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Date: Mon, 8 May 2017 12:41:24 +0100
From: Mark Rutland <mark.rutland@....com>
To: Daniel Micay <danielmicay@...il.com>
Cc: Kees Cook <keescook@...omium.org>, kernel-hardening@...ts.openwall.com,
	ard.biesheuvel@...aro.org, matt@...eblueprint.co.uk
Subject: Re: [PATCH] add the option of fortified string.h
 functions

On Fri, May 05, 2017 at 01:38:04PM -0400, Daniel Micay wrote:
> On Fri, 2017-05-05 at 11:38 +0100, Mark Rutland wrote:
> > On Thu, May 04, 2017 at 07:09:17PM +0100, Mark Rutland wrote:
> > > On Thu, May 04, 2017 at 01:49:44PM -0400, Daniel Micay wrote:
> > > > On Thu, 2017-05-04 at 16:48 +0100, Mark Rutland wrote:
> > > > > ... with an EFI stub fortify_panic() hacked in, I can build an
> > > > > arm64 kernel with this applied. It dies at some point after
> > > > > exiting EFI boot services; i don't know whether it made it out
> > > > > of the stub and into the kernel proper.
> > > > 
> > > > Could start with #define __NO_FORTIFY above the #include
> > > > sections there instead (or -D__NO_FORTIFY as a compiler flag),
> > > > which will skip fortifying those for now.
> > > 
> > > Neat. Given there are a few files, doing the latter for the stub
> > > is the simplest option.
> > > 
> > > > I'm successfully using this on a non-EFI ARM64 3.18 LTS kernel,
> > > > so it should be close to working on other systems (but not
> > > > necessarily with messy drivers). The x86 EFI workaround works.
> > > 
> > > FWIW, I've been playing atop of next-20170504, with a tonne of
> > > other debug options enabled (including KASAN_INLINE).
> > > 
> > > From a quick look with a JTAG debugger, the CPU got out of the
> > > stub and into the kernel. It looks like it's dying initialising
> > > KASAN, where the vectors appear to have been corrupted.
> > 
> > ... though it's a worring that __memcpy() is overridden. I think we
> > need to be more careful with the way we instrument the string
> > functions.
> 
> I don't think there's any way for the fortify code to be intercepting
> __memcpy. There's a memcpy function in arch/x86/boot/compressed/string.c
> defined via __memcpy and that appears to be working.

Just to check, are there additional patches to disable fortification of
the KASAN code? With that, things seem fine.

> A shot in the dark is that it might not happen if a __real_memcpy
> alias via __RENAME is used instead of __builtin_memcpy, but I'm not
> sure how or why this is breaking in the first place.

Using a RENAME(__real_memcpy), and a call to that didn't help.

With the rename removed (i.e. just an extern __real_memcpy()), it called
__real_memcpy as expected.

I think there's some unintended interaction with <asm/string.h>:

---->8----
#if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)

/*
 * For files that are not instrumented (e.g. mm/slub.c) we
 * should use not instrumented version of mem* functions.
 */

#define memcpy(dst, src, len) __memcpy(dst, src, len)
#define memmove(dst, src, len) __memmove(dst, src, len)
#define memset(s, c, n) __memset(s, c, n)
#endif
---->8----

If I *only* comment out the memcpy define above, KASAN's memcpy() calls
__memcpy() as we expect.

Looking at the assembly, I see memmove() and memset() have the same
issue, and messing with their <asm/string.h> defintion helps.

Looking at the preprocessed source, the fortified memcpy ends up as:

extern inline __attribute__((always_inline)) __attribute__((no_instrument_function)) __attribute__((always_inline)) __attribute__((gnu_inline)) void *__memcpy(void *p, const void *q, __kernel_size_t size)
{
 size_t p_size = __builtin_object_size(p, 0);
 size_t q_size = __builtin_object_size(q, 0);
 if (__builtin_constant_p(size) && (p_size < size || q_size < size))
  __buffer_overflow();
 if (p_size < size || q_size < size)
  fortify_panic(__func__);
 return __builtin_memcpy(p, q, size);
}

... i.e. we override __memcpy() rather than memcpy().

In KASAN, we undef memcpy before providing KASAN's version, so it keeps
its intended name, ending up as:

void *memcpy(void *dest, const void *src, size_t len)
{
 check_memory_region((unsigned long)src, len, false, (unsigned long)__builtin_return_address(0));
 check_memory_region((unsigned long)dest, len, true, (unsigned long)__builtin_return_address(0));

 return __memcpy(dest, src, len);
}

... then __memcpy() gets inlined and the builtin stuff resolved, calling
memcpy().

This'll require some thought.

> > FWIW, with that, and the previous bits, I can boot a next-20170504
> > kernel with this applied.
> > 
> > However, I get a KASAN splat from the SLUB init code, even though
> > that's deliberately not instrumented by KASAN:

[...]

> I'm not sure about this either. I'd like to avoid needing !KASAN since
> these are useful when paired together for finding bugs...

Likewise! I'd like to have both enabled for my fuzzing config.

Thanks,
Mark.

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