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

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Date:   Tue, 10 Jan 2017 19:30:44 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-unionfs@...r.kernel.org, Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH] ovl: do not ignore disk quota if current task is not
 privileged

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@...dex-team.ru>
>> Fixes: 1175b6b8d963 ("ovl: do operations on underlying file system in mounter's context")
>> Cc: Vivek Goyal <vgoyal@...hat.com>
>> Cc: Miklos Szeredi <mszeredi@...hat.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