Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

Dan Williams
2018-01-09T21:59:04Z

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.