linux-kernel - Re: [PATCH v2 33/39] objtool: Add IBT/ENDBR decoding

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Date:   Thu, 3 Mar 2022 11:06:43 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Miroslav Benes <mbenes@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alyssa.milburn@...el.com" <alyssa.milburn@...el.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>
Subject: Re: [PATCH v2 33/39] objtool: Add IBT/ENDBR decoding

On 03/03/2022 10:53, Miroslav Benes wrote:
> Hi,
>
> On Thu, 24 Feb 2022, Peter Zijlstra wrote:
>
>> Decode ENDBR instructions and WARN about NOTRACK prefixes.
> I guess it has been already mentioned somewhere, but could you explain 
> NOTRACK prefix here, please? If I understand it right, it disables IBT for 
> the indirect branch instruction meaning that its target does not have to 
> start with ENDBR?

CET-IBT has loads of get-out clauses.  The NOTRACK prefix is one; the
legacy code bitmap (implicit NOTRACK for whole libraries) is another.

And yes - the purpose of NOTRACK is to exempt a specific indirect branch
from checks.

GCC can emit NOTRACK'd calls in some cases when e.g. the programmer
launders a function pointer through (void *), or when
__attribute__((no_cf_check)) is used explicitly.

Each of the get-out clauses has separate enable bits, as each of them
reduces security.  In this series, Linux sets MSR_S_CET.ENDBR_EN but
specifically does not set NOTRACK_EN, so NOTRACK prefixes will be
ignored and suffer #CP if encountered.

~Andrew