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

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Message-ID: <CABCJKuf-wouSCh4u43GPQHPPgG=mqOGboSoHPQRfmgQSBkFrMg@mail.gmail.com>
Date:   Tue, 15 Feb 2022 08:56:03 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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?

> > 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?

> 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.

> > How would you like to deal with the 4-byte hashes in objtool? We
> > either need to annotate all function symbols in the kernel, or we need
> > a way to distinguish the hashes from random instructions, so we can
> > also have functions that don't have a type hash.
>
> Easiest would be to create a special section with all the hash offsets
> in I suppose. A bit like -mfentry-section=name.

OK, I'll take a look. With 64-bit hashes I was planning to use a known
prefix, but that's not really an option with a 32-bit hash.

Sami