On 14/11/2019 20:28, Amir Goldstein wrote:
> 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.
>
> There is an unhandled corner case described by Miklos this way:
> - two filesystems, A and B, both have null uuid
> - upper layer is on A
> - lower layer 1 is also on A
> - lower layer 2 is on B
>
> In this case bad_uuid won't be set for B, because the check only
> involves the list of lower fs. Hence we'll try to decode a layer 2
> origin on layer 1 and fail.
>
> We will deal with this corner case later.
>
> Reported-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Tested-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20191106234301.283006-1-colin.king@xxxxxxxxxxxxx/
> Fixes: 9df085f3c9a2 ("ovl: relax requirement for non null uuid ...")
> Cc: stable@xxxxxxxxxxxxxxx # v4.20+
> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> ---
>
> Changes since v4:
> - Go back to "falling back to index=off,nfs_export=off" warning
> - Mention remaining corner case in commit message
>
> fs/overlayfs/namei.c | 8 ++++++++
> fs/overlayfs/ovl_entry.h | 2 ++
> fs/overlayfs/super.c | 24 +++++++++++++++++-------
> 3 files changed, 27 insertions(+), 7 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 4a6b7870ba8e..cfb236e37fd9 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1255,7 +1255,7 @@ 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))
> + if (!ofs->config.nfs_export && !ofs->upper_mnt)
> return true;
>
> for (i = 0; i < ofs->numlowerfs; i++) {
> @@ -1263,9 +1263,13 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
> * 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 +1281,7 @@ 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)
> @@ -1288,11 +1293,15 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
> }
>
> if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
> - 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",
> - uuid_is_null(&sb->s_uuid) ? "null" : "conflicting",
> - path->dentry);
> + bad_uuid = true;
> + if (ofs->config.index || ofs->config.nfs_export) {
> + 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",
> + uuid_is_null(&sb->s_uuid) ? "null" :
> + "conflicting",
> + path->dentry);
> + }
> }
>
> err = get_anon_bdev(&dev);
> @@ -1303,6 +1312,7 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
>
> ofs->lower_fs[ofs->numlowerfs].sb = sb;
> ofs->lower_fs[ofs->numlowerfs].pseudo_dev = dev;
> + ofs->lower_fs[ofs->numlowerfs].bad_uuid = bad_uuid;
> ofs->numlowerfs++;
>
> return ofs->numlowerfs;
>
Looks good to me. Thanks Amir.
