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Date:   Tue, 15 Feb 2022 22:05:50 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        X86 ML <x86@kernel.org>, hjl.tools@gmail.com,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        andrew.cooper3@citrix.com, LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        llvm@lists.linux.dev
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
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On Tue, Feb 15, 2022 at 12:03:12PM -0800, Kees Cook wrote:
> On Tue, Feb 15, 2022 at 08:56:03AM -0800, Sami Tolvanen wrote:
> > On Mon, Feb 14, 2022 at 2:25 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > > On Mon, Feb 14, 2022 at 01:38:18PM -0800, Sami Tolvanen wrote:
> > > > I'm fine with adding a trap mode that's used by default, but having
> > > > more helpful diagnostics when something fails is useful even in
> > > > production systems in my experience. This change results in a vmlinux
> > > > that's another 0.92% smaller.
> > >
> > > You can easily have the exception generate a nice warning, you can even
> > > have it continue. You really don't need a call for that.
> > 
> > Sure, but wouldn't that require us to generate something like
> > __bug_table, so we know where the CFI specific traps are?
> 
> It also means the trap handler needs to do a bunch of instruction
> decoding to find the address that was going to be jumped to, etc.

arch/x86/kernel/alternative.c:apply_retpolines() has all that, since we
need to to know that to re-write the thunk-call.

> > > > In this case the function has two indirect calls and Clang seems to
> > > > prefer to emit just one ud2.
> > >
> > > That will not allow you to recover from the exception. UD2 is not an
> > > unconditional fail. It should have an out-going edge in this case too.
> > 
> > Yes, CFI failures are not recoverable in that code. In fact, LLVM
> > assumes that the llvm.trap intrinsic (i.e. ud2) never returns, but I
> > suppose we could just use an int3 instead. I assume that's sufficient
> > to stop speculation?
> 
> Peter, is there a reason you want things in the specific order of:
> 
> cmp, je-to-call, trap, call
> 
> Isn't it more run-time efficient to have an out-of-line failure of
> the form:
> 
> cmp, jne-to-trap, call, ...code..., trap, jmp-to-call
> 
> I thought the static label stuff allowed the "default out of line"
> option, as far as pessimizing certain states, etc? The former is certainly
> code-size smaller, though, yes, but doesn't it waste space in the cache
> line for the unlikely case, etc?

Mostly so that we can deduce the address of the trap from the retpoline
site, also the above has a fairly high chance of using jcc.d32 which is
actually larger than jcc.d8+ud2.