From: Kees Cook <keescook@chromium.org>
To: David Rientjes <rientjes@google.com>
Cc: linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Minchan Kim <minchan@kernel.org>, Joe Perches <joe@perches.com>,
Kautuk Consul <consul.kautuk@gmail.com>,
linux-mm@kvack.org, Brad Spengler <spender@grsecurity.net>
Subject: Re: [PATCH] mm: use %pK for /proc/vmallocinfo
Date: Wed, 3 Oct 2012 09:13:45 -0700 [thread overview]
Message-ID: <CAGXu5jL4Dd3jCusr+Du4q7tOhcsKaSQbW6u_ZN8ZSBry2AQARg@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.00.1210022236370.10573@chino.kir.corp.google.com>
On Tue, Oct 2, 2012 at 10:37 PM, David Rientjes <rientjes@google.com> wrote:
> On Tue, 2 Oct 2012, Kees Cook wrote:
>
>> >> In the paranoid case of sysctl kernel.kptr_restrict=2, mask the kernel
>> >> virtual addresses in /proc/vmallocinfo too.
>> >>
>> >> Reported-by: Brad Spengler <spender@grsecurity.net>
>> >> Signed-off-by: Kees Cook <keescook@chromium.org>
>> >
>> > /proc/vmallocinfo is S_IRUSR, not S_IRUGO, so exactly what are you trying
>> > to protect?
>>
>> Trying to block the root user from seeing virtual memory addresses
>> (mode 2 of kptr_restrict).
>>
>> Documentation/sysctl/kernel.txt:
>> "This toggle indicates whether restrictions are placed on
>> exposing kernel addresses via /proc and other interfaces. When
>> kptr_restrict is set to (0), there are no restrictions. When
>> kptr_restrict is set to (1), the default, kernel pointers
>> printed using the %pK format specifier will be replaced with 0's
>> unless the user has CAP_SYSLOG. When kptr_restrict is set to
>> (2), kernel pointers printed using %pK will be replaced with 0's
>> regardless of privileges."
>>
>> Even though it's S_IRUSR, it still needs %pK for the paranoid case.
>
> So root does echo 0 > /proc/sys/kernel/kptr_restrict first. Again: what
> are you trying to protect?
Only CAP_SYS_ADMIN can change the setting. This is, for example, for
containers, or other situations where a uid 0 process lacking
CAP_SYS_ADMIN cannot see virtual addresses. It's a very paranoid case,
yes, but it's part of how this feature was designed. Think of it as
supporting the recent uid 0 vs ring 0 boundary. :)
-Kees
--
Kees Cook
Chrome OS Security
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next prev parent reply other threads:[~2012-10-03 16:13 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-02 23:49 Kees Cook
2012-10-03 0:43 ` KOSAKI Motohiro
2012-10-03 5:12 ` David Rientjes
2012-10-03 5:15 ` Kees Cook
2012-10-03 5:37 ` David Rientjes
2012-10-03 16:13 ` Kees Cook [this message]
2012-10-03 18:02 ` David Rientjes
2012-10-03 18:39 ` Kees Cook
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