Re: [PATCH v4 02/11] x86, kfence: enable KFENCE for x86

Marco Elver
2020-10-07T14:43:06Z

On Wed, 7 Oct 2020 at 16:15, Jann Horn <jannh@google.com> wrote:
>
> On Wed, Oct 7, 2020 at 3:09 PM Marco Elver <elver@google.com> wrote:
> > On Fri, 2 Oct 2020 at 07:45, Jann Horn <jannh@google.com> wrote:
> > > On Tue, Sep 29, 2020 at 3:38 PM Marco Elver <elver@google.com> wrote:
> > > > Add architecture specific implementation details for KFENCE and enable
> > > > KFENCE for the x86 architecture. In particular, this implements the
> > > > required interface in <asm/kfence.h> for setting up the pool and
> > > > providing helper functions for protecting and unprotecting pages.
> > > >
> > > > For x86, we need to ensure that the pool uses 4K pages, which is done
> > > > using the set_memory_4k() helper function.
> > > [...]
> > > > diff --git a/arch/x86/include/asm/kfence.h b/arch/x86/include/asm/kfence.h
> > > [...]
> > > > +/* Protect the given page and flush TLBs. */
> > > > +static inline bool kfence_protect_page(unsigned long addr, bool protect)
> > > > +{
> > > > + unsigned int level;
> > > > + pte_t *pte = lookup_address(addr, &level);
> > > > +
> > > > + if (!pte || level != PG_LEVEL_4K)
> > >
> > > Do we actually expect this to happen, or is this just a "robustness"
> > > check? If we don't expect this to happen, there should be a WARN_ON()
> > > around the condition.
> >
> > It's not obvious here, but we already have this covered with a WARN:
> > the core.c code has a KFENCE_WARN_ON, which disables KFENCE on a
> > warning.
>
> So for this specific branch: Can it ever happen? If not, please either
> remove it or add WARN_ON(). That serves two functions: It ensures that
> if something unexpected happens, we see a warning, and it hints to
> people reading the code "this isn't actually expected to happen, you
> don't have to wrack your brain trying to figure out for which scenario
> this branch is intended".

Perhaps I could have been clearer: we already have this returning
false covered by a WARN+disable KFENCE in core.c.

We'll add another WARN_ON right here, as it doesn't hurt, and
hopefully improves readability.

> > > > + return false;
> > > > +
> > > > + if (protect)
> > > > + set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
> > > > + else
> > > > + set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
> > >
> > > Hmm... do we have this helper (instead of using the existing helpers
> > > for modifying memory permissions) to work around the allocation out of
> > > the data section?
> >
> > I just played around with using the set_memory.c functions, to remind
> > myself why this didn't work. I experimented with using
> > set_memory_{np,p}() functions; set_memory_p() isn't implemented, but
> > is easily added (which I did for below experiment). However, this
> > didn't quite work:
> [...]
> > For one, smp_call_function_many_cond() doesn't want to be called with
> > interrupts disabled, and we may very well get a KFENCE allocation or
> > page fault with interrupts disabled / within interrupts.
> >
> > Therefore, to be safe, we should avoid IPIs.
>
> set_direct_map_invalid_noflush() does that, too, I think? And that's
> already implemented for both arm64 and x86.

Sure, that works.

We still want the flush_tlb_one_kernel(), at least so the local CPU's
TLB is flushed.

> > It follows that setting
> > the page attribute is best-effort, and we can tolerate some
> > inaccuracy. Lazy fault handling should take care of faults after we
> > set the page as PRESENT.
> [...]
> > > Shouldn't kfence_handle_page_fault() happen after prefetch handling,
> > > at least? Maybe directly above the "oops" label?
> >
> > Good question. AFAIK it doesn't matter, as is_kfence_address() should
> > never apply for any of those that follow, right? In any case, it
> > shouldn't hurt to move it down.
>
> is_prefetch() ignores any #PF not caused by instruction fetch if it
> comes from kernel mode and the faulting instruction is one of the
> PREFETCH* instructions. (Which is not supposed to happen - the
> processor should just be ignoring the fault for PREFETCH instead of
> generating an exception AFAIK. But the comments say that this is about
> CPU bugs and stuff.) While this is probably not a big deal anymore
> partly because the kernel doesn't use software prefetching in many
> places anymore, it seems to me like, in principle, this could also
> cause page faults that should be ignored in KFENCE regions if someone
> tries to do PREFETCH on an out-of-bounds array element or a dangling
> pointer or something.

Thanks for the clarification.