Re: [PATCH RFC] x86/elf: Fix incorrect STACK_RND_MASK for x86_64

[Kees Cook <keescook@chromium.org>]

On Thu, Oct 14, 2021 at 09:27:36PM +0800, sxwjean@me.com wrote:
> From: Xiongwei Song <sxwjean@gmail.com>
> 
> According to the comment for stack randomization of x86, the range of
> randomization is 1GB, which occupies 30 valid bits in binary. And in
> x86_64, PAGE_SHIFT occupies 12 bits, then the STACK_RND_MASK should
> occupy 18 bits for 1GB.
> 
> The current range of randomization is :
> 	0x3fffff << PAGE_SHIFT
> , which occupies 34 bits.
> 
> This patch changed 0x3fffff to 0x3ffff, which makes the range of stack
> randomization is real 1GB.

Why do we want to _reduce_ entropy here? Perhaps adjust the comment
instead?

-Kees

> 
> Before this patch(bit0 ~ bit33 randomized):
> 	root@qemux86-64:~# for i in {1..20};do cat /proc/self/maps | grep stack;done
> 	7ffddf971000-7ffddf992000 rw-p 00000000 00:00 0                          [stack]
> 	7ffc4e0d4000-7ffc4e0f5000 rw-p 00000000 00:00 0                          [stack]
> 	7fff5898e000-7fff589af000 rw-p 00000000 00:00 0                          [stack]
> 	7ffcf910b000-7ffcf912c000 rw-p 00000000 00:00 0                          [stack]
> 	7ffce829d000-7ffce82be000 rw-p 00000000 00:00 0                          [stack]
> 	7ffec72d9000-7ffec72fa000 rw-p 00000000 00:00 0                          [stack]
> 	7ffd6e256000-7ffd6e277000 rw-p 00000000 00:00 0                          [stack]
> 	7ffe14120000-7ffe14141000 rw-p 00000000 00:00 0                          [stack]
> 	7ffe21549000-7ffe2156a000 rw-p 00000000 00:00 0                          [stack]
> 	7ffdc9d33000-7ffdc9d54000 rw-p 00000000 00:00 0                          [stack]
> 	7ffe1ced4000-7ffe1cef5000 rw-p 00000000 00:00 0                          [stack]
> 	7ffcb0440000-7ffcb0461000 rw-p 00000000 00:00 0                          [stack]
> 	7ffc84515000-7ffc84536000 rw-p 00000000 00:00 0                          [stack]
> 	7ffe557dd000-7ffe557fe000 rw-p 00000000 00:00 0                          [stack]
> 	7ffdcb7e4000-7ffdcb805000 rw-p 00000000 00:00 0                          [stack]
> 	7ffc6f989000-7ffc6f9aa000 rw-p 00000000 00:00 0                          [stack]
> 	7ffd9322a000-7ffd9324b000 rw-p 00000000 00:00 0                          [stack]
> 	7ffea8e34000-7ffea8e55000 rw-p 00000000 00:00 0                          [stack]
> 	7ffc594ff000-7ffc59520000 rw-p 00000000 00:00 0                          [stack]
> 	7ffe8db65000-7ffe8db86000 rw-p 00000000 00:00 0                          [stack]
> 
> After this patch(bit0 ~ bit29 randomized):
> 	root@qemux86-64:~# for i in {1..20};do cat /proc/self/maps | grep stack;done
> 	7fffd0ed2000-7fffd0ef3000 rw-p 00000000 00:00 0                          [stack]
> 	7fffdf555000-7fffdf576000 rw-p 00000000 00:00 0                          [stack]
> 	7ffffec0e000-7ffffec2f000 rw-p 00000000 00:00 0                          [stack]
> 	7fffcfa0f000-7fffcfa30000 rw-p 00000000 00:00 0                          [stack]
> 	7fffff75d000-7fffff77e000 rw-p 00000000 00:00 0                          [stack]
> 	7fffd1d11000-7fffd1d32000 rw-p 00000000 00:00 0                          [stack]
> 	7fffc7850000-7fffc7871000 rw-p 00000000 00:00 0                          [stack]
> 	7fffc956e000-7fffc958f000 rw-p 00000000 00:00 0                          [stack]
> 	7fffdf7be000-7fffdf7df000 rw-p 00000000 00:00 0                          [stack]
> 	7fffeffdf000-7ffff0000000 rw-p 00000000 00:00 0                          [stack]
> 	7fffd7d7f000-7fffd7da0000 rw-p 00000000 00:00 0                          [stack]
> 	7fffe3d23000-7fffe3d44000 rw-p 00000000 00:00 0                          [stack]
> 	7fffde4b7000-7fffde4d8000 rw-p 00000000 00:00 0                          [stack]
> 	7fffd477a000-7fffd479b000 rw-p 00000000 00:00 0                          [stack]
> 	7fffc0e95000-7fffc0eb6000 rw-p 00000000 00:00 0                          [stack]
> 	7fffdbdda000-7fffdbdfb000 rw-p 00000000 00:00 0                          [stack]
> 	7fffec737000-7fffec758000 rw-p 00000000 00:00 0                          [stack]
> 	7fffed163000-7fffed184000 rw-p 00000000 00:00 0                          [stack]
> 	7ffff9a9c000-7ffff9abd000 rw-p 00000000 00:00 0                          [stack]
> 	7fffff945000-7fffff966000 rw-p 00000000 00:00 0                          [stack]
> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Gabriel Krisman Bertazi <krisman@collabora.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: "Chang S. Bae" <chang.seok.bae@intel.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
> ---
>  arch/x86/include/asm/elf.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 29fea180a665..0b7a5510ebd5 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -343,7 +343,7 @@ extern unsigned long get_sigframe_size(void);
>  #else /* CONFIG_X86_32 */
>  
>  /* 1GB for 64bit, 8MB for 32bit */
> -#define __STACK_RND_MASK(is32bit) ((is32bit) ? 0x7ff : 0x3fffff)
> +#define __STACK_RND_MASK(is32bit) ((is32bit) ? 0x7ff : 0x3ffff)
>  #define STACK_RND_MASK __STACK_RND_MASK(mmap_is_ia32())
>  
>  #define ARCH_DLINFO							\
> -- 
> 2.30.2
> 

-- 
Kees Cook