Re: Questions about commit "ipc/shm: Fix shmat mmap nil-page protection"

[Joe Lawrence <joe.lawrence@redhat.com>]

On 09/25/2017 05:44 PM, Andrea Arcangeli wrote:
> Hello,
> 
> On Mon, Sep 25, 2017 at 03:38:07PM -0400, Joe Lawrence wrote:
>> Hi Davidlohr,
>>
>> I was looking into backporting commit 95e91b831f87 ("ipc/shm: Fix shmat
>> mmap nil-page protection") to a distro kernel and Andrea brought up some
>> interesting questions about that change.
>>
>> We saw that a LTP test [1] was added some time ago to reproduce behavior
>> matching that of the original report [2].  However, Andrea and I are a
>> little confused about that original report and what the upstream commit
>> was intended to fix.  A quick summary of our offlist discussion:
>>
>> - This is only about privileged users (and no SELinux).
>>
>> - We modified the 20170119_shmat_nullpage_poc.c reproducer from [2] to
>>   include MAP_FIXED to prove (as root, no SELinux):
>>
>>     It is possible to mmap 0
>>     It is NOT possible to mmap 1
>>
>> - Andrea points out that mmap(1, ...) fails not because of any
>>   mmap_min_addr checks, but for alignment reasons.
>>
>> - He also wonders about other bogus addr values above 4k, but below
>>   mmap_min_addr and whether this change misses those values
> 
> Yes, thanks for the accurate summary Joe.
> 
>> Is it possible that the original report noticed that shmat allowed
>> attach to an address of 1, and it was assumed that somehow mmap_min_addr
>> protections were circumvented?  Then commit 95e91b831f87 modified the
>> rounding in do_shmat() so that shmat would fail on similar input (but
>> for apparently different reasons)?
>>
>> I didn't see any discussion when looking up the original commit in the
>> list archives, so any explanations or pointers would be very helpful.
> 
> We identified only one positive side effect to such change, it is
> about the semantics of SHM_REMAP when addr < shmlba (and != 0). Before
> the patch SHM_REMAP was erroneously implicit for that virtual
> range. However that's not security related either, and there's no
> mention of SHM_REMAP in the commit message.
> 
> So then we wondered what this CVE is about in the first place, it
> looks a invalid CVE for a not existent security issue. The testcase at
> least shows no malfunction, mapping addr 0 is fine to succeed with
> CAP_SYS_RAWIO.
> 
> From the commit message, testcase and CVE I couldn't get what this
> commit is about.
> 
> Last but not the least, if there was a security problem in calling
> do_mmap_pgoff with addr=0, flags=MAP_FIXED|MAP_SHARED the fix would
> better be moved to do_mmap_pgoff, not in ipc/shm.c.

Gentle ping.

-- Joe