diff mbox series

[v2,3/7] fuse: Fix infinite loop in sget_fc()

Message ID 20210604161156.408496-4-groug@kaod.org (mailing list archive)
State New
Headers show
Series fuse: Some fixes for submounts | expand

Commit Message

Greg Kurz June 4, 2021, 4:11 p.m. UTC
We don't set the SB_BORN flag on submounts. This is wrong as these
superblocks are then considered as partially constructed or dying
in the rest of the code and can break some assumptions.

One such case is when you have a virtiofs filesystem with submounts
and you try to mount it again : virtio_fs_get_tree() tries to obtain
a superblock with sget_fc(). The logic in sget_fc() is to loop until
it has either found an existing matching superblock with SB_BORN set
or to create a brand new one. It is assumed that a superblock without
SB_BORN is transient and the loop is restarted. Forgetting to set
SB_BORN on submounts hence causes sget_fc() to retry forever.

Setting SB_BORN requires special care, i.e. a write barrier for
super_cache_count() which can check SB_BORN without taking any lock.
We should call vfs_get_tree() to deal with that but this requires
to have a proper ->get_tree() implementation for submounts, which
is a bigger piece of work. Go for a simple bug fix in the meatime.

Fixes: bf109c64040f ("fuse: implement crossmounts")
Cc: mreitz@redhat.com
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/dir.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

Comments

Max Reitz June 8, 2021, 3:12 p.m. UTC | #1
On 04.06.21 18:11, Greg Kurz wrote:
> We don't set the SB_BORN flag on submounts. This is wrong as these
> superblocks are then considered as partially constructed or dying
> in the rest of the code and can break some assumptions.
>
> One such case is when you have a virtiofs filesystem with submounts
> and you try to mount it again : virtio_fs_get_tree() tries to obtain
> a superblock with sget_fc(). The logic in sget_fc() is to loop until
> it has either found an existing matching superblock with SB_BORN set
> or to create a brand new one. It is assumed that a superblock without
> SB_BORN is transient and the loop is restarted. Forgetting to set
> SB_BORN on submounts hence causes sget_fc() to retry forever.
>
> Setting SB_BORN requires special care, i.e. a write barrier for
> super_cache_count() which can check SB_BORN without taking any lock.
> We should call vfs_get_tree() to deal with that but this requires
> to have a proper ->get_tree() implementation for submounts, which
> is a bigger piece of work. Go for a simple bug fix in the meatime.
>
> Fixes: bf109c64040f ("fuse: implement crossmounts")
> Cc: mreitz@redhat.com
> Cc: stable@vger.kernel.org # v5.10+
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>   fs/fuse/dir.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)

Reviewed-by: Max Reitz <mreitz@redhat.com>
diff mbox series

Patch

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 3fd1b71e546b..3fa8604c21d5 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -352,6 +352,17 @@  static struct vfsmount *fuse_dentry_automount(struct path *path)
 
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);
+
+	/*
+	 * FIXME: setting SB_BORN requires a write barrier for
+	 *        super_cache_count(). We should actually come
+	 *        up with a proper ->get_tree() implementation
+	 *        for submounts and call vfs_get_tree() to take
+	 *        care of the write barrier.
+	 */
+	smp_wmb();
+	sb->s_flags |= SB_BORN;
+
 	/* We are done configuring the superblock, so unlock it */
 	up_write(&sb->s_umount);