linux-kernel - [PATCHv1] kvm guest: fix uninitialized kvmclock read by KVM guest

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Date:	Sat, 15 Jun 2013 22:01:45 +0400
From:	Eugene Batalov <ebatalov@...allels.com>
To:	<kvm@...r.kernel.org>
CC:	Eugene Batalov <ebatalov@...allels.com>, <imammedo@...hat.com>,
	<mtosatti@...hat.com>, <linux-kernel@...r.kernel.org>,
	<eabatalov89@...il.com>
Subject: [PATCHv1] kvm guest: fix uninitialized kvmclock read by KVM guest

Due to unintialized kvmclock read KVM guest is hanging on SMP boot stage.
If unintialized memory contains fatal garbage then hang reproduction is 100%.
Unintialized memory is allocated by memblock_alloc. So the garbage values
depend on many many things.

See the detailed description of the bug and possible ways to fix it
in the kernel bug tracker.
"Bug 59521 - KVM linux guest reads uninitialized pvclock values before
executing rdmsr MSR_KVM_WALL_CLOCK"

I decided to fix it simply returning 0ULL from kvm_clock_read when
kvm clocksource is not initialized yet.
The same as kernel bootstrap CPU doesn on boot stage when kernel
clocksources are not initialized yet.

Signed-off-by: Eugene Batalov <ebatalov@...allels.com>
---
I dont' use kernel percpu variables because for each SMP CPU
their contents are copied from the bootstrap CPU. And I don't
think that fixing the value for each SMP CPU is a good style.
If you know a better approach to store the is_pv_clock_ready flags
I am ready to use it.

The patch applies cleanly to
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

I've tested the changes with Ubuntu 13.04 "raring" userspace and
Ubuntu-3.8.0.19-30 kernel tag.

 arch/x86/kernel/kvmclock.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 5bedbdd..a6e0af4 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -43,6 +43,9 @@ early_param("no-kvmclock", parse_no_kvmclock);
 static struct pvclock_vsyscall_time_info *hv_clock;
 static struct pvclock_wall_clock wall_clock;
 
+/* For each cpu store here a flag which tells whether pvclock is initialized */
+static int __cacheline_aligned_in_smp is_pv_clock_ready[NR_CPUS] = {};
+
 /*
  * The wallclock is the time of day when we booted. Since then, some time may
  * have elapsed since the hypervisor wrote the data. So we try to account for
@@ -84,8 +87,11 @@ static cycle_t kvm_clock_read(void)
 
 	preempt_disable_notrace();
 	cpu = smp_processor_id();
-	src = &hv_clock[cpu].pvti;
-	ret = pvclock_clocksource_read(src);
+	if (is_pv_clock_ready[cpu]) {
+		src = &hv_clock[cpu].pvti;
+		ret = pvclock_clocksource_read(src);
+	} else
+		ret = 0ULL;
 	preempt_enable_notrace();
 	return ret;
 }
@@ -168,6 +174,9 @@ int kvm_register_clock(char *txt)
 	printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n",
 	       cpu, high, low, txt);
 
+	if (!ret)
+		is_pv_clock_ready[cpu] = 1;
+
 	return ret;
 }
 
-- 
1.7.9.5

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