Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932639AbbD0MIs (ORCPT ); Mon, 27 Apr 2015 08:08:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56722 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932452AbbD0MIq (ORCPT ); Mon, 27 Apr 2015 08:08:46 -0400 Message-ID: <553E26C8.3020906@redhat.com> Date: Mon, 27 Apr 2015 14:08:40 +0200 From: Denys Vlasenko User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Borislav Petkov , Andy Lutomirski CC: Andy Lutomirski , X86 ML , "H. Peter Anvin" , Denys Vlasenko , Linus Torvalds , Brian Gerst , Ingo Molnar , Steven Rostedt , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , Linux Kernel Mailing List Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue References: <5d120f358612d73fc909f5bfa47e7bd082db0af0.1429841474.git.luto@kernel.org> <20150425211206.GE32099@pd.tnic> <20150427085305.GB6774@pd.tnic> <20150427113506.GG6774@pd.tnic> In-Reply-To: <20150427113506.GG6774@pd.tnic> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2494 Lines: 56 On 04/27/2015 01:35 PM, Borislav Petkov wrote: > On Mon, Apr 27, 2015 at 10:53:05AM +0200, Borislav Petkov wrote: >> ALTERNATIVE "", >> "shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \ >> sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \ >> cmpq %rcx, %r11 \ >> jne opportunistic_sysret_failed" >> X86_BUG_SYSRET_CANONICAL_RCX > > Right, so I can do this: > > /* > * Change top 16 bits to be the sign-extension of 47th bit, if this > * changed %rcx, it was not canonical. > */ > ALTERNATIVE "", \ > "shl $(64 - (47+1)), %rcx; \ > sar $(64 - (47+1)), %rcx; \ > cmpq %rcx, %r11; \ > jne opportunistic_sysret_failed", X86_BUG_SYSRET_CANON_RCX > > If I use the __VIRTUAL_MASK_SHIFT macro *in* the ALTERNATIVE macro, I get some > really cryptic gas error: > > arch/x86/kernel/entry_64.S: Assembler messages: > arch/x86/kernel/entry_64.S:441: Error: can't resolve `L0' {*ABS* section} - `L0' {*UND* section} > scripts/Makefile.build:294: recipe for target 'arch/x86/kernel/entry_64.o' failed > make[1]: *** [arch/x86/kernel/entry_64.o] Error 1 > Makefile:1536: recipe for target 'arch/x86/kernel/entry_64.o' failed > make: *** [arch/x86/kernel/entry_64.o] Error 2 > > but I guess we can simply use the naked "47" because a couple of lines > above, we already have the sanity-check: > > .ifne __VIRTUAL_MASK_SHIFT - 47 > .error "virtual address width changed -- SYSRET checks need update" > .endif > > so we should be guarded just fine. > > Anyway, if we do it this way, we get 17 NOPs added at build time which is the > length of the 4 instructions: > > ffffffff819ef40c: 48 c1 e1 10 shl $0x10,%rcx > ffffffff819ef410: 48 c1 f9 10 sar $0x10,%rcx > ffffffff819ef414: 49 39 cb cmp %rcx,%r11 > ffffffff819ef417: 0f 85 ff 9c bc ff jne ffffffff815b911c This looks strange. opportunistic_sysret_failed label is just a few instructions below. Why are you getting "ff 9c bc ff" offset in JNE instead of short jump of 0x5f bytes I see without ALTERNATIVE? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/