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

[Miroslav Benes]

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