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Date: Thu, 3 Mar 2022 13:33:06 +0100 (CET)
From: Miroslav Benes <mbenes@...e.cz>
To: Andrew Cooper <Andrew.Cooper3@...rix.com>
cc: Peter Zijlstra <peterz@...radead.org>,
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Subject: Re: [PATCH v2 33/39] objtool: Add IBT/ENDBR decoding
On Thu, 3 Mar 2022, Andrew Cooper wrote:
> 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.
Thanks for the explanation. I would be nice to include it somewhere so
that it is not lost.
Miroslav
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