linux-kernel - Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

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Date:   Tue, 15 Feb 2022 21:53:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Joao Moreira <joao@...rdrivepizza.com>,
        Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
        hjl.tools@...il.com, Josh Poimboeuf <jpoimboe@...hat.com>,
        andrew.cooper3@...rix.com, LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

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@...radead.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?

Yes, but since you're going to emit a retpoline, and objtool already
generates a list of retpoline sites, we can abuse that. If the
instruction after the trap is a retpoline site, we a CFI fail.

> > > 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?

It is. int3 is also smaller by one byte, but the exception is already
fairly complicated, but I can see if we can make it work.

> > Also, you really should add a CS prefix to the retpoline thunk call if
> > you insist on using r11 (or any of the higher regs).
> 
> I actually didn't touch the retpoline thunk call, that's exactly the
> code Clang normally generates.

Yeah, and it needs help, also see:

  https://lore.kernel.org/lkml/20211119165630.276205624@infradead.org/

Specifically, clang needs to:

 - CS prefix stuff the retpoline thunk call/jmp;
 - stop doing conditional indirect tail-calls.