Re: [Virtio-fs] [PATCH 1/4] fuse: Fix crash in fuse_dentry_automount() error path

[Max Reitz]

On 25.05.21 17:02, Greg Kurz wrote:
> 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@xxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx # v5.10+
> Signed-off-by: Greg Kurz <groug@xxxxxxxx>
> ---
>   fs/fuse/dir.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)

Reviewed-by: Max Reitz <mreitz@xxxxxxxxxx>