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

Julia Lawall
2018-01-04T06:28:45Z

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?

julia