fanotify: limit reporting of event with non-decodeable file handles

Commit a95aef69a740 ("fanotify: support reporting non-decodeable file
handles") merged in v6.5-rc1, added the ability to use an fanotify group
with FAN_REPORT_FID mode to watch filesystems that do not support nfs
export, but do know how to encode non-decodeable file handles, with the
newly introduced AT_HANDLE_FID flag.

At the time that this commit was merged, there were no filesystems
in-tree with those traits.

Commit 16aac5ad1fa9 ("ovl: support encoding non-decodable file handles"),
merged in v6.6-rc1, added this trait to overlayfs, thus allowing fanotify
watching of overlayfs with FAN_REPORT_FID mode.

In retrospect, allowing an fanotify filesystem/mount mark on such
filesystem in FAN_REPORT_FID mode will result in getting events with
file handles, without the ability to resolve the filesystem objects from
those file handles (i.e. no open_by_handle_at() support).

For v6.6, the safer option would be to allow this mode for inode marks
only, where the caller has the opportunity to use name_to_handle_at() at
the time of setting the mark. In the future we can revise this decision.

Fixes: a95aef69a740 ("fanotify: support reporting non-decodeable file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20231018100000.2453965-2-amir73il@gmail.com>

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index f69c451018e33defb493eef93b538b6507138422..62fe0b679e586ccbe181000fe94ed5e2b142b203 100644 (file)
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1585,16 +1585,25 @@ static int fanotify_test_fsid(struct dentry *dentry, __kernel_fsid_t *fsid)
 }
 
 /* Check if filesystem can encode a unique fid */
-static int fanotify_test_fid(struct dentry *dentry)
+static int fanotify_test_fid(struct dentry *dentry, unsigned int flags)
 {
+       unsigned int mark_type = flags & FANOTIFY_MARK_TYPE_BITS;
+       const struct export_operations *nop = dentry->d_sb->s_export_op;
+
+       /*
+        * We need to make sure that the filesystem supports encoding of
+        * file handles so user can use name_to_handle_at() to compare fids
+        * reported with events to the file handle of watched objects.
+        */
+       if (!nop)
+               return -EOPNOTSUPP;
+
        /*
-        * We need to make sure that the file system supports at least
-        * encoding a file handle so user can use name_to_handle_at() to
-        * compare fid returned with event to the file handle of watched
-        * objects. However, even the relaxed AT_HANDLE_FID flag requires
-        * at least empty export_operations for ecoding unique file ids.
+        * For sb/mount mark, we also need to make sure that the filesystem
+        * supports decoding file handles, so user has a way to map back the
+        * reported fids to filesystem objects.
         */
-       if (!dentry->d_sb->s_export_op)
+       if (mark_type != FAN_MARK_INODE && !nop->fh_to_dentry)
                return -EOPNOTSUPP;
 
        return 0;
@@ -1812,7 +1821,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
                if (ret)
                        goto path_put_and_out;
 
-               ret = fanotify_test_fid(path.dentry);
+               ret = fanotify_test_fid(path.dentry, flags);
                if (ret)
                        goto path_put_and_out;