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Subject: Re: [PATCH v5 2/3] overlayfs: check CAP_MKNOD before issuing
 vfs_whiteout
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Jonathan Corbet <corbet@lwn.net>,
        Vivek Goyal <vgoyal@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-doc@vger.kernel.org
References: <20180828165319.211563-1-salyzyn@android.com>
 <CAOQ4uxhRPOC_prhygrjndMMUrtU-S7FrLJ0tb4eJUV94GFj38w@mail.gmail.com>
 <CAOQ4uxg=maKZi=5Xet4mC1=uUXftyyKZagj0TBBm8emPDaLmtg@mail.gmail.com>
From:   Mark Salyzyn <salyzyn@android.com>
Message-ID: <004057f1-f0cd-5410-e4e3-a17287613f89@android.com>
Date:   Tue, 28 Aug 2018 11:51:55 -0700
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On 08/28/2018 11:42 AM, Amir Goldstein wrote:
> On Tue, Aug 28, 2018 at 8:43 PM Amir Goldstein <amir73il@gmail.com> wrote:
>> On Tue, Aug 28, 2018 at 7:53 PM Mark Salyzyn <salyzyn@android.com> wrote:
>>> Assumption never checked, should fail if the mounter creds are not
>>> sufficient.
>>>
>>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>>> Cc: Miklos Szeredi <miklos@szeredi.hu>
>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>> Cc: Vivek Goyal <vgoyal@redhat.com>
>>> Cc: Eric W. Biederman <ebiederm@xmission.com>
>>> Cc: Amir Goldstein <amir73il@gmail.com>
>>> Cc: Randy Dunlap <rdunlap@infradead.org>
>>> Cc: Stephen Smalley <sds@tycho.nsa.gov>
>>> Cc: linux-unionfs@vger.kernel.org
>>> Cc: linux-doc@vger.kernel.org
>>> Cc: linux-kernel@vger.kernel.org
>>>
>>> v5
>>> - dependency of "overlayfs: override_creds=off option bypass creator_cred"
>>> ---
>>>   fs/overlayfs/overlayfs.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
>>> index 7538b9b56237..bf3a80157d42 100644
>>> --- a/fs/overlayfs/overlayfs.h
>>> +++ b/fs/overlayfs/overlayfs.h
>>> @@ -176,7 +176,7 @@ static inline int ovl_do_rename(struct inode *olddir, struct dentry *olddentry,
>>>
>>>   static inline int ovl_do_whiteout(struct inode *dir, struct dentry *dentry)
>>>   {
>>> -       int err = vfs_whiteout(dir, dentry);
>>> +       int err = capable(CAP_MKNOD) ? vfs_whiteout(dir, dentry) : -EPERM;
>> Should that be ns_capable()? Should the test go into vfs_whiteout()?
>> I feel there is no convention at all.
>>
> Nevermind, I don't think creating a whiteout poses any risk, so don't think
> we need to worry about CAP_MKNOD.
>
> Thanks,
> Amir.

Ok, will discard from the set, we can address this later if it creates 
concern (as in, not a dependency to my proposed feature flag). So we 
feel that whiteout node in the writeable playground of workdir/upperdir 
is not in itself a security concern. Other (more dangerous) mknod will 
be checked against the caller's credentials coming in.

-- Mark