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 5D42B10E8 for ; Tue, 17 May 2022 09:38:49 +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=jwTi2Cazog8blP0yOGLXWlJcsxSDGZ2u4uY9kxsLkfk=; b=MTdKy2r7vz9qnE0wpTp07nMDEg ljCp1Zb2NriOnwiA/uzXXdtuSEEAt+8MmHDW386odcrFLXR7ho+3sO3bDv22akTTPVfMMBXnv+sYI g6gwbLw7UoFN9kxB7jC1PnSyV0mnJYnpYuoffSyjX5HIclZnpntQBYF3T/PccfzfZB5YTQq8536iO PNACVypbg/9epOBrixidP1XzJqc62oEdrDBFOtkey9RO2PV2y4tUg/sKetgnmz/oaloCU6sVTc76I GIFxs0GQBFaxTwXsfnWWxc12azbRX8zbFTrsQm1XHYjiH/MDS52S3YLfghOmjbjQalbuvQq7Rp/3m 60MDw/MA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nqtf5-00Aje0-Dt; Tue, 17 May 2022 09:38:39 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 7F7A130031C; Tue, 17 May 2022 11:38:36 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 66A62202BC439; Tue, 17 May 2022 11:38:36 +0200 (CEST) Date: Tue, 17 May 2022 11:38:36 +0200 From: Peter Zijlstra To: David Laight Cc: Kees Cook , Sami Tolvanen , "linux-kernel@vger.kernel.org" , Josh Poimboeuf , "x86@kernel.org" , Catalin Marinas , Will Deacon , Mark Rutland , Nathan Chancellor , Nick Desaulniers , Joao Moreira , Sedat Dilek , Steven Rostedt , "linux-hardening@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "llvm@lists.linux.dev" Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG Message-ID: References: <20220513202159.1550547-1-samitolvanen@google.com> <20220513202159.1550547-21-samitolvanen@google.com> <20220516183047.GM76023@worktop.programming.kicks-ass.net> <202205161531.3339CA95@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: On Tue, May 17, 2022 at 08:48:41AM +0000, David Laight wrote: > From: Peter Zijlstra > > Sent: 17 May 2022 09:41 > ... > > > If we use %eax instead of %r10d for the hash transfer (as per Joao), and > > > use int3 instead of ud2, then we can shrink the fineibt sequence to: > > > > > > __cfi_\func: > > > endbr # 4 > > > xorl $0x12345678, %eax # 5 > > > jz 1f # 2 > > > int3 # 1 > > > \func: > > > ... > > > > > > Which is 12 bytes, and needs a larger preamble (up from 9 in the current > > > proposal). > > > > On all that; perhaps it would be good to have a compiler option to > > specify the preamble size. It can enforce the minimum at 7 to have at > > least the required: > > > > movl $0x12345678, %eax > > int3 > > int3 > > > > but any larger number will just increase the preamble with int3 padding > > at the top. > > > > That can go right along with the option to supress endbr when preamble > > :-) > > You also need a compiler option to specify the register. > While (I think) %eax is usable in kernel, it isn't in userspace. > It is used in varargs calls to pass (IIRC) the number of fp > args that are passed in registers. You're mistaken, the compiler doesn't emit the FineIBT code *at*all*. That's all patched in later. For kCFI the mov is never executed and is only there to make it a valid instruction.