From: Seth Forshee <sforshee@xxxxxxxxxxxxxxxx>
[ Upstream commit 3e684903a8574ffc9475fdf13c4780a7adb506ad ]
A livepatch transition may stall indefinitely when a kvm vCPU is heavily
loaded. To the host, the vCPU task is a user thread which is spending a
very long time in the ioctl(KVM_RUN) syscall. During livepatch
transition, set_notify_signal() will be called on such tasks to
interrupt the syscall so that the task can be transitioned. This
interrupts guest execution, but when xfer_to_guest_mode_work() sees that
TIF_NOTIFY_SIGNAL is set but not TIF_SIGPENDING it concludes that an
exit to user mode is unnecessary, and guest execution is resumed without
transitioning the task for the livepatch.
This handling of TIF_NOTIFY_SIGNAL is incorrect, as set_notify_signal()
is expected to break tasks out of interruptible kernel loops and cause
them to return to userspace. Change xfer_to_guest_mode_work() to handle
TIF_NOTIFY_SIGNAL the same as TIF_SIGPENDING, signaling to the vCPU run
loop that an exit to userpsace is needed. Any pending task_work will be
run when get_signal() is called from exit_to_user_mode_loop(), so there
is no longer any need to run task work from xfer_to_guest_mode_work().
Suggested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Seth Forshee <sforshee@xxxxxxxxxxxxxxxx>
Message-Id: <20220504180840.2907296-1-sforshee@xxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/entry/kvm.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -8,10 +8,7 @@ static int xfer_to_guest_mode_work(struc
do {
int ret;
- if (ti_work & _TIF_NOTIFY_SIGNAL)
- tracehook_notify_signal();
-
- if (ti_work & _TIF_SIGPENDING) {
+ if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) {
kvm_handle_signal_exit(vcpu);
return -EINTR;
}
