From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 42670186C for ; Tue, 15 Feb 2022 21:06:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=uvMluQdN0GSPuEJBcDu2YtcMnFGhEbHkw4SETZERvO0=; b=Rt5N2dtwUV7yCxceA3dKBG7Jec DN0oXg1X/475ee26Wl+i80jXWzL36PEZHfWtFuolqqKIAmfheMVxDPoaAdPrZIc7XRpqmyJJWIcfn Tc5jGPiry2KXhNSK++FitBh3laLYzLUOq1JapJoodBvcjrJrs1xQpBrmXc8vlkso96NNzKlQ6NDOn PdhT9FvvKCy+vJTylcgyFZ3YEGMYOWjhCkvcIPMbHsUCoxtXCGTNCzjmTrFjOBVGCVZwUiU8svxrl Yp51OVSq4cQvGa8BRlbEbg8C4oJYGzALwc2ucGwqleaBJyBq4Mkx4VqAHi3fedOUaQbuNZPxSRR1R QhMyXUUg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nK51D-00EB2o-Ti; Tue, 15 Feb 2022 21:05:52 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id 6B154987956; Tue, 15 Feb 2022 22:05:50 +0100 (CET) Date: Tue, 15 Feb 2022 22:05:50 +0100 From: Peter Zijlstra To: Kees Cook Cc: Sami Tolvanen , Joao Moreira , X86 ML , hjl.tools@gmail.com, Josh Poimboeuf , andrew.cooper3@citrix.com, LKML , Nick Desaulniers , llvm@lists.linux.dev Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups Message-ID: <20220215210550.GD23216@worktop.programming.kicks-ass.net> References: <6ebb0ab131c522f20c094294d49091fc@overdrivepizza.com> <202202081541.900F9E1B@keescook> <202202082003.FA77867@keescook> <9ea50c51ee8db366430c9dc697a83923@overdrivepizza.com> <20220211133803.GV23216@worktop.programming.kicks-ass.net> <20220214222550.GB23216@worktop.programming.kicks-ass.net> <202202151129.1A5C5FE42@keescook> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202202151129.1A5C5FE42@keescook> 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 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.