[kernel-hardening] Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

H. Peter Anvin
2017-03-22T20:21:38Z

On 03/22/17 12:15, Thomas Garnier wrote:
> On Wed, Mar 15, 2017 at 10:43 AM, Thomas Garnier <thgarnie@google.com> wrote:
>> Thanks for the feedback. I will look into inlining by default (looking
>> at code size on different arch), the updated patch for x86 in the
>> meantime:
>
> I did couple checks and it doesn't seem worth it. I will send a v4
> with the change below for additional feedback.

Can you specify what that means?

On x86, where there is only one caller of this, it really seems like it
ought to reduce the overhead to almost zero (since it most likely is
hidden in the pipeline.)

I would like to suggest defining it inline if
CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE is set; I really don't
care about an architecture which doesn't have it.

Note that the upcoming 5-level paging (LA57) support will make
TASK_SIZE_MAX dependent on the CPU. The best way to handle that is
probably to tag this immediate with an assembly alternative rather than
loading it from a memory variable (most likely taking a cache hit.)

-hpa