Re: [PATCH v2 1/3] x86/uprobes: Fix not using prefixes.nbytes for loop over prefixes.bytes

Tom Lendacky
2020-12-03T18:50:58Z

On 12/3/20 12:17 PM, Borislav Petkov wrote:
> On Thu, Dec 03, 2020 at 12:10:10PM -0600, Tom Lendacky wrote:
>> Since that struct is used in multiple places, I think basing it on the array
>> size is the best way to go. The main point of the check is just to be sure
>> you don't read outside of the array.
>
> Well, what happens if someone increases the array size of:
>
> struct insn_field {
> union {
> insn_byte_t bytes[4];
> ^^^^
>
> ?

I think we need to keep the parsing of the instruction separate from
accessing the prefixes after (successfully) parsing it. This fix is merely
making sure that we don't read outside the bounds of the array that
currently holds the legacy prefixes.

>
> That's why a separate array only for legacy prefixes would be better
> in the long run. The array size check is good as a short-term fix for
> stable.
>
> I'd say.

According to Volume 3 of the AMD APM (Figure 1-2 on page 5), there could
be as many as 5 legacy prefixes and it says that more than one prefix from
each group is undefined behavior. The instruction parsing code doesn't
seem to take into account the different prefix groups. So I agree with you
that short term the array size check works, and long term, the legacy
prefix support probably needs a closer look.

Thanks,
Tom

>