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

[Kees Cook]

On Tue, Oct 2, 2012 at 10:37 PM, David Rientjes <rientjes@xxxxxxxxxx> 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@xxxxxxxxxxxxxx>
>> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> >
>> > /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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