[PATCH v2 1/7] fuse: Fix crash in fuse_dentry_automount() error path

[PATCH v2 1/7] fuse: Fix crash in fuse_dentry_automount() error path

[Greg Kurz]

If fuse_fill_super_submount() returns an error, the error path
triggers a crash:

[   26.206673] BUG: kernel NULL pointer dereference, address: 0000000000000000
[...]
[   26.226362] RIP: 0010:__list_del_entry_valid+0x25/0x90
[...]
[   26.247938] Call Trace:
[   26.248300]  fuse_mount_remove+0x2c/0x70 [fuse]
[   26.248892]  virtio_kill_sb+0x22/0x160 [virtiofs]
[   26.249487]  deactivate_locked_super+0x36/0xa0
[   26.250077]  fuse_dentry_automount+0x178/0x1a0 [fuse]

The crash happens because fuse_mount_remove() assumes that the FUSE
mount was already added to list under the FUSE connection, but this
only done after fuse_fill_super_submount() has returned success.

This means that until fuse_fill_super_submount() has returned success,
the FUSE mount isn't actually owned by the superblock. We should thus
reclaim ownership by clearing sb->s_fs_info, which will skip the call
to fuse_mount_remove(), and perform rollback, like virtio_fs_get_tree()
already does for the root sb.

Fixes: bf109c64040f ("fuse: implement crossmounts")
Cc: mreitz@redhat.com
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 fs/fuse/dir.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 1b6c001a7dd1..01559061cbfb 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -339,8 +339,12 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 
 	/* Initialize superblock, making @mp_fi its root */
 	err = fuse_fill_super_submount(sb, mp_fi);
-	if (err)
+	if (err) {
+		fuse_conn_put(fc);
+		kfree(fm);
+		sb->s_fs_info = NULL;
 		goto out_put_sb;
+	}
 
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);
-- 
2.31.1

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

[Greg Kurz]

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(+)

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);
 
-- 
2.31.1

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

[Max Reitz]

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>