Re: [PATCH 3.19 v2 3/3] x86: Enforce MAX_INSN_SIZE in the instruction decoder

[Masami Hiramatsu]

(2015/01/13 8:04), Andy Lutomirski wrote:
> The instruction decoder used to assume that the input buffer was
> exactly MAX_INSN_SIZE bytes long.  Now that the input buffer has
> variable length, even if the input buffer is longer than
> MAX_INSN_SIZE, we should still reject instructions longer than
> MAX_INSN_SIZE, since a real CPU will reject them even if they're
> otherwise valid.
> 
> Other than potentially confusing some of the decoder sanity checks,
> I'm not aware of any actual problems that omitting this check would
> cause.
> 
> It's worth noting that MAX_INSN_SIZE is incorrectly set to 16.  This
> patch doesn't change that.  I'll submit a fix for that later.

Hm, this patch logic is OK, but the comment is a bit odd.
As you said, if the current code incorrectly set MAX_INSN_SIZE to 16,
the comment should not say "15" without changing MAX_INSN_SIZE itself.

Others are OK for me.

> 
> Fixes: 6ba48ff46f76 x86: Remove arbitrary instruction size limit in instruction decoder
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
> 
> Arguably, the limit could be hard-coded as 15 instead of relying on the
> macro.
> 
> arch/x86/lib/insn.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 1313ae6b478b..c5912d7a4a15 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 15 bytes are invalid even if the

I meant you'd better use MAX_INSN_SIZE instead of "15 bytes" here.

Thank you,

> +	 * 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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