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Date: Thu, 21 Jul 2016 11:34:25 -0700
From: Kees Cook <keescook@...omium.org>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: LKML <linux-kernel@...r.kernel.org>, Balbir Singh <bsingharora@...il.com>,
Daniel Micay <danielmicay@...il.com>, Josh Poimboeuf <jpoimboe@...hat.com>,
Rik van Riel <riel@...hat.com>, Casey Schaufler <casey@...aufler-ca.com>,
PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>,
Russell King <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>, Tony Luck <tony.luck@...el.com>,
Fenghua Yu <fenghua.yu@...el.com>, "David S. Miller" <davem@...emloft.net>,
"x86@...nel.org" <x86@...nel.org>, Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>,
Borislav Petkov <bp@...e.de>, Mathias Krause <minipli@...glemail.com>, Jan Kara <jack@...e.cz>,
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"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, linux-ia64@...r.kernel.org,
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linux-arch <linux-arch@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 02/11] mm: Hardened usercopy
On Wed, Jul 20, 2016 at 11:52 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> Kees Cook <keescook@...omium.org> writes:
>
>> diff --git a/mm/usercopy.c b/mm/usercopy.c
>> new file mode 100644
>> index 000000000000..e4bf4e7ccdf6
>> --- /dev/null
>> +++ b/mm/usercopy.c
>> @@ -0,0 +1,234 @@
> ...
>> +
>> +/*
>> + * Checks if a given pointer and length is contained by the current
>> + * stack frame (if possible).
>> + *
>> + * 0: not at all on the stack
>> + * 1: fully within a valid stack frame
>> + * 2: fully on the stack (when can't do frame-checking)
>> + * -1: error condition (invalid stack position or bad stack frame)
>> + */
>> +static noinline int check_stack_object(const void *obj, unsigned long len)
>> +{
>> + const void * const stack = task_stack_page(current);
>> + const void * const stackend = stack + THREAD_SIZE;
>
> That allows access to the entire stack, including the struct thread_info,
> is that what we want - it seems dangerous? Or did I miss a check
> somewhere else?
That seems like a nice improvement to make, yeah.
> We have end_of_stack() which computes the end of the stack taking
> thread_info into account (end being the opposite of your end above).
Amusingly, the object_is_on_stack() check in sched.h doesn't take
thread_info into account either. :P Regardless, I think using
end_of_stack() may not be best. To tighten the check, I think we could
add this after checking that the object is on the stack:
#ifdef CONFIG_STACK_GROWSUP
stackend -= sizeof(struct thread_info);
#else
stack += sizeof(struct thread_info);
#endif
e.g. then if the pointer was in the thread_info, the second test would
fail, triggering the protection.
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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