Re: [PATCH v6 0/6] KASAN for arm64

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

On Fri, Oct 09, 2015 at 01:18:09PM +0300, Andrey Ryabinin wrote:
> 2015-10-09 12:48 GMT+03:00 Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>:
> > On Fri, Oct 09, 2015 at 12:32:18PM +0300, Andrey Ryabinin wrote:
> > [...]
> >
> >> I thought the EFI stub isolation patches create a copy of mem*() functions in the stub,
> >> but they are just create aliases with __efistub_ prefix.
> >>
> >> We only need to create some more aliases for KASAN.
> >> The following patch on top of the EFI stub isolation series works for me.
> >>
> >>
> >> Signed-off-by: Andrey Ryabinin <ryabinin.a.a-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> >> ---
> >>  arch/arm64/kernel/image.h | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
> >> index e083af0..6eb8fee 100644
> >> --- a/arch/arm64/kernel/image.h
> >> +++ b/arch/arm64/kernel/image.h
> >> @@ -80,6 +80,12 @@ __efistub_strcmp           = __pi_strcmp;
> >>  __efistub_strncmp            = __pi_strncmp;
> >>  __efistub___flush_dcache_area        = __pi___flush_dcache_area;
> >>
> >> +#ifdef CONFIG_KASAN
> >> +__efistub___memcpy           = __pi_memcpy;
> >> +__efistub___memmove          = __pi_memmove;
> >> +__efistub___memset           = __pi_memset;
> >> +#endif
> >
> > Ard's v4 stub isolation series has these aliases [1], as the stub
> > requires these aliases regardless of KASAN in order to link.
> 
> Stub isolation series has __efistub_memcpy, not __efistub___memcpy
> (two additional '_').

Ah, I see, sorry for my sloppy reading.

> The thing is, KASAN provides own implementation of memcpy() which
> checks memory before access.
> The original 'memcpy()' becomes __memcpy(), so we could still use it.

Ok.

> In code that not instrumented by KASAN (like the EFI stub) we replace
> KASAN's memcpy() with the original __mempcy():
> #define memcpy() __memcpy()

I'm a little confused by this. Surely that doesn't override implicit
calls generated by the compiler, leaving us with a mixture of calls to
memcpy and __memcpy?

That doesn't matter for the stub, as both __efistub_mem* and
__efistub___mem* would point at __pe_mem*, but doesn't that matter for
other users that shouldn't be instrumented?

Is that not a problem, or do we inhibit/override that somehow?

> So with CONFIG_KASAN=y the EFI stub uses __memcpy, thus we need to
> create the __efistub___memcpy alias.

Ok, that makes sense to me.

Thanks,
Mark.