Date:   Mon, 30 Apr 2018 10:21:52 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Joe Lawrence <joe.lawrence@...hat.com>, akpm@...ux-foundation.org,
        gareth.evans@...textis.co.uk,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org
Subject: Re: Questions about commit "ipc/shm: Fix shmat mmap nil-page
 protection"

On Mon, 25 Sep 2017, Andrea Arcangeli wrote:

Sorry this took so long guys. I had forgotten about this until it recently
resurfaced.

>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.

Coincidence. I didn't notice the SHM_REMAP, but after looking at it
you appear to be right. I'll send a patch along with the revert
(see below).

>
>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.

This is exactly the issue. I thought mapping addr=0 with MAP_FIXED
was an issue, including for root. Hence avoiding the round off from
1 to 0. If this is legal, then this commit needs reverted.

In fact, X11[1] seems to rely on this _exact_ case; and this change
breaks semantics.

>
>>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.

Yeah at the time, akpm and I wondered why this was special to security.

[1] https://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/os-support/linux/int10/linux.c#n347

Thanks,
Davidlohr

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