linux-kernel - [PATCH] x86/kvmclock: set offset for kvm unstable clock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20190126174956.22769-1-pasha.tatashin@soleen.com>
Date:   Sat, 26 Jan 2019 12:49:56 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     pbonzini@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pasha.tatashin@...een.com, nuxi@...lt24.org, carnil@...ian.org,
        t.glaser@...ent.de, asmadeus@...ewreck.org
Subject: [PATCH] x86/kvmclock: set offset for kvm unstable clock

VMs may show incorrect uptime and dmesg printk offsets on hypervisors with
unstable clock. The problem is produced when VM is rebooted without exiting
from qemu.

The fix is to calculate clock offset not only for stable clock but for
unstable clock as well, and use kvm_sched_clock_read() which substracts
the offset for both clocks.

This is safe, because pvclock_clocksource_read() does the right thing and
makes sure that clock always goes forward, so once offset is calculated
with unstable clock, we won't get new reads that are smaller than offset,
and thus won't get negative results.

Thank you Jon DeVree for helping to reproduce this issue.

Fixes: 857baa87b642 ("sched/clock: Enable sched clock early")

Reported-by: Dominique Martinet <asmadeus@...ewreck.org>
Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
---
 arch/x86/kernel/kvmclock.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index e811d4d1c824..d908a37bf3f3 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -104,12 +104,8 @@ static u64 kvm_sched_clock_read(void)

 static inline void kvm_sched_clock_init(bool stable)
 {
-	if (!stable) {
-		pv_ops.time.sched_clock = kvm_clock_read;
+	if (!stable)
 		clear_sched_clock_stable();
-		return;
-	}
-
 	kvm_sched_clock_offset = kvm_clock_read();
 	pv_ops.time.sched_clock = kvm_sched_clock_read;

-- 
2.20.1