From: Seth Forshee sforshee@digitalocean.com
[ 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@xmission.com Cc: Petr Mladek pmladek@suse.com Signed-off-by: Seth Forshee sforshee@digitalocean.com Message-Id: 20220504180840.2907296-1-sforshee@digitalocean.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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; }