Re: [PATCH][V3] ovl: fix lookup failure on multi lower squashfs

[Colin Ian King <colin.king@canonical.com>]

On 13/11/2019 18:34, Amir Goldstein wrote:
> On Wed, Nov 13, 2019 at 7:57 PM Colin King <colin.king@canonical.com> wrote:
>>
>> From: Amir Goldstein <amir73il@gmail.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.
>>
>> [Colin Ian King] fixed the case of index=off,nfs_export=off
>>
>> 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>
>> Tested-by: Colin Ian King <colin.king@canonical.com>
>> Signed-off-by: Colin Ian King <colin.king@canonical.com>`
>> ---
>>
>> V3: fix the following check:
>>   -       if (!ofs->config.nfs_export && !(ofs->config.index && ofs->upper_mnt))
>>   +       if (!ofs->config.nfs_export && !ofs->upper_mnt)
>>
>> Add the index=off,nfs_export=off comment in the commit message
>>
>> ---
>>  fs/overlayfs/namei.c     |  8 ++++++++
>>  fs/overlayfs/ovl_entry.h |  2 ++
>>  fs/overlayfs/super.c     | 15 +++++++++++----
>>  3 files changed, 21 insertions(+), 4 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..e53d399ce0af 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)
>> @@ -1287,10 +1292,11 @@ 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)) {


> 
> Sorry I wasn't clear.
> Miklos is right. the test ofs->upper_mnt here is bogus because
> nfs_export could be enabled without upper.
> The change you made in v3 to ovl_lower_uuid_ok() should be enough.

OK, so the following is required for V4:

+	if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {

> 
>> +               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",
>>                         uuid_is_null(&sb->s_uuid) ? "null" : "conflicting",
>>                         path->dentry);
>>         }
>> @@ -1303,6 +1309,7 @@ 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;
>> --
>> 2.20.1
>>