Re: [PATCH v6 2/9] x86, kfence: enable KFENCE for x86

[Jann Horn <jannh@google.com>]

On Fri, Oct 30, 2020 at 2:00 PM Marco Elver <elver@google.com> wrote:
> On Fri, 30 Oct 2020 at 03:49, Jann Horn <jannh@google.com> wrote:
> > On Thu, Oct 29, 2020 at 2:17 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.
> > >
> > > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > > Co-developed-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Alexander Potapenko <glider@google.com>
> > [...]
> > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > [...]
> > > @@ -725,6 +726,9 @@ no_context(struct pt_regs *regs, unsigned long error_code,
> > >         if (IS_ENABLED(CONFIG_EFI))
> > >                 efi_recover_from_page_fault(address);
> > >
> > > +       if (kfence_handle_page_fault(address))
> > > +               return;
[...]
> > Unrelated sidenote: Since we're hooking after exception fixup
> > handling, the debug-only KFENCE_STRESS_TEST_FAULTS can probably still
> > cause some behavioral differences through spurious faults in places
> > like copy_user_enhanced_fast_string (where the exception table entries
> > are used even if the *kernel* pointer, not the user pointer, causes a
> > fault). But since KFENCE_STRESS_TEST_FAULTS is exclusively for KFENCE
> > development, the difference might not matter. And ordering them the
> > other way around definitely isn't possible, because the kernel relies
> > on being able to fixup OOB reads. So there probably isn't really
> > anything we can do better here; it's just something to keep in mind.
> > Maybe you can add a little warning to the help text for that Kconfig
> > entry that warns people about this?
>
> Thanks for pointing it out, but that option really is *only* to stress
> kfence with concurrent allocations/frees/page faults. If anybody
> enables this option for anything other than testing kfence, it's their
> own fault. ;-)

Sounds fair. :P

> I'll try to add a generic note to the Kconfig entry, but what you
> mention here seems quite x86-specific.

(FWIW, I think it could currently also happen on arm64 in the rare
cases where KERNEL_DS is used. But luckily Christoph Hellwig has
already gotten rid of most places that did that.)