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

[Kees Cook <keescook@chromium.org>]

On Fri, Apr 29, 2016 at 12:57 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.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@kernel.org>
>> Signed-off-by: Kees Cook <keescook@chromium.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]
>
> :-)

Argh, I didn't get that warning when I built. But yes, this patch
hates me too, it seems.

>
> 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"

Hrm, let me poke at it. I think the better thing to do would be to
split up header files. I would rather have the warn/error/etc
functions in a separate header than to put the memcpy code in a
header.

-Kees

>
> ?
>
> 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)
>  {



-- 
Kees Cook
Chrome OS & Brillo Security