Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751472AbdHNGm2 (ORCPT ); Mon, 14 Aug 2017 02:42:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:56032 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750981AbdHNGm1 (ORCPT ); Mon, 14 Aug 2017 02:42:27 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BFBF623A02 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=luto@kernel.org MIME-Version: 1.0 In-Reply-To: References: <7c88ed36805d36841ab03ec3b48b4122c4418d71.1502164668.git.luto@kernel.org> From: Andy Lutomirski Date: Sun, 13 Aug 2017 23:42:05 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] x86/xen/64: Rearrange the SYSCALL entries To: Andy Lutomirski Cc: Brian Gerst , Andrew Cooper , X86 ML , Juergen Gross , Borislav Petkov , Linux Kernel Mailing List , "xen-devel@lists.xenproject.org" , Boris Ostrovsky , "H. Peter Anvin" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2932 Lines: 76 On Sun, Aug 13, 2017 at 10:53 PM, Andy Lutomirski wrote: > On Sun, Aug 13, 2017 at 7:44 PM, Brian Gerst wrote: >> On Mon, Aug 7, 2017 at 11:59 PM, Andy Lutomirski wrote: >>> /* Normal 64-bit system call target */ >>> ENTRY(xen_syscall_target) >>> - undo_xen_syscall >>> - jmp entry_SYSCALL_64_after_swapgs >>> + popq %rcx >>> + popq %r11 >>> + jmp entry_SYSCALL_64_after_hwframe >>> ENDPROC(xen_syscall_target) >>> >>> #ifdef CONFIG_IA32_EMULATION >>> >>> /* 32-bit compat syscall target */ >>> ENTRY(xen_syscall32_target) >>> - undo_xen_syscall >>> - jmp entry_SYSCALL_compat >>> + popq %rcx >>> + popq %r11 >>> + jmp entry_SYSCALL_compat_after_hwframe >>> ENDPROC(xen_syscall32_target) >>> >>> /* 32-bit compat sysenter target */ >>> ENTRY(xen_sysenter_target) >>> - undo_xen_syscall >>> + mov 0*8(%rsp), %rcx >>> + mov 1*8(%rsp), %r11 >>> + mov 5*8(%rsp), %rsp >>> jmp entry_SYSENTER_compat >>> ENDPROC(xen_sysenter_target) >> >> This patch causes the iopl_32 and ioperm_32 self-tests to fail on a >> 64-bit PV kernel. The 64-bit versions pass. It gets a seg fault after >> "parent: write to 0x80 (should fail)", and the fault isn't caught by >> the signal handler. It just dumps back to the shell. The tests pass >> after reverting this. > > I can reproduce it if I emulate an AMD machine. I can "fix" it like this: > > diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S > index a8a4f4c460a6..6255e00f425e 100644 > --- a/arch/x86/xen/xen-asm_64.S > +++ b/arch/x86/xen/xen-asm_64.S > @@ -97,6 +97,9 @@ ENDPROC(xen_syscall_target) > ENTRY(xen_syscall32_target) > popq %rcx > popq %r11 > + movq $__USER32_DS, 4*8(%rsp) > + movq $__USER32_CS, 1*8(%rsp) > + movq %r11, 2*8(%rsp) > jmp entry_SYSCALL_compat_after_hwframe > ENDPROC(xen_syscall32_target) > > but I haven't tried to diagnose precisely what's going on. > > Xen seems to be putting the 0xe0?? values in ss and cs, which oughtn't > to be a problem, but it kills opportunistic sysretl. Maybe that's > triggering a preexisting bug? It is indeed triggering an existing but, but that bug is not a kernel bug :) It's this thing: https://sourceware.org/bugzilla/show_bug.cgi?id=21269 See, we have this old legacy garbage in which, when running with nonstandard SS, a certain special, otherwise nonsensical input to sigaction() causes a stack switch. Xen PV runs user code with a nonstandard SS, and glibc accidentally passes this weird parameter to sigaction() on a regular basis. With this patch applied, the kernel suddenly starts to *realize* that ss is weird, and boom. (Or maybe it increases the chance that SS is actually weird, since I'd expect this to trip on #GP, not SYSCALL. But I don't care quite enough to dig further.) Patch coming.