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

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Date:   Thu, 3 Dec 2020 12:49:46 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "H . Peter Anvin" <hpa@...or.com>, Joerg Roedel <jroedel@...e.de>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] x86/uprobes: Fix not using prefixes.nbytes for
 loop over prefixes.bytes

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

>