From mboxrd@z Thu Jan 1 00:00:00 1970 From: KarimAllah Ahmed Subject: [RFC 04/10] x86/mm: Only flush indirect branches when switching into non dumpable process Date: Sat, 20 Jan 2018 20:22:55 +0100 Message-ID: <1516476182-5153-5-git-send-email-karahmed@amazon.de> References: <1516476182-5153-1-git-send-email-karahmed@amazon.de> Cc: KarimAllah Ahmed , Andi Kleen , Andrea Arcangeli , Andy Lutomirski , Arjan van de Ven , Ashok Raj , Asit Mallick , Borislav Petkov , Dan Williams , Dave Hansen , David Woodhouse , Greg Kroah-Hartman , "H . Peter Anvin" , Ingo Molnar , Janakarajan Natarajan , Joerg Roedel , Jun Nakajima , Laura Abbott , Linus Torvalds , Masami Hiramatsu Received: from smtp-fw-9102.amazon.com ([207.171.184.29]:1106 "EHLO smtp-fw-9102.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932246AbeATTXj (ORCPT ); Sat, 20 Jan 2018 14:23:39 -0500 In-Reply-To: <1516476182-5153-1-git-send-email-karahmed@amazon.de> Sender: kvm-owner@vger.kernel.org List-ID: From: Tim Chen Flush indirect branches when switching into a process that marked itself non dumpable. This protects high value processes like gpg better, without having too high performance overhead. Signed-off-by: Andi Kleen Signed-off-by: David Woodhouse Signed-off-by: KarimAllah Ahmed --- arch/x86/mm/tlb.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 304de7d..f64e80c 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -225,8 +225,19 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, * Avoid user/user BTB poisoning by flushing the branch predictor * when switching between processes. This stops one process from * doing Spectre-v2 attacks on another. + * + * As an optimization: Flush indirect branches only when + * switching into processes that disable dumping. + * + * This will not flush when switching into kernel threads. + * But it would flush when switching into idle and back + * + * It might be useful to have a one-off cache here + * to also not flush the idle case, but we would need some + * kind of stable sequence number to remember the previous mm. */ - indirect_branch_prediction_barrier(); + if (tsk && tsk->mm && get_dumpable(tsk->mm) != SUID_DUMP_USER) + indirect_branch_prediction_barrier(); if (IS_ENABLED(CONFIG_VMAP_STACK)) { /* -- 2.7.4