linux-kernel - Re: [PATCH] mm: use %pK for /proc/vmallocinfo

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAGXu5jL4Dd3jCusr+Du4q7tOhcsKaSQbW6u_ZN8ZSBry2AQARg@mail.gmail.com>
Date:	Wed, 3 Oct 2012 09:13:45 -0700
From:	Kees Cook <keescook@...omium.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Joe Perches <joe@...ches.com>,
	Kautuk Consul <consul.kautuk@...il.com>, linux-mm@...ck.org,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH] mm: use %pK for /proc/vmallocinfo

On Tue, Oct 2, 2012 at 10:37 PM, David Rientjes <rientjes@...gle.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@...ecurity.net>
>> >> Signed-off-by: Kees Cook <keescook@...omium.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/