Merge tag 'fs.fixes.v5.19-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

Pull vfs idmapping fix from Christian Brauner:
 "This fixes an issue where we fail to change the group of a file when
  the caller owns the file and is a member of the group to change to.

  This is only relevant on idmapped mounts.

  There's a detailed description in the commit message and regression
  tests have been added to xfstests"

* tag 'fs.fixes.v5.19-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  fs: account for group membership

Author: torvalds

fs/attr.c

@@ -61,9 +61,15 @@ static bool chgrp_ok(struct user_namespace *mnt_userns,
 		     const struct inode *inode, kgid_t gid)
 {
 	kgid_t kgid = i_gid_into_mnt(mnt_userns, inode);
-	if (uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) &&
-	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
-		return true;
+	if (uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode))) {
+		kgid_t mapped_gid;
+
+		if (gid_eq(gid, inode->i_gid))
+			return true;
+		mapped_gid = mapped_kgid_fs(mnt_userns, i_user_ns(inode), gid);
+		if (in_group_p(mapped_gid))
+			return true;
+	}
 	if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_CHOWN))
 		return true;
 	if (gid_eq(kgid, INVALID_GID) &&
@@ -123,12 +129,20 @@ int setattr_prepare(struct user_namespace *mnt_userns, struct dentry *dentry,
 
 	/* Make sure a caller can chmod. */
 	if (ia_valid & ATTR_MODE) {
+		kgid_t mapped_gid;
+
 		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EPERM;
+
+		if (ia_valid & ATTR_GID)
+			mapped_gid = mapped_kgid_fs(mnt_userns,
+						i_user_ns(inode), attr->ia_gid);
+		else
+			mapped_gid = i_gid_into_mnt(mnt_userns, inode);
+
 		/* Also check the setgid bit! */
-               if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
-                                i_gid_into_mnt(mnt_userns, inode)) &&
-                    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
+		if (!in_group_p(mapped_gid) &&
+		    !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
 			attr->ia_mode &= ~S_ISGID;
 	}