Re: [PATCH] x86: Fix and enforce max instruction size in the insn decoder

[Masami Hiramatsu]

(2015/01/28 9:06), Andy Lutomirski wrote:
> x86 instructions cannot exceed 15 bytes, and the instruction decoder
> should enforce that.  Prior to 6ba48ff46f76, 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.
> 

Thank you for updating. This looks good to me :)

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>

Ingo, could you pull this in -tip tree?

Thank you,

> Fixes: 6ba48ff46f76 x86: Remove arbitrary instruction size limit in instruction decoder
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
> 
> Changes from old versions: folded
> 
> arch/x86/include/asm/insn.h | 2 +-
>  arch/x86/lib/insn.c         | 7 +++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/insn.h b/arch/x86/include/asm/insn.h
> index 47f29b1d1846..e7814b74caf8 100644
> --- a/arch/x86/include/asm/insn.h
> +++ b/arch/x86/include/asm/insn.h
> @@ -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)
> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 2480978b31cc..bf1f227cf706 100644
> --- a/arch/x86/lib/insn.c
> +++ b/arch/x86/lib/insn.c
> @@ -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;
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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