Re: [PATCH -tip] kasan: Emit different calls for instrumentable memintrinsics

[Marco Elver]

On Thu, 9 Feb 2023 at 23:43, Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
>
> On Wed, Feb 8, 2023 at 7:42 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > Clang 15 will provide an option to prefix calls to memcpy/memset/memmove
> > with __asan_ in instrumented functions: https://reviews.llvm.org/D122724
>
> Hi Marco,
>
> Does this option affect all functions or only the ones that are marked
> with no_sanitize?

Only functions that are instrumented, i.e. wherever
fsanitize=kernel-address inserts instrumentation.

> Based on the LLVM patch description, should we also change the normal
> memcpy/memset/memmove to be noninstrumented?

They are no longer instrumented as of 69d4c0d32186 (for
CONFIG_GENERIC_ENTRY arches).

> These __asan_mem* functions are not defined in the kernel AFAICS.
> Should we add them?

Peter introduced them in 69d4c0d32186, and we effectively have no
mem*() instrumentation on x86 w/o the compiler-enablement patch here.

> Or maybe we should just use "__" as the prefix, as right now __mem*
> functions are the ones that are not instrumented?

__asan_mem* is for instrumented code, just like ASan userspace does
(actually ASan userspace has been doing it like this forever, just the
kernel was somehow special).

[...]
> > Fixes: 69d4c0d32186 ("entry, kasan, x86: Disallow overriding mem*() functions")
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > ---
> >
> > The Fixes tag is just there to show the dependency, and that people
> > shouldn't apply this patch without 69d4c0d32186.

^^^ Depends on this commit, which is only in -tip.

> > +ifdef CONFIG_GENERIC_ENTRY

It also only affects GENERIC_ENTRY arches.

> > +# Instrument memcpy/memset/memmove calls by using instrumented __asan_mem*()
> > +# instead. With compilers that don't support this option, compiler-inserted
> > +# memintrinsics won't be checked by KASAN.
> > +CFLAGS_KASAN += $(call cc-param,asan-kernel-mem-intrinsic-prefix)
> > +endif

Probably the same should be done for SW_TAGS, because arm64 will be
GENERIC_ENTRY at one point or another as well.

KASAN + GCC on x86 will have no mem*() instrumentation after
69d4c0d32186, which is sad, so somebody ought to teach it the same
param as above.

Thanks,
-- Marco