[lustre-devel] [PATCH] staging: luster: llite: fix a potential missing-check bug when copying lumv

[Dilger, Andreas]

On Apr 29, 2018, at 07:20, Greg Kroah-Hartman <gregkh at linuxfoundation.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 at umn.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,

I don't think it fixes a real bug, but it makes the code a bit more clear,
so I'm OK to land it (with minor corrections to commit message per above).

Cheers, Andreas
--
Andreas Dilger
Lustre Principal Architect
Intel Corporation