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authorDavid Woodhouse <dwmw@amazon.co.uk>2023-01-11 21:06:49 +0300
committerPaolo Bonzini <pbonzini@redhat.com>2023-01-11 21:32:21 +0300
commitbbe17c625d6843e9cdf14d81fbece1b0f0c3fb2f (patch)
treeaba7b08f42d952a673d1ee91a1111bb620e44184 /arch
parent23e60258aeafb04e5dd813f03cb0c8ab7b01462a (diff)
downloadlinux-bbe17c625d6843e9cdf14d81fbece1b0f0c3fb2f.tar.xz
KVM: x86/xen: Fix potential deadlock in kvm_xen_update_runstate_guest()
The kvm_xen_update_runstate_guest() function can be called when the vCPU is being scheduled out, from a preempt notifier. It *opportunistically* updates the runstate area in the guest memory, if the gfn_to_pfn_cache which caches the appropriate address is still valid. If there is *contention* when it attempts to obtain gpc->lock, then locking inside the priority inheritance checks may cause a deadlock. Lockdep reports: [13890.148997] Chain exists of: &gpc->lock --> &p->pi_lock --> &rq->__lock [13890.149002] Possible unsafe locking scenario: [13890.149003] CPU0 CPU1 [13890.149004] ---- ---- [13890.149005] lock(&rq->__lock); [13890.149007] lock(&p->pi_lock); [13890.149009] lock(&rq->__lock); [13890.149011] lock(&gpc->lock); [13890.149013] *** DEADLOCK *** In the general case, if there's contention for a read lock on gpc->lock, that's going to be because something else is either invalidating or revalidating the cache. Either way, we've raced with seeing it in an invalid state, in which case we would have aborted the opportunistic update anyway. So in the 'atomic' case when called from the preempt notifier, just switch to using read_trylock() and avoid the PI handling altogether. Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Message-Id: <20230111180651.14394-2-dwmw2@infradead.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kvm/xen.c19
1 files changed, 17 insertions, 2 deletions
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index bfa9809721b5..651f9c5b873d 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -271,7 +271,15 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
* Attempt to obtain the GPC lock on *both* (if there are two)
* gfn_to_pfn caches that cover the region.
*/
- read_lock_irqsave(&gpc1->lock, flags);
+ if (atomic) {
+ local_irq_save(flags);
+ if (!read_trylock(&gpc1->lock)) {
+ local_irq_restore(flags);
+ return;
+ }
+ } else {
+ read_lock_irqsave(&gpc1->lock, flags);
+ }
while (!kvm_gpc_check(gpc1, user_len1)) {
read_unlock_irqrestore(&gpc1->lock, flags);
@@ -308,7 +316,14 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
* gpc1 lock to make lockdep shut up about it.
*/
lock_set_subclass(&gpc1->lock.dep_map, 1, _THIS_IP_);
- read_lock(&gpc2->lock);
+ if (atomic) {
+ if (!read_trylock(&gpc2->lock)) {
+ read_unlock_irqrestore(&gpc1->lock, flags);
+ return;
+ }
+ } else {
+ read_lock(&gpc2->lock);
+ }
if (!kvm_gpc_check(gpc2, user_len2)) {
read_unlock(&gpc2->lock);