Messages in this thread | | | From | Andrew Cooper <> | Subject | Re: [PATCH v2 33/39] objtool: Add IBT/ENDBR decoding | Date | Thu, 3 Mar 2022 11:06:43 +0000 |
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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
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