Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
causes application-level breakage
[Posted April 28, 2015 by corbet]
| From: |
| Andy Lutomirski <luto-AT-amacapital.net> |
| To: |
| Linus Torvalds <torvalds-AT-linux-foundation.org> |
| Subject: |
| Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage |
| Date: |
| Fri, 24 Apr 2015 09:10:45 -0700 |
| Message-ID: |
| <CALCETrUkkbZaNGkcZMenciC7o9BO7U52LPXQwT+Q5TT8W2=uKQ@mail.gmail.com> |
| Cc: |
| Mark Williamson <mwilliamson-AT-undo-software.com>, Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>, "Kirill A. Shutemov" <kirill.shutemov-AT-linux.intel.com>, Pavel Emelyanov <xemul-AT-parallels.com>, Konstantin Khlebnikov <khlebnikov-AT-openvz.org>, Andrew Morton <akpm-AT-linux-foundation.org>, Mark Seaborn <mseaborn-AT-chromium.org>, Linux API <linux-api-AT-vger.kernel.org>, Finn Grimwood <fgrimwood-AT-undo-software.com>, Daniel James <djames-AT-undo-software.com> |
| Archive‑link: | |
Article |
On Fri, Apr 24, 2015 at 9:08 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Apr 24, 2015 at 7:55 AM, Mark Williamson
> <mwilliamson@undo-software.com> wrote:
>>
>> Although I've marked this as a "Regression", we do realise there are
>> legitimate security concerns over the original implementation of this
>> interface. Still, given the kernel's strong stance on preserving userspace
>> interfaces, we thought we ought to flag this quickly as something that has
>> changed application-relevant behaviour.
>
> So the one exception to the regression rule is "security fixes", but
> even for security fixes we do try to be as reasonable as humanly
> possible to make them not break things.
>
> Now, as you mentioned, one option is to not outright disallow accesses
> to the /proc/PID/pagemap, but to at least hide the page frame numbers.
> However, I don't believe that we have a good enough scrambling model
> to make that reasonable. Remember: any attacker will be able to see
> our scrambling code, so it would need to be both cryptographically
> secure *and* use a truly random per-VM secret key. Quite frankly,
> that's a _lot_ of effort for dubious gain...
Even though I've been accused (correctly?) of suggesting that, I'm not
sure I like it anymore. Suppose I map some anonymous memory, learn
its (scrambled) pfn, then unmap it and remap a setuid file. Now I can
tell whether I've mapped the setuid file at the same pfn that was
mapped as my anonymous memory. IIRC that's sufficient for one of the
variants of Mark's attack.
--Andy
