Messages in this thread | | | Date | Thu, 3 Mar 2022 13:33:06 +0100 (CET) | From | Miroslav Benes <> | 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 | |