Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Tue, 15 Feb 2022 15:05:57 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.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
Message-ID: <202202151503.91E9B76B@keescook>
References: <202202081541.900F9E1B@keescook>
 <ad6c2633f39e39583bc5c5eaf7ccbe52@overdrivepizza.com>
 <202202082003.FA77867@keescook>
 <9ea50c51ee8db366430c9dc697a83923@overdrivepizza.com>
 <20220211133803.GV23216@worktop.programming.kicks-ass.net>
 <CABCJKuciRBnz4JxBDJC=+kuJn4pU2uBkWPBov7-VL2o2j0F4SA@mail.gmail.com>
 <20220214222550.GB23216@worktop.programming.kicks-ass.net>
 <CABCJKuf-wouSCh4u43GPQHPPgG=mqOGboSoHPQRfmgQSBkFrMg@mail.gmail.com>
 <202202151129.1A5C5FE42@keescook>
 <20220215210550.GD23216@worktop.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220215210550.GD23216@worktop.programming.kicks-ass.net>
Precedence: bulk

On Tue, Feb 15, 2022 at 10:05:50PM +0100, Peter Zijlstra wrote:
> 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.

Ah, okay, well that makes things easier. :)

> > > > > 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.

Ah, yeah, that's an interesting point.

Still, I worry about finding ways to convinces Clang to emit precisely
cmp/je/trap/call, but I guess we'll catch it immediately if it doesn't.
:P

-- 
Kees Cook