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

[Alexei Starovoitov]

On Sat, Jan 20, 2018 at 8:56 AM, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
> On Fri, Jan 19, 2018 at 10:58:44PM -0800, Dan Williams wrote:
>> On Thu, Jan 18, 2018 at 4:01 PM, Dan Williams <dan.j.williams@xxxxxxxxx> 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;                                                       \
>
> hmm. I'm not sure it's the right thing to do, since the macro
> is forcing cpu to speculate subsequent load from null instead
> of valid pointer.
> As Linus said: "
>  So that __u._bit masking wasn't masking
>  the pointer, it was masking the value that was *read* from the
>  pointer, so that you could know that an invalid access returned
>  0/NULL, not just the first value in the array.
> "
> imo just
>   return _arr + (_i & _mask);
> is enough. No need for union games.
> The cpu will speculate the load from _arr[0] if _i is out of bounds
> which is the same as if user passed _i == 0 which would have passed
> bounds check anyway, so I don't see any data leak from populating
> cache with _arr[0] data. In-bounds access can do that just as well
> without any speculation.

scratch that. It's array_ptr, not array_access.
The code will do if (!ptr) later, so yeah this api is fine.