Re: [PATCH v5 15/30] arm64/sve: Signal handling support

Kees Cook
2017-12-12T19:32:51Z

On Tue, Dec 12, 2017 at 3:11 AM, Dave Martin <Dave.Martin@arm.com> wrote:
> On Tue, Dec 12, 2017 at 10:40:30AM +0000, Will Deacon wrote:
>> On Mon, Dec 11, 2017 at 11:23:09AM -0800, Kees Cook wrote:
>> > On Mon, Dec 11, 2017 at 6:07 AM, Will Deacon <will.deacon@arm.com> wrote:
>> > > On Thu, Dec 07, 2017 at 10:50:38AM -0800, Kees Cook wrote:
>> > >> My question is mainly: why not just use copy_*() everywhere instead?
>> > >> Having these things so spread out makes it fragile, and there's very
>> > >> little performance benefit from using __copy_*() over copy_*().
>> > >
>> > > I think that's more of a general question. Why not just remove the __
>> > > versions from the kernel entirely if they're not worth the perf?
>> >
>> > That has been something Linus has strongly suggested in the past, so
>> > I've kind of been looking for easy places to drop the __copy_*
>> > versions. :)
>>
>> Tell you what then: I'll Ack the arm64 patch if it's part of a series
>> removing the thing entirely :p
>>
>> I guess we'd still want to the validation of the whole sigframe though,
>> so we don't end up pushing half a signal stack before running into an
>> access_ok failure?
>
> That's an interesting question. In many cases access_ok() might become
> redundant, but for syscalls that you don't want to have side-effects
> on user memory on failure it's still relevant.
>
> In the signal case we'd still an encompassing access_ok() to prevent
> stack guard overruns, because the signal frame can be large and isn't
> written or read contiguously or in a well-defined order.

Yeah, I think bailing early is fine. I think the existing access_ok()
checks are fine; I wouldn't want to drop those.

-Kees

--
Kees Cook
Pixel Security