Commit 91e5ed49 authored Jan 27, 2015 by Andy Lutomirski Committed by Ingo Molnar Feb 19, 2015

x86/asm/decoder: Fix and enforce max instruction size in the insn decoder



x86 instructions cannot exceed 15 bytes, and the instruction
decoder should enforce that.  Prior to 6ba48ff4, the
instruction length limit was implicitly set to 16, which was an
approximation of 15, but there is currently no limit at all.

Fix MAX_INSN_SIZE (it should be 15, not 16), and fix the decoder
to reject instructions that exceed MAX_INSN_SIZE.

Other than potentially confusing some of the decoder sanity
checks, I'm not aware of any actual problems that omitting this
check would cause, nor am I aware of any practical problems
caused by the MAX_INSN_SIZE error.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Fixes: 6ba48ff4 ("x86: Remove arbitrary instruction size limit ...
Link: http://lkml.kernel.org/r/f8f0bc9b8c58cfd6830f7d88400bf1396cbdcd0f.1422403511.git.luto@amacapital.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent cbb53b96

arch/x86/include/asm/insn.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -69,7 +69,7 @@ struct insn {
		const insn_byte_t *next_byte;
		};

		#define MAX_INSN_SIZE 16
		#define MAX_INSN_SIZE 15

		#define X86_MODRM_MOD(modrm) (((modrm) & 0xc0) >> 6)
		#define X86_MODRM_REG(modrm) (((modrm) & 0x38) >> 3)

arch/x86/lib/insn.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -52,6 +52,13 @@
		*/
		void insn_init(struct insn insn, const void kaddr, int buf_len, int x86_64)
		{
		/*
		* Instructions longer than MAX_INSN_SIZE (15 bytes) are invalid
		* even if the input buffer is long enough to hold them.
		*/
		if (buf_len > MAX_INSN_SIZE)
		buf_len = MAX_INSN_SIZE;

		memset(insn, 0, sizeof(*insn));
		insn->kaddr = kaddr;
		insn->end_kaddr = kaddr + buf_len;