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

From: Colin Ian King
Date: Wed Nov 13 2019 - 14:06:06 EST

On 13/11/2019 18:34, Amir Goldstein wrote:
> On Wed, Nov 13, 2019 at 7:57 PM Colin King <colin.king@xxxxxxxxxxxxx> wrote:
>>
>> From: Amir Goldstein <amir73il@xxxxxxxxx>
>>
>> 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@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>
>> Tested-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>`
>> ---
>>
>> 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
>>