linux-kernel - Re: [PATCH v7] add param that allows bootline control of hardened usercopy

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Date:   Wed, 4 Jul 2018 19:47:38 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Kees Cook <keescook@...omium.org>
Cc:     Chris von Recklinghausen <crecklin@...hat.com>,
        Laura Abbott <labbott@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v7] add param that allows bootline control of hardened
 usercopy

On 07/04/2018 06:52 PM, Kees Cook wrote:
> On Wed, Jul 4, 2018 at 6:43 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
>> On 07/03/2018 09:43 PM, Chris von Recklinghausen wrote:
>>
>> Subject: [PATCH v7] add param that allows bootline control of hardened
>> usercopy
>>
>> s/bootline/boot time/ ?
>>
>>> v1->v2:
>>>       remove CONFIG_HUC_DEFAULT_OFF
>>>       default is now enabled, boot param disables
>>>       move check to __check_object_size so as to not break optimization of
>>>               __builtin_constant_p()
>>
>> Sorry for late and drive-by suggestion, but I think the change above is
>> kind of a waste because there's a function call overhead only to return
>> immediately.
>>
>> Something like this should work and keep benefits of both the built-in
>> check and avoiding function call?
>>
>> static __always_inline void check_object_size(const void *ptr, unsigned
>> long n, bool to_user)
>> {
>>         if (!__builtin_constant_p(n) &&
>>                         static_branch_likely(&bypass_usercopy_checks))
>>                 __check_object_size(ptr, n, to_user);
>> }
> 
> This produces less efficient code in the general case, and I'd like to
> keep the general case (hardening enabled) as fast as possible.

How specifically is the code less efficient? It should be always a
static key check (no-op thanks to the code patching involved) and a
function call in the "hardening enabled" case, just in different order.
And in either case compiled out if it's a constant.

> -Kees
>