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

Dan Williams
2018-01-04T01:13:46Z

[ 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.