UPSTREAM: entry/kvm: Exit to user mode when TIF_NOTIFY_SIGNAL is set

[ 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" <[email protected]>
Cc: Petr Mladek <[email protected]>
Change-Id: If14e86a516403671ccb122cea32cc704f774e8ce
Signed-off-by: Seth Forshee <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
(cherry picked from commit 000de389ad7b8094bcf714ee27e2362eb5054a1a)
Bug: 268174392
Signed-off-by: Greg Kroah-Hartman <[email protected]>

kernel/entry/kvm.c

@@ -8,10 +8,7 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
 	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;
 		}