linux-kernel - [RFC 04/10] x86/mm: Only flush indirect branches when switching into non dumpable process

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Message-Id: <1516476182-5153-5-git-send-email-karahmed@amazon.de>
Date:   Sat, 20 Jan 2018 20:22:55 +0100
From:   KarimAllah Ahmed <karahmed@...zon.de>
To:     linux-kernel@...r.kernel.org
Cc:     KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        x86@...nel.org
Subject: [RFC 04/10] x86/mm: Only flush indirect branches when switching into non dumpable process

From: Tim Chen <tim.c.chen@...ux.intel.com>

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 <ak@...ux.intel.com>
Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
---
 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