Re: [PATCH] ovl: fix oops in ovl_indexdir_cleanup() with nfs_export=on

[Amir Goldstein <amir73il@gmail.com>]

On Sun, Jun 21, 2020 at 9:38 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> Mounting with nfs_export=on, xfstests overlay/031 triggers a kernel panic
> since v5.8-rc1 overlayfs updates.

Ping.

This is a kernel OOPS regression in v5.8-rc1.

Push tested branch ovl-fixes based on v5.8-rc3 with this change.
It contains two other non-critical fixes, which are pretty obvious,
so you may want to consider taking them as well.

Thanks,
Amir.

>
>  overlayfs: orphan index entry (index/00fb1..., ftype=4000, nlink=2)
>  BUG: kernel NULL pointer dereference, address: 0000000000000030
>  RIP: 0010:ovl_cleanup_and_whiteout+0x28/0x220 [overlay]
>
> Bisect point at commit c21c839b8448 ("ovl: whiteout inode sharing")
>
> Minimal reproducer:
> --------------------------------------------------
> rm -rf l u w m
> mkdir -p l u w m
> mkdir -p l/testdir
> touch l/testdir/testfile
> mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
> echo 1 > m/testdir/testfile
> umount m
> rm -rf u/testdir
> mount -t overlay -o lowerdir=l,upperdir=u,workdir=w,nfs_export=on overlay m
> umount m
> --------------------------------------------------
>
> When mount with nfs_export=on, and fail to verify an orphan index, we're
> cleaning this index from indexdir by calling ovl_cleanup_and_whiteout().
> This dereferences ofs->workdir, that was earlier set to NULL.
>
> The design was that ovl->workdir will point at ovl->indexdir, but we are
> assigning ofs->indexdir to ofs->workdir only after ovl_indexdir_cleanup().
> There is no reason not to do it sooner, because once we get success from
> ofs->indexdir = ovl_workdir_create(... there is no turning back.
>
> Reported-and-tested-by: Murphy Zhou <jencce.kernel@gmail.com>
> Fixes: commit c21c839b8448 ("ovl: whiteout inode sharing")
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>  fs/overlayfs/super.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 91476bc422f9..15939ab39c1c 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1354,6 +1354,12 @@ static int ovl_get_indexdir(struct super_block *sb, struct ovl_fs *ofs,
>
>         ofs->indexdir = ovl_workdir_create(ofs, OVL_INDEXDIR_NAME, true);
>         if (ofs->indexdir) {
> +               /* index dir will act also as workdir */
> +               iput(ofs->workdir_trap);
> +               ofs->workdir_trap = NULL;
> +               dput(ofs->workdir);
> +               ofs->workdir = dget(ofs->indexdir);
> +
>                 err = ovl_setup_trap(sb, ofs->indexdir, &ofs->indexdir_trap,
>                                      "indexdir");
>                 if (err)
> @@ -1843,20 +1849,12 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>                 sb->s_flags |= SB_RDONLY;
>
>         if (!(ovl_force_readonly(ofs)) && ofs->config.index) {
> -               /* index dir will act also as workdir */
> -               dput(ofs->workdir);
> -               ofs->workdir = NULL;
> -               iput(ofs->workdir_trap);
> -               ofs->workdir_trap = NULL;
> -
>                 err = ovl_get_indexdir(sb, ofs, oe, &upperpath);
>                 if (err)
>                         goto out_free_oe;
>
>                 /* Force r/o mount with no index dir */
> -               if (ofs->indexdir)
> -                       ofs->workdir = dget(ofs->indexdir);
> -               else
> +               if (!ofs->indexdir)
>                         sb->s_flags |= SB_RDONLY;
>         }
>
> --
> 2.17.1
>