Re: [PATCH] ovl: do not ignore disk quota if current task is not privileged

Konstantin Khlebnikov
2017-01-10T16:30:44Z

On 10.01.2017 19:06, Vivek Goyal wrote:
> On Tue, Jan 10, 2017 at 02:26:48PM +0300, Konstantin Khlebnikov wrote:
>> If overlay was mounted by root then quota set for upper layer does not work
>> because overlay now always use mounter's credentials for operations.
>>
>
> Hi Konstantin,
>
> So CAP_SYS_RESOURCE bypasses the quota checks?

Yep. See in fs/quota/dquot.c

static int ignore_hardlimit(struct dquot *dquot)
{
struct mem_dqinfo *info = &sb_dqopt(dquot->dq_sb)->info[dquot->dq_id.type];

return capable(CAP_SYS_RESOURCE) &&
(info->dqi_format->qf_fmt_id != QFMT_VFS_OLD ||
!(info->dqi_flags & DQF_ROOT_SQUASH));
}

Feature DQF_ROOT_SQUASH which disables this bypassing is obsoleted in modern quota formats.

>
> I just created dir upper on xfs filesystem and defined quota of 1G and
> as root user (with cap_sys_resoureces), I am not able to create file
> bigger than 1g in that dir. So looks like xfs quota took affect even
> for privileged user with CAP_SYS_RESOURCE set.
>
> What am I missing?

XFS has its own quota and I cannot find any capable(CAP_SYS_RESOURCE) here.
So, probably XFS ignores this capability and always limits root user.

>
> Vivek
>
>> This patch adds second copy of credentials without CAP_SYS_RESOURCE and
>> use it if current task doesn't have this capability in mounter's user-ns.
>> This affects creation new files, whiteouts, and copy-up operations.
>>
>> Now quota limits are ignored only if both mounter and current task have
>> capability CAP_SYS_RESOURCE in root user namespace.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> Fixes: 1175b6b8d963 ("ovl: do operations on underlying file system in mounter's context")
>> Cc: Vivek Goyal <vgoyal@redhat.com>
>> Cc: Miklos Szeredi <mszeredi@redhat.com>
>> ---
>> fs/overlayfs/ovl_entry.h | 2 ++
>> fs/overlayfs/super.c | 13 ++++++++++++-
>> fs/overlayfs/util.c | 10 +++++++++-
>> 3 files changed, 23 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
>> index d14bca1850d9..55eb3b08e292 100644
>> --- a/fs/overlayfs/ovl_entry.h
>> +++ b/fs/overlayfs/ovl_entry.h
>> @@ -27,6 +27,8 @@ struct ovl_fs {
>> struct ovl_config config;
>> /* creds of process who forced instantiation of super block */
>> const struct cred *creator_cred;
>> + /* the same credentials without CAP_SYS_RESOURCE */
>> + const struct cred *creator_cred_unpriv;
>> };
>>
>> /* private information held for every overlayfs dentry */
>> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
>> index 20f48abbb82f..6a15693641e0 100644
>> --- a/fs/overlayfs/super.c
>> +++ b/fs/overlayfs/super.c
>> @@ -157,6 +157,7 @@ static void ovl_put_super(struct super_block *sb)
>> kfree(ufs->config.upperdir);
>> kfree(ufs->config.workdir);
>> put_cred(ufs->creator_cred);
>> + put_cred(ufs->creator_cred_unpriv);
>> kfree(ufs);
>> }
>>
>> @@ -701,6 +702,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>> unsigned int stacklen = 0;
>> unsigned int i;
>> bool remote = false;
>> + struct cred *cred;
>> int err;
>>
>> err = -ENOMEM;
>> @@ -874,10 +876,17 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>> if (!ufs->creator_cred)
>> goto out_put_lower_mnt;
>>
>> + cred = prepare_creds();
>> + if (!cred)
>> + goto out_put_cred;
>> +
>> + ufs->creator_cred_unpriv = cred;
>> + cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);
>> +
>> err = -ENOMEM;
>> oe = ovl_alloc_entry(numlower);
>> if (!oe)
>> - goto out_put_cred;
>> + goto out_put_cred_unpriv;
>>
>> sb->s_magic = OVERLAYFS_SUPER_MAGIC;
>> sb->s_op = &ovl_super_operations;
>> @@ -914,6 +923,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>>
>> out_free_oe:
>> kfree(oe);
>> +out_put_cred_unpriv:
>> + put_cred(ufs->creator_cred_unpriv);
>> out_put_cred:
>> put_cred(ufs->creator_cred);
>> out_put_lower_mnt:
>> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
>> index 952286f4826c..92f60096c5da 100644
>> --- a/fs/overlayfs/util.c
>> +++ b/fs/overlayfs/util.c
>> @@ -35,8 +35,16 @@ struct dentry *ovl_workdir(struct dentry *dentry)
>> const struct cred *ovl_override_creds(struct super_block *sb)
>> {
>> struct ovl_fs *ofs = sb->s_fs_info;
>> + const struct cred *cred = ofs->creator_cred;
>>
>> - return override_creds(ofs->creator_cred);
>> + /*
>> + * Do not override quota inode limit if current task is not
>> + * capable to do that in mounter's user namespace.
>> + */
>> + if (!ns_capable_noaudit(cred->user_ns, CAP_SYS_RESOURCE))
>> + cred = ofs->creator_cred_unpriv;
>> +
>> + return override_creds(cred);
>> }
>>
>> struct ovl_entry *ovl_alloc_entry(unsigned int numlower)

--
Konstantin