fanotify: Disallow permission events for proc filesystem

Proc filesystem has special locking rules for various files. Thus
fanotify which opens files on event delivery can easily deadlock
against another process that waits for fanotify permission event to be
handled. Since permission events on /proc have doubtful value anyway,
just disallow them.

Link: https://lore.kernel.org/linux-fsdevel/20190320131642.GE9485@quack2.suse.cz/
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index a90bb19dcfa287c34234b2954524d7047481fe40..91006f47e42054716e00be18ccdb4723d3a34bf6 100644 (file)
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -920,6 +920,22 @@ static int fanotify_test_fid(struct path *path, __kernel_fsid_t *fsid)
        return 0;
 }
 
+static int fanotify_events_supported(struct path *path, __u64 mask)
+{
+       /*
+        * Some filesystems such as 'proc' acquire unusual locks when opening
+        * files. For them fanotify permission events have high chances of
+        * deadlocking the system - open done when reporting fanotify event
+        * blocks on this "unusual" lock while another process holding the lock
+        * waits for fanotify permission event to be answered. Just disallow
+        * permission events for such filesystems.
+        */
+       if (mask & FANOTIFY_PERM_EVENTS &&
+           path->mnt->mnt_sb->s_type->fs_flags & FS_DISALLOW_NOTIFY_PERM)
+               return -EINVAL;
+       return 0;
+}
+
 static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
                            int dfd, const char  __user *pathname)
 {
@@ -1018,6 +1034,12 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
        if (ret)
                goto fput_and_out;
 
+       if (flags & FAN_MARK_ADD) {
+               ret = fanotify_events_supported(&path, mask);
+               if (ret)
+                       goto path_put_and_out;
+       }
+
        if (FAN_GROUP_FLAG(group, FAN_REPORT_FID)) {
                ret = fanotify_test_fid(&path, &__fsid);
                if (ret)

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 8b145e7b96614a8da1f5df306fae771539a613a3..522199e9525e5485d0ce3805d1e962b3b254a329 100644 (file)
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -211,7 +211,7 @@ static struct file_system_type proc_fs_type = {
        .init_fs_context        = proc_init_fs_context,
        .parameters             = &proc_fs_parameters,
        .kill_sb                = proc_kill_sb,
-       .fs_flags               = FS_USERNS_MOUNT,
+       .fs_flags               = FS_USERNS_MOUNT | FS_DISALLOW_NOTIFY_PERM,
 };
 
 void __init proc_root_init(void)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index f7fdfe93e25d3e94cc4bc7b053ead12667607cc9..c7136c98b5baf4fb63422a462d5387d261af2278 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2184,6 +2184,7 @@ struct file_system_type {
 #define FS_BINARY_MOUNTDATA    2
 #define FS_HAS_SUBTYPE         4
 #define FS_USERNS_MOUNT                8       /* Can be mounted by userns root */
+#define FS_DISALLOW_NOTIFY_PERM        16      /* Disable fanotify permission events */
 #define FS_RENAME_DOES_D_MOVE  32768   /* FS will handle d_move() during rename() internally. */
        int (*init_fs_context)(struct fs_context *);
        const struct fs_parameter_description *parameters;