Re: [PATCH][V2] ovl: fix lookup failure on multi lower squashfs

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, Nov 7, 2019 at 11:50 AM Colin King <colin.king@canonical.com> wrote:
>
> From: Colin Ian King <colin.king@canonical.com>
>
> In the past, overlayfs required that lower fs have non null uuid in
> order to support nfs export and decode copy up origin file handles.
>
> Commit 9df085f3c9a2 ("ovl: relax requirement for non null uuid of
> lower fs") relaxed this requirement for nfs export support, as long
> as uuid (even if null) is unique among all lower fs.
>
> However, said commit unintentionally also relaxed the non null uuid
> requirement for decoding copy up origin file handles, regardless of
> the unique uuid requirement.
>
> Amend this mistake by disabling decoding of copy up origin file handle
> from lower fs with a conflicting uuid.
>
> We still encode copy up origin file handles from those fs, because
> file handles like those already exist in the wild and because they
> might provide useful information in the future.
>
> Reported-by: Colin Ian King <colin.king@canonical.com>
> Link: https://lore.kernel.org/lkml/20191106234301.283006-1-colin.king@canonical.com/
> Fixes: 9df085f3c9a2 ("ovl: relax requirement for non null uuid ...")
> Cc: stable@vger.kernel.org # v4.20+
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  fs/overlayfs/namei.c     |  8 ++++++++
>  fs/overlayfs/ovl_entry.h |  2 ++
>  fs/overlayfs/super.c     | 16 ++++++++++------
>  3 files changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index e9717c2f7d45..f47c591402d7 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -325,6 +325,14 @@ int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
>         int i;
>
>         for (i = 0; i < ofs->numlower; i++) {
> +               /*
> +                * If lower fs uuid is not unique among lower fs we cannot match
> +                * fh->uuid to layer.
> +                */
> +               if (ofs->lower_layers[i].fsid &&
> +                   ofs->lower_layers[i].fs->bad_uuid)
> +                       continue;
> +
>                 origin = ovl_decode_real_fh(fh, ofs->lower_layers[i].mnt,
>                                             connected);
>                 if (origin)
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index a8279280e88d..28348c44ea5b 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -22,6 +22,8 @@ struct ovl_config {
>  struct ovl_sb {
>         struct super_block *sb;
>         dev_t pseudo_dev;
> +       /* Unusable (conflicting) uuid */
> +       bool bad_uuid;
>  };
>
>  struct ovl_layer {
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index afbcb116a7f1..5d4faab57ba0 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1255,17 +1255,18 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
>  {
>         unsigned int i;
>
> -       if (!ofs->config.nfs_export && !(ofs->config.index && ofs->upper_mnt))
> -               return true;
> -
>         for (i = 0; i < ofs->numlowerfs; i++) {
>                 /*
>                  * We use uuid to associate an overlay lower file handle with a
>                  * lower layer, so we can accept lower fs with null uuid as long
>                  * as all lower layers with null uuid are on the same fs.
> +                * if we detect multiple lower fs with the same uuid, we
> +                * disable lower file handle decoding on all of them.
>                  */
> -               if (uuid_equal(&ofs->lower_fs[i].sb->s_uuid, uuid))
> +               if (uuid_equal(&ofs->lower_fs[i].sb->s_uuid, uuid)) {
> +                       ofs->lower_fs[i].bad_uuid = true;
>                         return false;
> +               }
>         }
>         return true;
>  }
> @@ -1277,6 +1278,7 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
>         unsigned int i;
>         dev_t dev;
>         int err;
> +       bool bad_uuid = false;
>
>         /* fsid 0 is reserved for upper fs even with non upper overlay */
>         if (ofs->upper_mnt && ofs->upper_mnt->mnt_sb == sb)
> @@ -1287,10 +1289,11 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
>                         return i + 1;
>         }
>
> -       if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
> +       if (ofs->upper_mnt && !ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {

This seems bogus: why only check conflicting lower layers if there's
an upper layer?

> +               bad_uuid = true;
>                 ofs->config.index = false;
>                 ofs->config.nfs_export = false;
> -               pr_warn("overlayfs: %s uuid detected in lower fs '%pd2', falling back to index=off,nfs_export=off.\n",
> +               pr_warn("overlayfs: %s uuid detected in lower fs '%pd2', enforcing index=off,nfs_export=off.\n",

And this while this makes sense, it doesn't really fit into this patch
(no change of behavior regarding how index and nfs_export are
handled).

Thanks,
Miklos