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

[Marco Elver]

On Fri, 30 Oct 2020 at 03:49, Jann Horn <jannh@xxxxxxxxxx> wrote:
> On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Co-developed-by: Marco Elver <elver@xxxxxxxxxx>
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> [...]
> > 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;
>
> We can also get to this point due to an attempt to execute a data
> page. That's very unlikely (given that the same thing would also crash
> if you tried to do it with normal heap memory, and KFENCE allocations
> are extremely rare); but we might want to try to avoid handling such
> faults as KFENCE faults, since KFENCE will assume that it has resolved
> the fault and retry execution of the faulting instruction. Once kernel
> protection keys are introduced, those might cause the same kind of
> trouble.
>
> So we might want to gate this on a check like "if ((error_code &
> X86_PF_PROT) == 0)" (meaning "only handle the fault if the fault was
> caused by no page being present", see enum x86_pf_error_code).

Good point. Will fix in v7.

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

Thanks,
-- Marco