fuse: Send security context of inode on file creation

When a new inode is created, send its security context to server along
with creation request (FUSE_CREAT, FUSE_MKNOD, FUSE_MKDIR and FUSE_SYMLINK).
This gives server an opportunity to create new file and set security
context (possibly atomically). In all the configurations it might not
be possible to set context atomically.

Like nfs and ceph, use security_dentry_init_security() to dermine security
context of inode and send it with create, mkdir, mknod, and symlink requests.

Following is the information sent to server.

- struct fuse_secctxs.
  This contains total number of security contexts being sent.

- struct fuse_secctx.
  This contains total size of security context which follows this structure.
  There is one fuse_secctx instance per security context.

- xattr name string.
  This string represents name of xattr which should be used while setting
  security context. As of now it is hardcoded to "security.selinux".

- security context.
  This is the actual security context whose size is specified in fuse_secctx
  struct.

This patch is modified version of patch from
Chirantan Ekbote <chirantan@chromium.org>

v2:
- Added "fuse_secctxs" structure where one can specify how many security
  contexts are being sent. This can be useful down the line if we
  have more than one security contexts being set.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

fs/fuse/dir.c

@@ -17,6 +17,9 @@
 #include <linux/xattr.h>
 #include <linux/iversion.h>
 #include <linux/posix_acl.h>
+#include <linux/security.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
 
 static void fuse_advise_use_readdirplus(struct inode *dir)
 {
@@ -456,6 +459,66 @@ static struct dentry *fuse_lookup(struct inode *dir, struct dentry *entry,
 	return ERR_PTR(err);
 }
 
+static int get_security_context(struct dentry *entry, umode_t mode,
+				void **security_ctx, u32 *security_ctxlen)
+{
+	struct fuse_secctx *fsecctx;
+	struct fuse_secctxs *fsecctxs;
+	void *ctx, *full_ctx;
+	u32 ctxlen, full_ctxlen;
+	int err = 0;
+	const char *name;
+
+	err = security_dentry_init_security(entry, mode, &entry->d_name,
+					    &name, &ctx, &ctxlen);
+	if (err) {
+		if (err != -EOPNOTSUPP)
+			goto out_err;
+		/* No LSM is supporting this security hook. Ignore error */
+		err = 0;
+		ctxlen = 0;
+	}
+
+	if (ctxlen > 0) {
+		void *ptr;
+
+		full_ctxlen = sizeof(*fsecctxs) + sizeof(*fsecctx) +
+			      strlen(name) + ctxlen + 1;
+		full_ctx = kzalloc(full_ctxlen, GFP_KERNEL);
+		if (!full_ctx) {
+			err = -ENOMEM;
+			kfree(ctx);
+			goto out_err;
+		}
+
+		ptr = full_ctx;
+		fsecctxs = (struct fuse_secctxs*) ptr;
+		fsecctxs->nr_secctx = 1;
+		ptr += sizeof(*fsecctxs);
+
+		fsecctx = (struct fuse_secctx*) ptr;
+		fsecctx->size = ctxlen;
+		ptr += sizeof(*fsecctx);
+
+		strcpy(ptr, name);
+		ptr += strlen(name) + 1;
+		memcpy(ptr, ctx, ctxlen);
+		kfree(ctx);
+	} else {
+		full_ctxlen = sizeof(*fsecctxs);
+		full_ctx = kzalloc(full_ctxlen, GFP_KERNEL);
+		if (!full_ctx) {
+			err = -ENOMEM;
+			goto out_err;
+		}
+	}
+
+	*security_ctxlen = full_ctxlen;
+	*security_ctx = full_ctx;
+out_err:
+	return err;
+}
+
 /*
  * Atomic create+open operation
  *
@@ -476,6 +539,8 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	struct fuse_entry_out outentry;
 	struct fuse_inode *fi;
 	struct fuse_file *ff;
+	void *security_ctx = NULL;
+	u32 security_ctxlen;
 
 	/* Userspace expects S_IFREG in create mode */
 	BUG_ON((mode & S_IFMT) != S_IFREG);
@@ -517,6 +582,18 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	args.out_args[0].value = &outentry;
 	args.out_args[1].size = sizeof(outopen);
 	args.out_args[1].value = &outopen;
+
+	if (fm->fc->init_security) {
+		err = get_security_context(entry, mode, &security_ctx,
+					   &security_ctxlen);
+		if (err)
+			goto out_put_forget_req;
+
+		args.in_numargs = 3;
+		args.in_args[2].size = security_ctxlen;
+		args.in_args[2].value = security_ctx;
+	}
+
 	err = fuse_simple_request(fm, &args);
 	if (err)
 		goto out_free_ff;
@@ -554,6 +631,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 
 out_free_ff:
 	fuse_file_free(ff);
+	kfree(security_ctx);
 out_put_forget_req:
 	kfree(forget);
 out_err:
@@ -613,13 +691,15 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
  */
 static int create_new_entry(struct fuse_mount *fm, struct fuse_args *args,
 			    struct inode *dir, struct dentry *entry,
-			    umode_t mode)
+			    umode_t mode, bool init_security)
 {
 	struct fuse_entry_out outarg;
 	struct inode *inode;
 	struct dentry *d;
 	int err;
 	struct fuse_forget_link *forget;
+	void *security_ctx = NULL;
+	u32 security_ctxlen = 0;
 
 	if (fuse_is_bad(dir))
 		return -EIO;
@@ -633,7 +713,29 @@ static int create_new_entry(struct fuse_mount *fm, struct fuse_args *args,
 	args->out_numargs = 1;
 	args->out_args[0].size = sizeof(outarg);
 	args->out_args[0].value = &outarg;
+
+	if (init_security) {
+		unsigned short idx = args->in_numargs;
+
+		if ((size_t)idx >= ARRAY_SIZE(args->in_args)) {
+			err = -ENOMEM;
+			goto out_put_forget_req;
+		}
+
+		err = get_security_context(entry, mode, &security_ctx,
+					   &security_ctxlen);
+		if (err)
+			goto out_put_forget_req;
+
+		if (security_ctxlen > 0) {
+			args->in_args[idx].size = security_ctxlen;
+			args->in_args[idx].value = security_ctx;
+			args->in_numargs++;
+		}
+	}
+
 	err = fuse_simple_request(fm, args);
+	kfree(security_ctx);
 	if (err)
 		goto out_put_forget_req;
 
@@ -691,7 +793,7 @@ static int fuse_mknod(struct user_namespace *mnt_userns, struct inode *dir,
 	args.in_args[0].value = &inarg;
 	args.in_args[1].size = entry->d_name.len + 1;
 	args.in_args[1].value = entry->d_name.name;
-	return create_new_entry(fm, &args, dir, entry, mode);
+	return create_new_entry(fm, &args, dir, entry, mode, fm->fc->init_security);
 }
 
 static int fuse_create(struct user_namespace *mnt_userns, struct inode *dir,
@@ -719,7 +821,8 @@ static int fuse_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	args.in_args[0].value = &inarg;
 	args.in_args[1].size = entry->d_name.len + 1;
 	args.in_args[1].value = entry->d_name.name;
-	return create_new_entry(fm, &args, dir, entry, S_IFDIR);
+	return create_new_entry(fm, &args, dir, entry, S_IFDIR,
+				fm->fc->init_security);
 }
 
 static int fuse_symlink(struct user_namespace *mnt_userns, struct inode *dir,
@@ -735,7 +838,8 @@ static int fuse_symlink(struct user_namespace *mnt_userns, struct inode *dir,
 	args.in_args[0].value = entry->d_name.name;
 	args.in_args[1].size = len;
 	args.in_args[1].value = link;
-	return create_new_entry(fm, &args, dir, entry, S_IFLNK);
+	return create_new_entry(fm, &args, dir, entry, S_IFLNK,
+				fm->fc->init_security);
 }
 
 void fuse_update_ctime(struct inode *inode)
@@ -915,7 +1019,8 @@ static int fuse_link(struct dentry *entry, struct inode *newdir,
 	args.in_args[0].value = &inarg;
 	args.in_args[1].size = newent->d_name.len + 1;
 	args.in_args[1].value = newent->d_name.name;
-	err = create_new_entry(fm, &args, newdir, newent, inode->i_mode);
+	err = create_new_entry(fm, &args, newdir, newent, inode->i_mode,
+			       false);
 	/* Contrary to "normal" filesystems it can happen that link
 	   makes two "logical" inodes point to the same "physical"
 	   inode.  We invalidate the attributes of the old one, so it

fs/fuse/fuse_i.h

@@ -765,6 +765,9 @@ struct fuse_conn {
 	/* Propagate syncfs() to server */
 	unsigned int sync_fs:1;
 
+	/* Initialize security xattrs when creating a new inode */
+	unsigned int init_security:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 

fs/fuse/inode.c

@@ -1152,6 +1152,8 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 			}
 			if (arg->flags & FUSE_SETXATTR_EXT)
 				fc->setxattr_ext = 1;
+			if (arg->flags & FUSE_SECURITY_CTX)
+				fc->init_security = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1195,7 +1197,7 @@ void fuse_send_init(struct fuse_mount *fm)
 		FUSE_PARALLEL_DIROPS | FUSE_HANDLE_KILLPRIV | FUSE_POSIX_ACL |
 		FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
 		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
-		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT;
+		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_SECURITY_CTX;
 #ifdef CONFIG_FUSE_DAX
 	if (fm->fc->dax)
 		ia->in.flags |= FUSE_MAP_ALIGNMENT;

include/uapi/linux/fuse.h

@@ -986,4 +986,24 @@ struct fuse_syncfs_in {
 	uint64_t	padding;
 };
 
+/*
+ * For each security context, send fuse_secctx with size of security context
+ * fuse_secctx will be followed by security context name and this in turn
+ * will be followed by actual context label.
+ * fuse_secctx, name, context
+ * */
+struct fuse_secctx {
+	uint32_t	size;
+	uint32_t	padding;
+};
+
+/*
+ * Contains the information about how many fuse_secctx structures are being
+ * sent.
+ */
+struct fuse_secctxs {
+	uint32_t	nr_secctx;
+	uint32_t	padding;
+};
+
 #endif /* _LINUX_FUSE_H */