value;
 	int error;
 
 	if (size && strcmp(name, XATTR_NAME_CAPS) == 0) {
@@ -298,6 +304,9 @@ vfs_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 		size = error;
 	}
 
+	if (size && is_posix_acl_xattr(name))
+		posix_acl_setxattr_idmapped_mnt(mnt_userns, inode, value, size);
+
 retry_deleg:
 	inode_lock(inode);
 	error = __vfs_setxattr_locked(mnt_userns, dentry, name, value, size,
@@ -431,7 +440,10 @@ vfs_getxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 		return ret;
 	}
 nolsm:
-	return __vfs_getxattr(dentry, inode, name, value, size);
+	error = __vfs_getxattr(dentry, inode, name, value, size);
+	if (error > 0 && is_posix_acl_xattr(name))
+		posix_acl_getxattr_idmapped_mnt(mnt_userns, inode, value, size);
+	return error;
 }
 EXPORT_SYMBOL_GPL(vfs_getxattr);
 
@@ -577,8 +589,7 @@ static void setxattr_convert(struct user_namespace *mnt_userns,
 	if (ctx->size &&
 		((strcmp(ctx->kname->name, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
 		(strcmp(ctx->kname->name, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)))
-		posix_acl_fix_xattr_from_user(mnt_userns, d_inode(d),
-						ctx->kvalue, ctx->size);
+		posix_acl_fix_xattr_from_user(ctx->kvalue, ctx->size);
 }
 
 int do_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
@@ -695,8 +706,7 @@ do_getxattr(struct user_namespace *mnt_userns, struct dentry *d,
 	if (error > 0) {
 		if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
 		    (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
-			posix_acl_fix_xattr_to_user(mnt_userns, d_inode(d),
-							ctx->kvalue, error);
+			posix_acl_fix_xattr_to_user(ctx->kvalue, error);
 		if (ctx->size && copy_to_user(ctx->value, ctx->kvalue, error))
 			error = -EFAULT;
 	} else if (error == -ERANGE && ctx->size >= XATTR_SIZE_MAX) {

include/linux/capability.h

@@ -276,6 +276,6 @@ int get_vfs_caps_from_disk(struct user_namespace *mnt_userns,
 			   struct cpu_vfs_cap_data *cpu_caps);
 
 int cap_convert_nscap(struct user_namespace *mnt_userns, struct dentry *dentry,
-		      const void **ivalue, size_t size);
+		      void **ivalue, size_t size);
 
 #endif /* !_LINUX_CAPABILITY_H */

include/linux/posix_acl_xattr.h

@@ -33,21 +33,29 @@ posix_acl_xattr_count(size_t size)
 }
 
 #ifdef CONFIG_FS_POSIX_ACL
-void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
-				   struct inode *inode,
-				   void *value, size_t size);
-void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
-				   struct inode *inode,
-				 void *value, size_t size);
+void posix_acl_fix_xattr_from_user(void *value, size_t size);
+void posix_acl_fix_xattr_to_user(void *value, size_t size);
+void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
+				     const struct inode *inode,
+				     void *value, size_t size);
+void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
+				     const struct inode *inode,
+				     void *value, size_t size);
 #else
-static inline void posix_acl_fix_xattr_from_user(struct user_namespace *mnt_userns,
-						 struct inode *inode,
-						 void *value, size_t size)
+static inline void posix_acl_fix_xattr_from_user(void *value, size_t size)
 {
 }
-static inline void posix_acl_fix_xattr_to_user(struct user_namespace *mnt_userns,
-					       struct inode *inode,
-					       void *value, size_t size)
+static inline void posix_acl_fix_xattr_to_user(void *value, size_t size)
+{
+}
+void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
+				     const struct inode *inode,
+				     void *value, size_t size)
+{
+}
+void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
+				     const struct inode *inode,
+				     void *value, size_t size)
 {
 }
 #endif

include/linux/xattr.h

@@ -61,7 +61,7 @@ int __vfs_setxattr_locked(struct user_namespace *, struct dentry *,
 			  const char *, const void *, size_t, int,
 			  struct inode **);
 int vfs_setxattr(struct user_namespace *, struct dentry *, const char *,
-		 const void *, size_t, int);
+		 void *, size_t, int);
 int __vfs_removexattr(struct user_namespace *, struct dentry *, const char *);
 int __vfs_removexattr_locked(struct user_namespace *, struct dentry *,
 			     const char *, struct inode **);

security/commoncap.c

@@ -538,7 +538,7 @@ static bool validheader(size_t size, const struct vfs_cap_data *cap)
  * Return: On success, return the new size; on error, return < 0.
  */
 int cap_convert_nscap(struct user_namespace *mnt_userns, struct dentry *dentry,
-		      const void **ivalue, size_t size)
+		      void **ivalue, size_t size)
 {
 	struct vfs_ns_cap_data *nscap;
 	uid_t nsrootid;