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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
Date: Tue, 15 Feb 2022 15:05:57 -0800	[thread overview]
Message-ID: <202202151503.91E9B76B@keescook> (raw)
In-Reply-To: <20220215210550.GD23216@worktop.programming.kicks-ass.net>

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

  reply	other threads:[~2022-02-15 23:06 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-22 17:03 [RFC][PATCH 0/6] x86: Kernel IBT beginnings Peter Zijlstra
2021-11-22 17:03 ` [RFC][PATCH 1/6] x86: Annotate _THIS_IP_ Peter Zijlstra
2021-11-23 13:53   ` Mark Rutland
2021-11-23 14:14     ` Peter Zijlstra
2021-11-24 18:18       ` Josh Poimboeuf
2021-11-22 17:03 ` [RFC][PATCH 2/6] x86: Base IBT bits Peter Zijlstra
2022-02-08 23:32   ` Kees Cook
2021-11-22 17:03 ` [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self Peter Zijlstra
2021-11-22 18:09   ` Peter Zijlstra
2022-02-08 23:33     ` Kees Cook
2021-11-22 17:03 ` [RFC][PATCH 4/6] objtool: Read the _THIS_IP_ hints Peter Zijlstra
2021-11-22 17:03 ` [RFC][PATCH 5/6] x86: Sprinkle ENDBR dust Peter Zijlstra
2021-11-23 14:00   ` Mark Rutland
2021-11-23 14:21     ` Peter Zijlstra
2022-02-08 23:38     ` Kees Cook
2021-11-22 17:03 ` [RFC][PATCH 6/6] objtool: Add IBT validation / fixups Peter Zijlstra
2021-11-24 19:30   ` Josh Poimboeuf
2022-02-08 23:43     ` Kees Cook
2022-02-09  5:09       ` Josh Poimboeuf
2022-02-09 11:41       ` Peter Zijlstra
2022-02-09 11:45         ` Peter Zijlstra
2021-12-24  2:05   ` joao
2022-02-08 23:42     ` Kees Cook
2022-02-09  2:21       ` Joao Moreira
2022-02-09  4:05         ` Kees Cook
2022-02-09  5:18           ` Joao Moreira
2022-02-11 13:38             ` Peter Zijlstra
2022-02-14 21:38               ` Sami Tolvanen
2022-02-14 22:25                 ` Peter Zijlstra
2022-02-15 16:56                   ` Sami Tolvanen
2022-02-15 20:03                     ` Kees Cook
2022-02-15 21:05                       ` Peter Zijlstra
2022-02-15 23:05                         ` Kees Cook [this message]
2022-02-15 23:38                           ` Joao Moreira
2022-02-16 12:24                         ` Peter Zijlstra
2022-02-15 20:53                     ` Peter Zijlstra
2022-02-15 22:45               ` Joao Moreira
2022-02-16  0:57               ` Andrew Cooper
2022-03-02  3:06               ` Peter Collingbourne
2022-03-02  3:32                 ` Joao Moreira
2022-06-08 17:53                 ` Fāng-ruì Sòng
2022-06-09  0:05                   ` Sami Tolvanen
2021-11-23  7:58 ` [RFC][PATCH 0/6] x86: Kernel IBT beginnings Christoph Hellwig
2021-11-23  9:02   ` Peter Zijlstra
2022-02-08 23:48 ` Kees Cook
2022-02-09  0:09 ` Nick Desaulniers

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