linux-kernel - Re: [PATCH] staging: luster: llite: fix a potential missing-check bug when copying lumv

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Date:   Sun, 29 Apr 2018 15:58:55 -0500
From:   Wenwen Wang <wang6495@....edu>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Dilger, Andreas" <andreas.dilger@...el.com>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        Ben Evans <bevans@...y.com>, Jeff Layton <jlayton@...hat.com>,
        Aastha Gupta <aastha.gupta4104@...il.com>,
        "kjlu@....edu" <kjlu@....edu>, NeilBrown <neilb@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Drokin, Oleg" <oleg.drokin@...el.com>,
        "lustre-devel@...ts.lustre.org" <lustre-devel@...ts.lustre.org>,
        Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH] staging: luster: llite: fix a potential missing-check bug
 when copying lumv

On Sun, Apr 29, 2018 at 8:20 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Sat, Apr 28, 2018 at 04:04:25PM +0000, Dilger, Andreas wrote:
>> On Apr 27, 2018, at 17:45, Wenwen Wang <wang6495@....edu> wrote:
>> > [PATCH] staging: luster: llite: fix potential missing-check bug when copying lumv
>>
>> (typo) s/luster/lustre/
>>
>> > In ll_dir_ioctl(), the object lumv3 is firstly copied from the user space
>> > using Its address, i.e., lumv1 = &lumv3. If the lmm_magic field of lumv3 is
>> > LOV_USER_MAGIV_V3, lumv3 will be modified by the second copy from the user
>>
>> (typo) s/MAGIV/MAGIC/
>>
>> > space. The second copy is necessary, because the two versions (i.e.,
>> > lov_user_md_v1 and lov_user_md_v3) have different data formats and lengths.
>> > However, given that the user data resides in the user space, a malicious
>> > user-space process can race to change the data between the two copies. By
>> > doing so, the attacker can provide a data with an inconsistent version,
>> > e.g., v1 version + v3 data. This can lead to logical errors in the
>> > following execution in ll_dir_setstripe(), which performs different actions
>> > according to the version specified by the field lmm_magic.
>>
>> This isn't a serious bug in the end.  The LOV_USER_MAGIC_V3 check just copies
>> a bit more data from userspace (the lmm_pool field).  It would be more of a
>> problem if the reverse was possible (copy smaller V1 buffer, but change the
>> magic to LOV_USER_MAGIC_V3 afterward), but this isn't possible since the second
>> copy is not done if there is a V1 magic.  If the user changes from V3 magic
>> to V1 in a racy manner it means less data will be used than copied, which
>> is harmless.
>>
>> > This patch rechecks the version field lmm_magic in the second copy.  If the
>> > version is not as expected, i.e., LOV_USER_MAGIC_V3, an error code will be
>> > returned: -EINVAL.
>>
>> This isn't a bad idea in any case, since it verifies the data copied from
>> userspace is still valid.
>
> So you agree with this patch?  Or do not?
>
> confused,
>
> greg k-h

It is worth fixing this bug, since it offers an opportunity for adversaries
to provide inconsistent user data. In addition to the unwanted version
LOV_USER_MAGIC_V1, a malicious user can also use the version
LMV_USER_MAGIC, which is also unexpected but allowed in the function
ll_dir_setstripe(). These inconsistent data can cause potential logical
errors in the following execution. Hence it is necessary to re-verify the
data copied from userspace.

Thanks!
Wenwen