linux-kernel - Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use

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Date:	Fri, 29 Apr 2016 09:57:45 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Lasse Collin <lasse.collin@...aani.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yinghai@...nel.org>, Baoquan He <bhe@...hat.com>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH v4] x86/boot: Warn on future overlapping memcpy() use


* Kees Cook <keescook@...omium.org> wrote:

> If an overlapping memcpy() is ever attempted, we should at least report
> it, in case it might lead to problems, so it could be changed to a
> memmove() call instead.
> 
> Suggested-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v4:
> - use __memcpy not memcpy since we've already done the check.
> v3:
> - call memmove in addition to doing the warning
> v2:
> - warn about overlapping region
> ---
>  arch/x86/boot/compressed/string.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

I think you'll hate this patch some more:

 arch/x86/boot/compressed/string.c:68:3: warning: implicit declaration of function ‘warn’ [-Wimplicit-function-declaration]

:-)

Can we do the trick below? Because misc.h also includes the regular kernel memcpy 
functions, we can remove the decompressor specific __memcpy() - but the question 
is, is it safe to do?

If it's not safe to do, we are playing with fire already I suspect:

 arch/x86/boot/compressed/cmdline.c:#include "misc.h"
 arch/x86/boot/compressed/early_serial_console.c:#include "misc.h"
 arch/x86/boot/compressed/kaslr.c:#include "misc.h"
 arch/x86/boot/compressed/misc.c:#include "misc.h"

?

Thanks,

	Ingo

 arch/x86/boot/compressed/string.c | 31 +------------------------------
 1 file changed, 1 insertion(+), 30 deletions(-)

diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c
index 952510976732..f4b95ed4e7a2 100644
--- a/arch/x86/boot/compressed/string.c
+++ b/arch/x86/boot/compressed/string.c
@@ -6,37 +6,8 @@
  * (e.g. FPU ops) in the minimal decompression stub execution environment.
  */
 #include "../string.c"
-#include "misc.h"
-
-#ifdef CONFIG_X86_32
-static void *__memcpy(void *dest, const void *src, size_t n)
-{
-	int d0, d1, d2;
-	asm volatile(
-		"rep ; movsl\n\t"
-		"movl %4,%%ecx\n\t"
-		"rep ; movsb\n\t"
-		: "=&c" (d0), "=&D" (d1), "=&S" (d2)
-		: "0" (n >> 2), "g" (n & 3), "1" (dest), "2" (src)
-		: "memory");
-
-	return dest;
-}
-#else
-static void *__memcpy(void *dest, const void *src, size_t n)
-{
-	long d0, d1, d2;
-	asm volatile(
-		"rep ; movsq\n\t"
-		"movq %4,%%rcx\n\t"
-		"rep ; movsb\n\t"
-		: "=&c" (d0), "=&D" (d1), "=&S" (d2)
-		: "0" (n >> 3), "g" (n & 7), "1" (dest), "2" (src)
-		: "memory");
 
-	return dest;
-}
-#endif
+#include "misc.h"
 
 void *memset(void *s, int c, size_t n)
 {