Re: [PATCH v4 00/10] prevent bounds-check bypass via speculative execution

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Jan 18, 2018 at 4:01 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> Changes since v3 [1]
> * Drop 'ifence_array_ptr' and associated compile-time + run-time
>   switching and just use the masking approach all the time.
>
> * Convert 'get_user' to use pointer sanitization via masking rather than
>   lfence. '__get_user' and associated paths still rely on
>   lfence. (Linus)
>
>       "Basically, the rule is trivial: find all 'stac' users, and use
>        address masking if those users already integrate the limit
>        check, and lfence they don't."
>
> * At syscall entry sanitize the syscall number under speculation
>   to remove a user controlled pointer de-reference in kernel
>   space.  (Linus)
>
> * Fix a raw lfence in the kvm code (added for v4.15-rc8) to use
>   'array_ptr'.
>
> * Propose 'array_idx' as a way to sanitize user input that is
>   later used as an array index, but where the validation is
>   happening in a different code block than the array reference.
>   (Christian).
>
> * Fix grammar in speculation.txt (Kees)
>
> ---
>
> 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 [2]
> and the Documentation patch in this series."
>
> A precondition of using this attack on the kernel is to get a user
> controlled pointer de-referenced (under speculation) in privileged code.
> The primary source of user controlled pointers in the kernel is the
> arguments passed to 'get_user' and '__get_user'. An example of other
> user controlled pointers are user-controlled array / pointer offsets.
>
> Better tooling is needed to find more arrays / pointers with user
> controlled indices / offsets that can be converted to use 'array_ptr' or
> 'array_idx'. A few are included in this set, and these are not expected
> to be complete. That said, the 'get_user' protections raise the bar on
> finding a vulnerable gadget in the kernel.
>
> These patches are also available via the 'nospec-v4' git branch here:
>
>     git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4

I've pushed out a nospec-v4.1 with the below minor cleanup, a fixup of
the changelog for "kvm, x86: fix spectre-v1 mitigation", and added
Paolo's ack.

     git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4.1

diff --git a/include/linux/nospec.h b/include/linux/nospec.h
index 8af35be1869e..b8a9222e34d1 100644
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -37,7 +37,7 @@ static inline unsigned long array_ptr_mask(unsigned
long idx, unsigned long sz)
        unsigned long _i = (idx);                                       \
        unsigned long _mask = array_ptr_mask(_i, (sz));                 \
                                                                        \
-       __u._ptr = _arr + (_i & _mask);                                 \
+       __u._ptr = _arr + _i;                                           \
        __u._bit &= _mask;                                              \
        __u._ptr;                                                       \
 })