[PATCH 5/6] ovl: fix corner case of conflicting lower layer uuid

Amir Goldstein
2019-11-17T15:43:48Z

This fixes ovl_lower_uuid_ok() to correctly detect the corner case:
- 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 would not have been set for B, because the check
only involved the list of lower fs. Hence we'll try to decode a layer 2
origin on layer 1 and fail.

We check for conflicting (and null) uuid among all lower layers,
including those layers that are on the same fs as the upper layer.

Reported-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
fs/overlayfs/ovl_entry.h | 2 ++
fs/overlayfs/super.c | 8 ++++++--
2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 93e9fd5fba9d..6c04b6a4e6bf 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -24,6 +24,8 @@ struct ovl_sb {
dev_t pseudo_dev;
/* Unusable (conflicting) uuid */
bool bad_uuid;
+ /* Used as a lower layer (but maybe also as upper) */
+ bool is_lower;
};

struct ovl_layer {
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 09e53780d9bc..e80f79bb8a4e 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1256,7 +1256,7 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
if (!ofs->config.nfs_export && !ofs->upper_mnt)
return true;

- for (i = 1; i <= ofs->maxfsid; i++) {
+ for (i = 0; i <= ofs->maxfsid; 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
@@ -1264,7 +1264,8 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
* if we detect multiple lower fs with the same uuid, we
* disable lower file handle decoding on all of them.
*/
- if (uuid_equal(&ofs->fs[i].sb->s_uuid, uuid)) {
+ if (ofs->fs[i].is_lower &&
+ uuid_equal(&ofs->fs[i].sb->s_uuid, uuid)) {
ofs->fs[i].bad_uuid = true;
return false;
}
@@ -1336,10 +1337,12 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
/*
* All lower layers that share the same fs as upper layer, use the real
* upper st_dev.
+ * is_lower will be set if upper fs is shared with a lower layer.
*/
if (ofs->upper_mnt) {
ofs->fs[0].sb = ofs->upper_mnt->mnt_sb;
ofs->fs[0].pseudo_dev = ofs->upper_mnt->mnt_sb->s_dev;
+ ofs->fs[0].is_lower = false;
}

for (i = 0; i < numlower; i++) {
@@ -1381,6 +1384,7 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
ofs->layers[ofs->numlower].idx = ofs->numlower;
ofs->layers[ofs->numlower].fsid = fsid;
ofs->layers[ofs->numlower].fs = &ofs->fs[fsid];
+ ofs->fs[fsid].is_lower = true;
}

/*
--
2.17.1