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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id b8sm3256350pfv.56.2021.10.14.09.10.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 Oct 2021 09:10:04 -0700 (PDT) Date: Thu, 14 Oct 2021 09:10:03 -0700 From: Kees Cook To: sxwjean@me.com Cc: x86@kernel.org, Xiongwei Song , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Gabriel Krisman Bertazi , Andy Lutomirski , "Chang S. Bae" , Al Viro , linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] x86/elf: Fix incorrect STACK_RND_MASK for x86_64 Message-ID: <202110140909.8CFD5377@keescook> References: <20211014132736.266553-1-sxwjean@me.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211014132736.266553-1-sxwjean@me.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 14, 2021 at 09:27:36PM +0800, sxwjean@me.com wrote: > From: Xiongwei Song > > 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 > Cc: Ingo Molnar > Cc: Borislav Petkov > Cc: "H. Peter Anvin" > Cc: Gabriel Krisman Bertazi > Cc: Andy Lutomirski > Cc: Kees Cook > Cc: "Chang S. Bae" > Cc: Al Viro > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Xiongwei Song > --- > 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