Re: [PATCH 1/3] x86/sev-es: Fix not using prefixes.nbytes for loop over prefixes.bytes

[Tom Lendacky]

On 12/2/20 2:51 AM, Masami Hiramatsu wrote:
> Since the insn.prefixes.nbytes can be bigger than the size of
> insn.prefixes.bytes[] when a same prefix is repeated, we have to
> check whether the insn.prefixes.bytes[i] != 0 and i < 4 instead
> of insn.prefixes.nbytes.
>
> Fixes: 25189d08e516 ("x86/sev-es: Add support for handling IOIO exceptions")
> Reported-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>   arch/x86/boot/compressed/sev-es.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/sev-es.c b/arch/x86/boot/compressed/sev-es.c
> index 954cb2702e23..6a7a3027c9ac 100644
> --- a/arch/x86/boot/compressed/sev-es.c
> +++ b/arch/x86/boot/compressed/sev-es.c
> @@ -36,7 +36,7 @@ static bool insn_has_rep_prefix(struct insn *insn)
>   
>   	insn_get_prefixes(insn);
>   
> -	for (i = 0; i < insn->prefixes.nbytes; i++) {
> +	for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {

Wouldn't it be better to create a #define for the size rather than hard 
coding 4 in the various files? That would protect everything should the 
bytes array size ever change in the future.

Thanks,
Tom

>   		insn_byte_t p = insn->prefixes.bytes[i];
>   
>   		if (p == 0xf2 || p == 0xf3)