Messages in this thread | | | Date | Mon, 30 Apr 2018 10:21:52 -0700 | From | Davidlohr Bueso <> | Subject | Re: Questions about commit "ipc/shm: Fix shmat mmap nil-page protection" |
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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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