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    SubjectRe: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok
    On Tue, Jan 9, 2018 at 1:49 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
    > On Tue, Jan 09, 2018 at 01:47:09PM -0800, Dan Williams wrote:
    >> On Tue, Jan 9, 2018 at 1:41 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
    >> > On Fri, Jan 05, 2018 at 06:52:07PM -0800, Linus Torvalds wrote:
    >> >> On Fri, Jan 5, 2018 at 5:10 PM, Dan Williams <dan.j.williams@intel.com> wrote:
    >> >> > From: Andi Kleen <ak@linux.intel.com>
    >> >> >
    >> >> > When access_ok fails we should always stop speculating.
    >> >> > Add the required barriers to the x86 access_ok macro.
    >> >>
    >> >> Honestly, this seems completely bogus.
    >> >>
    >> >> The description is pure garbage afaik.
    >> >>
    >> >> The fact is, we have to stop speculating when access_ok() does *not*
    >> >> fail - because that's when we'll actually do the access. And it's that
    >> >> access that needs to be non-speculative.
    >> >>
    >> >> That actually seems to be what the code does (it stops speculation
    >> >> when __range_not_ok() returns false, but access_ok() is
    >> >> !__range_not_ok()). But the explanation is crap, and dangerous.
    >> >
    >> > The description also seems to be missing the "why", as it's not
    >> > self-evident (to me, at least).
    >> >
    >> > Isn't this (access_ok/uaccess_begin/ASM_STAC) too early for the lfence?
    >> >
    >> > i.e., wouldn't the pattern be:
    >> >
    >> > get_user(uval, uptr);
    >> > if (uval < array_size) {
    >> > lfence();
    >> > foo = a2[a1[uval] * 256];
    >> > }
    >> >
    >> > Shouldn't the lfence come much later, *after* reading the variable and
    >> > comparing it and branching accordingly?
    >>
    >> The goal of putting the lfence in uaccess_begin() is to prevent
    >> speculation past access_ok().
    >
    > Right, but what's the purpose of preventing speculation past
    > access_ok()?

    Caution. It's the same rationale for the nospec_array_ptr() patches.
    If we, kernel community, can identify any possible speculation past a
    bounds check we should inject a speculation mitigation. Unless there's
    a way to be 100% certain that the first unwanted speculation can be
    turned into a gadget later on in the instruction stream, err on the
    side of shutting it down early.

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