linux-kernel - Re: [PATCH] ovl: fix uid/gid when creating over whiteout

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Message-ID: <CAJfpegvopC3wA9YQ9J5khG3nCJ9zeXTpZ-SMj515vF5RMXEhNQ@mail.gmail.com>
Date:	Wed, 15 Jun 2016 16:09:47 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Stephen Smalley <sds@...ho.nsa.gov>,
	"linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ovl: fix uid/gid when creating over whiteout

On Wed, Jun 15, 2016 at 4:01 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Wed, Jun 15, 2016 at 03:30:02PM +0200, Miklos Szeredi wrote:

>> --- a/fs/overlayfs/dir.c
>> +++ b/fs/overlayfs/dir.c
>> @@ -405,12 +405,21 @@ static int ovl_create_or_link(struct den
>>               err = ovl_create_upper(dentry, inode, &stat, link, hardlink);
>>       } else {
>>               const struct cred *old_cred;
>> +             struct cred *override_cred;
>>
>>               old_cred = ovl_override_creds(dentry->d_sb);
>>
>> -             err = ovl_create_over_whiteout(dentry, inode, &stat, link,
>> -                                            hardlink);
>> +             err = -ENOMEM;
>> +             override_cred = prepare_creds();
>> +             if (override_cred) {
>> +                     override_cred->fsuid = old_cred->fsuid;
>> +                     override_cred->fsgid = old_cred->fsgid;
>
> Hi Miklos,
>
> I am wondering if we are switching to tasks's ->fsuid and ->fsgid too
> early. ovl_create_over_whiteout() calls ovl_lookup_temp(workdir) and
> IIUC, task might not have permission to do lookup in workdir.
>
> Should we switch to this override_cred, just before ovl_create_real()
> so that task ->fsuid and ->fsgid are used only for creation purposes
> only.

For lookup in workdir only CAP_DAC_OVERRIDE should matter, the actual
value of the fsuid and fsgid should be irrelevant (user, group and
others all have zero permission on workdir).

Thanks,
Miklos