From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f177.google.com (mail-pf1-f177.google.com [209.85.210.177]) (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 17E427C for ; Tue, 15 Feb 2022 20:03:13 +0000 (UTC) Received: by mail-pf1-f177.google.com with SMTP id i6so150376pfc.9 for ; Tue, 15 Feb 2022 12:03:13 -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=BYpYOt247YYcQkSbIW220dgIYhgVR6WY6sT8wWi/ZtE=; b=EJvqgsGqpY5ntsB8smDSnvlbX6MMh2tLmKHk2kpn+WLLX26Qg46icNudHQswUgDvDG 2XCYZgLfLV3ryQ7MFgZo/SptowPM72fkx86+kCCAcd0DDtlbTkL6uvbqPIJNRGFPanJ5 4xh874w3o9d/Mal+b4CGcSbfDAxCP1FsuU+TA= 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=BYpYOt247YYcQkSbIW220dgIYhgVR6WY6sT8wWi/ZtE=; b=J9fgWeCC497B7JUY+bvGDbMiswDkhtnwH2upPKXCI+9YfMeGnvXdFujYIF17eg2+k6 pAUZ3i1tzw802PSBgmC/3f09gSlW7HFuJ5GyMwKe/ePPcBnYQp104S3x/qVa9K3laP5d Fh+rkDwXSphUi1DMLxWBf8qTI/311OnqLwlouDJ7l+qNvWaGXkVLMGgHr8XK+EZgFjJW ZGx0wZN9Vd56l5JqWhmGACT6rL1SS5bUjKduxh2Xr5SvJAiioxI2cq5gev8ToYn1msp7 hKXB4IZzMdWIQDYvqvm0wS5sAZw6W1/46NZd8ec5bl3dukGNsTkM0R+8oQ6JiXr+r7lV SgpQ== X-Gm-Message-State: AOAM531AOe3THzdPuueFVaScxhvhP2KKmn69ii08XK4WJu3vf2/N2tVy 5tsJToUxeVyeQrT96m2kEelDZQ== X-Google-Smtp-Source: ABdhPJymds6XPIxfBbmKquoj/dCf0PdF93RyQzyKRSTKGOGU+jlmGBmvXoixB2ASM9RaOeug7YSqcA== X-Received: by 2002:a63:4854:: with SMTP id x20mr449831pgk.178.1644955393489; Tue, 15 Feb 2022 12:03:13 -0800 (PST) Received: from www.outflux.net (smtp.outflux.net. [198.145.64.163]) by smtp.gmail.com with ESMTPSA id q16sm42222151pfu.194.2022.02.15.12.03.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Feb 2022 12:03:13 -0800 (PST) Date: Tue, 15 Feb 2022 12:03:12 -0800 From: Kees Cook To: Sami Tolvanen Cc: Peter Zijlstra , 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: <202202151129.1A5C5FE42@keescook> References: <20211122170805.338489412@infradead.org> <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> 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: 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. > > > 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? > > 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. 32-bit hashes would have both code size and runtime benefits: fewer instructions for the compare therefore a smaller set of instructions added. -- Kees Cook