Re: [PATCH v2] fork: Unconditionally clear stack on fork

[Kees Cook]

On Wed, Feb 21, 2018 at 12:59 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 21 Feb 2018 11:29:33 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
>> On Tue 20-02-18 18:16:59, Kees Cook wrote:
>> > One of the classes of kernel stack content leaks[1] is exposing the
>> > contents of prior heap or stack contents when a new process stack is
>> > allocated. Normally, those stacks are not zeroed, and the old contents
>> > remain in place. In the face of stack content exposure flaws, those
>> > contents can leak to userspace.
>> >
>> > Fixing this will make the kernel no longer vulnerable to these flaws,
>> > as the stack will be wiped each time a stack is assigned to a new
>> > process. There's not a meaningful change in runtime performance; it
>> > almost looks like it provides a benefit.
>> >
>> > Performing back-to-back kernel builds before:
>> >     Run times: 157.86 157.09 158.90 160.94 160.80
>> >     Mean: 159.12
>> >     Std Dev: 1.54
>> >
>> > and after:
>> >     Run times: 159.31 157.34 156.71 158.15 160.81
>> >     Mean: 158.46
>> >     Std Dev: 1.46
>>
>> /bin/true or similar would be more representative for the worst case
>> but it is good to see that this doesn't have any visible effect on
>> a more real usecase.
>
> Yes, that's a pretty large memset.  And while it will populate the CPU
> cache with the stack contents, doing so will evict other things.
>
> So some quite careful quantitative testing is needed here, methinks.

Well, I did some more with perf and cycle counts on running 100,000
execs of /bin/true.

before:
Cycles: 218858861551 218853036130 214727610969 227656844122 224980542841
Mean:  221015379122.60
Std Dev: 4662486552.47

after:
Cycles: 213868945060 213119275204 211820169456 224426673259 225489986348
Mean:  217745009865.40
Std Dev: 5935559279.99

It continues to look like it's faster, though the deviation is rather
wide, but I'm not sure what I could do that would be less noisy. I'm
open to ideas!

-Kees

-- 
Kees Cook
Pixel Security