[3.19.y-ckt stable] Patch "x86: vdso: fix pvclock races with task migration" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    x86: vdso: fix pvclock races with task migration

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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>From 3517103bc1d574c8e8449439d6571b8be677dac3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar at redhat.com>
Date: Thu, 2 Apr 2015 20:44:23 +0200
Subject: x86: vdso: fix pvclock races with task migration
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 80f7fdb1c7f0f9266421f823964fd1962681f6ce upstream.

If we were migrated right after __getcpu, but before reading the
migration_count, we wouldn't notice that we read TSC of a different
VCPU, nor that KVM's bug made pvti invalid, as only migration_count
on source VCPU is increased.

Change vdso instead of updating migration_count on destination.

Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
Fixes: 0a4e6be9ca17 ("x86: kvm: Revert "remove sched notifier for cross-cpu migrations"")
Message-Id: <1428000263-11892-1-git-send-email-rkrcmar at redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/vdso/vclock_gettime.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 3093376..40d2473 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -99,21 +99,25 @@ static notrace cycle_t vread_pvclock(int *mode)
 		 * __getcpu() calls (Gleb).
 		 */

-		pvti = get_pvti(cpu);
+		/* Make sure migrate_count will change if we leave the VCPU. */
+		do {
+			pvti = get_pvti(cpu);
+			migrate_count = pvti->migrate_count;

-		migrate_count = pvti->migrate_count;
+			cpu1 = cpu;
+			cpu = __getcpu() & VGETCPU_CPU_MASK;
+		} while (unlikely(cpu != cpu1));

 		version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);

 		/*
 		 * Test we're still on the cpu as well as the version.
-		 * We could have been migrated just after the first
-		 * vgetcpu but before fetching the version, so we
-		 * wouldn't notice a version change.
+		 * - We must read TSC of pvti's VCPU.
+		 * - KVM doesn't follow the versioning protocol, so data could
+		 *   change before version if we left the VCPU.
 		 */
-		cpu1 = __getcpu() & VGETCPU_CPU_MASK;
-	} while (unlikely(cpu != cpu1 ||
-			  (pvti->pvti.version & 1) ||
+		smp_rmb();
+	} while (unlikely((pvti->pvti.version & 1) ||
 			  pvti->pvti.version != version ||
 			  pvti->migrate_count != migrate_count));

--
1.9.1