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Date: Wed, 4 Jul 2018 09:52:35 -0700
From: Kees Cook <>
To: Vlastimil Babka <>
Cc: Chris von Recklinghausen <>, Laura Abbott <>, 
	Paolo Abeni <>, LKML <>, 
	Linux-MM <>, Kernel Hardening <>
Subject: Re: [PATCH v7] add param that allows bootline control of hardened usercopy

On Wed, Jul 4, 2018 at 6:43 AM, Vlastimil Babka <> 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.


Kees Cook
Pixel Security

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