From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752229AbdHOFgX (ORCPT ); Tue, 15 Aug 2017 01:36:23 -0400 Received: from mail.kernel.org ([198.145.29.99]:33614 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751652AbdHOFgW (ORCPT ); Tue, 15 Aug 2017 01:36:22 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BF73522DA6 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 From: Andy Lutomirski To: x86@kernel.org Cc: linux-kernel@vger.kernel.org, Borislav Petkov , Brian Gerst , Andrew Cooper , Juergen Gross , "xen-devel@lists.xenproject.org" , Boris Ostrovsky , Andy Lutomirski Subject: [PATCH] x86/xen/64: Fix the reported SS and CS in SYSCALL Date: Mon, 14 Aug 2017 22:36:19 -0700 Message-Id: X-Mailer: git-send-email 2.13.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When I cleaned up the Xen SYSCALL entries, I inadvertently changed the reported segment registers. Before my patch, regs->ss was __USER(32)_DS and regs->cs was __USER(32)_CS. After the patch, they are FLAT_USER_CS/DS(32). This had a couple unfortunate effects. It confused the opportunistic fast return logic. It also significantly increased the risk of triggering a nasty glibc bug: https://sourceware.org/bugzilla/show_bug.cgi?id=21269 Update the Xen entry code to change it back. Reported-by: Brian Gerst Fixes: 8a9949bc71a7 ("x86/xen/64: Rearrange the SYSCALL entries") Signed-off-by: Andy Lutomirski --- arch/x86/xen/xen-asm_64.S | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S index a8a4f4c460a6..c5fee2680abc 100644 --- a/arch/x86/xen/xen-asm_64.S +++ b/arch/x86/xen/xen-asm_64.S @@ -88,6 +88,15 @@ RELOC(xen_sysret64, 1b+1) ENTRY(xen_syscall_target) popq %rcx popq %r11 + + /* + * Neither Xen nor the kernel really knows what the old SS and + * CS were. The kernel expects __USER_DS and __USER_CS, so + * report those values even though Xen will guess its own values. + */ + movq $__USER_DS, 4*8(%rsp) + movq $__USER_CS, 1*8(%rsp) + jmp entry_SYSCALL_64_after_hwframe ENDPROC(xen_syscall_target) @@ -97,6 +106,15 @@ ENDPROC(xen_syscall_target) ENTRY(xen_syscall32_target) popq %rcx popq %r11 + + /* + * Neither Xen nor the kernel really knows what the old SS and + * CS were. The kernel expects __USER32_DS and __USER32_CS, so + * report those values even though Xen will guess its own values. + */ + movq $__USER32_DS, 4*8(%rsp) + movq $__USER32_CS, 1*8(%rsp) + jmp entry_SYSCALL_compat_after_hwframe ENDPROC(xen_syscall32_target) -- 2.13.3