linux-kernel - [PATCH v2 8/8] x86/mm: Only flush indirect branches when switching into non dumpable process

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Date:   Sun, 21 Jan 2018 09:49:09 +0000
From:   David Woodhouse <dwmw@...zon.co.uk>
To:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, bp@...en8.de, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org
Subject: [PATCH v2 8/8] x86/mm: Only flush indirect branches when switching into non dumpable process

From: Andi Kleen <ak@...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