Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

Dan Williams
2018-01-04T17:58:41Z

On Wed, Jan 3, 2018 at 10:28 PM, Julia Lawall <julia.lawall@lip6.fr> wrote:
>
>
> On Wed, 3 Jan 2018, Dan Williams wrote:
>
>> [ adding Julia and Dan ]
>>
>> On Wed, Jan 3, 2018 at 5:07 PM, Alan Cox <gnomes@lxorguk.ukuu.org.uk> wrote:
>> > On Wed, 3 Jan 2018 16:39:31 -0800
>> > Linus Torvalds <torvalds@linux-foundation.org> wrote:
>> >
>> >> On Wed, Jan 3, 2018 at 4:15 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>> >> > The 'if_nospec' primitive marks locations where the kernel is disabling
>> >> > speculative execution that could potentially access privileged data. It
>> >> > is expected to be paired with a 'nospec_{ptr,load}' where the user
>> >> > controlled value is actually consumed.
>> >>
>> >> I'm much less worried about these "nospec_load/if" macros, than I am
>> >> about having a sane way to determine when they should be needed.
>> >>
>> >> Is there such a sane model right now, or are we talking "people will
>> >> randomly add these based on strong feelings"?
>> >
>> > There are people trying to tune coverity and other tool rules to identify
>> > cases, and some of the work so far was done that way. For x86 we didn't
>> > find too many so far so either the needed pattern is uncommon or .... 8)
>> >
>> > Given you can execute over a hundred basic instructions in a speculation
>> > window it does need to be a tool that can explore not just in function
>> > but across functions. That's really tough for the compiler itself to do
>> > without help.
>> >
>> > What remains to be seen is if there are other patterns that affect
>> > different processors.
>> >
>> > In the longer term the compiler itself needs to know what is and isn't
>> > safe (ie you need to be able to write things like
>> >
>> > void foo(tainted __user int *x)
>> >
>> > and have the compiler figure out what level of speculation it can do and
>> > (on processors with those features like IA64) when it can and can't do
>> > various kinds of non-trapping loads.
>> >
>>
>> It would be great if coccinelle and/or smatch could be taught to catch
>> some of these case at least as a first pass "please audit this code
>> block" type of notification.
>>
>
> What should one be looking for. Do you have a typical example?
>

See "Exploiting Conditional Branch Misprediction" from the paper [1].

The typical example is an attacker controlled index used to trigger a
dependent read near a branch. Where an example of "near" from the
paper is "up to 188 simple instructions inserted in the source code
between the ‘if’ statement and the line accessing array...".

if (attacker_controlled_index < bound)
val = array[attacker_controlled_index];
else
return error;

...when the cpu speculates that the 'index < bound' branch is taken it
reads index and uses that value to read array[index]. The result of an
'array' relative read is potentially observable in the cache.

[1]: https://spectreattack.com/spectre.pdf