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SubjectRe: [PATCH 00/18] prevent bounds-check bypass via speculative execution
On Sat, Jan 6, 2018 at 11:37 AM, Dan Williams <> wrote:
> On Fri, Jan 5, 2018 at 5:09 PM, Dan Williams <> wrote:
>> Quoting Mark's original RFC:
>> "Recently, Google Project Zero discovered several classes of attack
>> against speculative execution. One of these, known as variant-1, allows
>> explicit bounds checks to be bypassed under speculation, providing an
>> arbitrary read gadget. Further details can be found on the GPZ blog [1]
>> and the Documentation patch in this series."
>> This series incorporates Mark Rutland's latest api and adds the x86
>> specific implementation of nospec_barrier. The
>> nospec_{array_ptr,ptr,barrier} helpers are then combined with a kernel
>> wide analysis performed by Elena Reshetova to address static analysis
>> reports where speculative execution on a userspace controlled value
>> could bypass a bounds check. The patches address a precondition for the
>> attack discussed in the Spectre paper [2].
>> A consideration worth noting for reviewing these patches is to weigh the
>> dramatic cost of being wrong about whether a given report is exploitable
>> vs the overhead nospec_{array_ptr,ptr} may introduce. In other words,
>> lets make the bar for applying these patches be "can you prove that the
>> bounds check bypass is *not* exploitable". Consider that the Spectre
>> paper reports one example of a speculation window being ~180 cycles.
>> Note that there is also a proposal from Linus, array_access [3], that
>> attempts to quash speculative execution past a bounds check without
>> introducing an lfence instruction. That may be a future optimization
>> possibility that is compatible with this api, but it would appear to
>> need guarantees from the compiler that it is not clear the kernel can
>> rely on at this point. It is also not clear that it would be a
>> significant performance win vs lfence.
>> These patches also will also be available via the 'nospec' git branch
>> here:
>> git:// nospec
> It appears that has not mirrored out the new branch. In
> the meantime here's an alternative location:
> nospec
> If there are updates to these patches they will appear in nospec-v2,
> nospec-v3, etc... branches.

For completeness I appended the bpf fix [1] to the git branch.

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