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    SubjectRe: [PATCH] fork: Allow stack to be wiped on fork
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    On 01/17/2018 01:17 AM, Michal Hocko wrote:
    > On Tue 16-01-18 21:50:15, Kees Cook wrote:
    >> One of the classes of kernel stack content leaks 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. With some types of stack content exposure flaws, those contents
    >> can leak to userspace. Kernels built with CONFIG_CLEAR_STACK_FORK will
    >> no longer be vulnerable to this, 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.
    >
    > Have you tried something as simple as /bin/true in a loop. kbuild will
    > certainly amortize few cycles for the clearing and I would expect, most
    > reasonable applications would do as well. But it would be better to know
    > the worst case scenario IMHO.
    >

    I tried /bin/true in a loop in my QEMU setup and didn't see a difference
    there.

    >> 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
    >>
    >> With CONFIG_CLEAR_STACK_FORK=y:
    >> Run times: 159.31 157.34 156.71 158.15 160.81
    >> Mean: 158.46
    >> Std Dev: 1.46
    >>
    >> Signed-off-by: Kees Cook <keescook@chromium.org>
    >
    > The change seems reasonable to me. Although it would be better to extend
    > on the types of attacks this prevents from, with some examples ideally.
    > How many attacks of that kind we had in the past and how often they
    > appear. That might help people to decide whether to deserve few cycles
    > on each fork. Also the config option sounds rather limiting. Consider
    > distros, should they enable it just to be on the safe side? This is kind
    > of generic concern with other hardening options though.
    >

    Agreed this could use a few more words, but it looks good to me overall.

    Thanks,
    Laura

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