ovl: fix lookup failure on multi lower squashfs

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@canonical.com>
Tested-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: Miklos Szeredi <mszeredi@redhat.com>

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index e9717c2f7d455902415d2c1c88f9ab15e872085c..f47c591402d76556dd5118288c8b84d8ef380f82 100644 (file)
--- 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 a8279280e88dd89a5acdbda5f7acd179d5b81569..28348c44ea5b2a808277598bc6206168955d4c50 100644 (file)
--- 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 afbcb116a7f1b8d1f26eafcd94c7064bd58b56ef..7621ff176d15ce6920170f6c5064e63f07816522 100644 (file)
--- 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;