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

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Message-ID: <d9b47fc6-6d9d-b966-30df-9ef8c83b46e3@amd.com>
Date:   Thu, 3 Dec 2020 12:10:10 -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 11:01 AM, Borislav Petkov wrote:
> On Thu, Dec 03, 2020 at 05:54:20PM +0100, Borislav Petkov wrote:
>> On Thu, Dec 03, 2020 at 10:45:48AM -0600, Tom Lendacky wrote:
>>> Since this is based on the array size, can
>>>
>>> 	idx < NUM_LEGACY_PREFIXES
>>>
>>> be replaced with:
>>>
>>> 	idx < ARRAY_SIZE(insn->prefixes.bytes)
>>
>> Actually, this needs another change:
>>
>> struct insn_field {
>>          union {
>>                  insn_value_t value;
>>                  insn_byte_t bytes[NUM_LEGACY_PREFIXES];
> 
> Blergh, spoke too soon. All those struct insn members are struct
> insn_field.
> 
> insn.prefixes should probably be a separate array of explicit size
> NUM_LEGACY_PREFIXES, not that insn_byte_t bytes[] gets enlarged in the
> future for whatever reason, while the max legacy prefixes count will
> remain 4.

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.

Thanks,
Tom

>