From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f42.google.com (mail-pj1-f42.google.com [209.85.216.42]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9DED21B85 for ; Tue, 15 Feb 2022 23:05:59 +0000 (UTC) Received: by mail-pj1-f42.google.com with SMTP id r64-20020a17090a43c600b001b8854e682eso725663pjg.0 for ; Tue, 15 Feb 2022 15:05:59 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=xFoXp1iXxwPnqPJm6+R4iAU7d160QC7MYGNnmZh/iBg=; b=kQ3XkCI8aIjA5Ugam7/F4NKdZ3ib0Caocmpb4cMY+oXEk7UdngLyy7C5ZthE2yi4T+ 8ancU/wgUoU8Ie/JRkBYI1vQi5P7ZRB8cOA+jrBJRKXuxHUy9R1SJCWzN2TK/GFlCF5V XSHRhRkKiZFOBTRHm3pDqNPDRTvyKEPS1YWEU= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=xFoXp1iXxwPnqPJm6+R4iAU7d160QC7MYGNnmZh/iBg=; b=SDm13wPk+4tOySpev1DIFLT9tsAhg0gWspyOnWAbreR1D36aafT+cFWwWk1FSuew3L 0qn99gKuFBCbwENAo4i8jwYt6F3FSXb3sdEGJQBV2oYd68GHya8KAZD4AJBvHfjBEVdV omh98+pgNP4/HWkE93vhd5y75hpP9RXFvXCTch0A5w5/Hd4efXWk61xS/rMtxsJTF777 Lo10Z+2LcLIxXKeVNZyu8CLdYxoEsVgbGZiYXnb6sBSV31ToKLKkwpk8zpiT8mqW6j7g yYpEYBXOa8Ldm61stsZf/LQqJ4nlbXdktarK6sZfiBhqeoviNrSmNNlL41ZKFBxhdkjn DMDw== X-Gm-Message-State: AOAM532r5SqzwnhIEi1p+Y2Ip58FutxmmB2qgYsPoLb9yfGvF2PMBgvy /zIfFNxEKd9bclINiW4xflndQg== X-Google-Smtp-Source: ABdhPJxpn8JTzN/X/F3Jqc5MxaE4ad0yPJLGCRgLTY9lFi2ex5Wt3LFUet9W6MXnAIXxEQ2uWDtQUQ== X-Received: by 2002:a17:902:cf07:: with SMTP id i7mr1278462plg.137.1644966359102; Tue, 15 Feb 2022 15:05:59 -0800 (PST) Received: from www.outflux.net (smtp.outflux.net. [198.145.64.163]) by smtp.gmail.com with ESMTPSA id pj8sm9590566pjb.54.2022.02.15.15.05.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Feb 2022 15:05:58 -0800 (PST) Date: Tue, 15 Feb 2022 15:05:57 -0800 From: Kees Cook To: Peter Zijlstra 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: <202202151503.91E9B76B@keescook> References: <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> <20220215210550.GD23216@worktop.programming.kicks-ass.net> 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: <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 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