From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757579AbcHCK2l (ORCPT ); Wed, 3 Aug 2016 06:28:41 -0400 Received: from mail.skyhub.de ([78.46.96.112]:37290 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755293AbcHCK2i (ORCPT ); Wed, 3 Aug 2016 06:28:38 -0400 From: Borislav Petkov To: X86 ML Cc: LKML , Andy Lutomirski Subject: [PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET Date: Wed, 3 Aug 2016 12:19:24 +0200 Message-Id: <20160803101924.27247-1-bp@alien8.de> X-Mailer: git-send-email 2.8.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Borislav Petkov Clarify why exactly RF cannot be restored properly by SYSRET to avoid confusion. No functionality change. Signed-off-by: Borislav Petkov Cc: Andy Lutomirski --- arch/x86/entry/entry_64.S | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 8956eae04c25..80ad6d0fe38b 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -288,11 +288,15 @@ return_from_SYSCALL_64: jne opportunistic_sysret_failed /* - * SYSRET can't restore RF. SYSRET can restore TF, but unlike IRET, - * restoring TF results in a trap from userspace immediately after - * SYSRET. This would cause an infinite loop whenever #DB happens - * with register state that satisfies the opportunistic SYSRET - * conditions. For example, single-stepping this user code: + * SYSCALL clears RF when it saves rFLAGS in R11 so SYSRET cannot + * restore RF properly. If the slowpath sets it for whatever reason, we + * need to restore it correctly. + * + * SYSRET can restore TF, but unlike IRET, restoring TF results in a + * trap from userspace immediately after SYSRET. This would cause an + * infinite loop whenever #DB happens with register state that satisfies + * the opportunistic SYSRET conditions. For example, single-stepping + * this user code: * * movq $stuck_here, %rcx * pushfq -- 2.8.4