On Tue, Jun 16, 2020 at 07:40:17AM -0700, Dave Hansen wrote:
> On 6/16/20 12:49 AM, Kees Cook wrote:
> > + /* Mark the second page as untouched (i.e. "old") */
> > + preempt_disable();
> > + set_pte_at(&init_mm, vaddr, ptep, pte_mkold(*(READ_ONCE(ptep))));
> > + local_flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE);
> > + preempt_enable();
>
> If you can, I'd wrap that nugget up in a helper. I'd also suggest being
> very explicit in a comment about what it is trying to do: ensure no TLB
> entries exist so that a future access will always set the Accessed bit.
Yeah, good idea!
>
> > + /* Make sure the PTE agrees that it is untouched. */
> > + if (WARN_ON_ONCE(sd_touched(ptep)))
> > + return;
> > + /* Read a portion of struct seccomp_data from the second page. */
> > + check = sd->instruction_pointer;
> > + /* First, verify the contents are zero from vzalloc(). */
> > + if (WARN_ON_ONCE(check))
> > + return;
> > + /* Now make sure the ACCESSED bit has been set after the read. */
> > + if (!sd_touched(ptep)) {
> > + /*
> > + * If autodetection fails, fall back to standard beahavior by
> > + * clearing the entire "allow" bitmap.
> > + */
> > + pr_warn_once("seccomp: cannot build automatic syscall filters\n");
> > + bitmap_zero(bitmaps->allow, NR_syscalls);
> > + return;
> > + }
>
> I can't find any big holes with this. It's the kind of code that makes
> me nervous, but mostly because it's pretty different that anything else
> we have in the kernel.
>
> It's also clear to me here that you probably have a slightly different
> expectation of what the PTE accessed flag means versus the hardware
> guys. What you are looking for it to mean is roughly: "a retired
> instruction touched this page".
>
> The hardware guys would probably say it's closer to "a TLB entry was
> established for this page." Remember that TLB entries can be
> established speculatively or from things like prefetchers. While I
> don't know of anything microarchitectural today which would trip this
> mechanism, it's entirely possible that something in the future might.
> Accessing close to the page boundary is the exact kind of place folks
> might want to optimize.
Yeah, and to that end, going the cBPF emulator route removes this kind
of "weird" behavior.
>
> *But*, at least it would err in the direction of being conservative. It
> would say "somebody touched the page!" more often than it should, but
> never _less_ often than it should.
Right -- I made sure to design the bitmaps and the direction of the
checking to fail towards running the filter instead of bypassing it.
> One thing about the implementation (which is roughly):
>
> // Touch the data:
> check = sd->instruction_pointer;
> // Examine the PTE mapping that data:
> if (!sd_touched(ptep)) {
> // something
> }
>
> There aren't any barriers in there, which could lead to the sd_touched()
> check being ordered before the data touch. I think a rmb() will
> suffice. You could even do it inside sd_touched().
Ah yeah, I had convinced myself that READ_ONCE() gained me that
coverage, but I guess that's not actually true here.
> Was there a reason you chose to export a ranged TLB flush? I probably
> would have just used the single-page flush_tlb_one_kernel() for this
> purpose if I were working in arch-specific code.
No particular reason -- it just seemed easiest to make available given
the interfaces. I could do the single-page version instead, if this way
of doing things survives review. ;)
Thanks for looking at it!
--
Kees Cook
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