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

[Peter Zijlstra]

On Thu, Mar 03, 2022 at 01:33:06PM +0100, Miroslav Benes wrote:
> 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.

I'll add something to the Changelog. Thanks!