Re: [PATCH v2] entry/kvm: Make vCPU tasks exit to userspace when a livepatch is pending

[Seth Forshee]

On Wed, May 04, 2022 at 03:07:53PM +0200, Petr Mladek wrote:
> On Tue 2022-05-03 12:49:34, Seth Forshee wrote:
> > A task can be livepatched only when it is sleeping or it exits to
> > userspace. This may happen infrequently for a heavily loaded vCPU task,
> > leading to livepatch transition failures.
> 
> This is misleading.
> 
> First, the problem is not a loaded CPU. The problem is that the
> task might spend very long time in the kernel when handling
> some syscall.

It's a fully loaded vCPU, which yes to the host looks like spending a
very long time in the ioctl(KVM_RUN) syscall. I can reword to clarify.

> Second, there is no timeout for the transition in the kernel code.
> It might take very long time but it will not fail.

I suppose the timeout is in kpatch then. I didn't check what implemented
the timeout. I'll remove the statement about timing out.

> > Fake signals will be sent to tasks which fail patching via stack
> > checking. This will cause running vCPU tasks to exit guest mode, but
> > since no signal is pending they return to guest execution without
> > exiting to userspace. Fix this by treating a pending livepatch migration
> > like a pending signal, exiting to userspace with EINTR. This allows the
> > task to be patched, and userspace should re-excecute KVM_RUN to resume
> > guest execution.
> 
> It seems that the patch works as expected but it is far from clear.
> And the above description helps only partially. Let me try to
> explain it for dummies like me ;-)
> 
> <explanation>
> The problem was solved by sending a fake signal, see the commit
> 0b3d52790e1cfd6b80b826 ("livepatch: Remove signal sysfs attribute").
> It was achieved by calling signal_wake_up(). It set TIF_SIGPENDING
> and woke the task. It interrupted the syscall and the task was
> transitioned when leaving to the userspace.
> 
> signal_wake_up() was later replaced by set_notify_signal(),
> see the commit 8df1947c71ee53c7e21 ("livepatch: Replace
> the fake signal sending with TIF_NOTIFY_SIGNAL infrastructure").
> The difference is that set_notify_signal() uses TIF_NOTIFY_SIGNAL
> instead of TIF_SIGPENDING.
> 
> The effect is the same when running on a real hardware. The syscall
> gets interrupted and exit_to_user_mode_loop() is called where
> the livepatch state is updated (task migrated).
> 
> But it works a different way in kvm where the task works are
> called in the guest mode and the task does not return into
> the user space in the host mode.
> </explanation>

Thanks, I can update the commit message to include more of this
background.

> 
> The solution provided by this patch is a bit weird, see below.
> 
> 
> > In my testing, systems where livepatching would timeout after 60 seconds
> > were able to load livepatches within a couple of seconds with this
> > change.
> > 
> > Signed-off-by: Seth Forshee <sforshee@xxxxxxxxxxxxxxxx>
> > ---
> > Changes in v2:
> >  - Added _TIF_SIGPENDING to XFER_TO_GUEST_MODE_WORK
> >  - Reworded commit message and comments to avoid confusion around the
> >    term "migrate"
> > 
> >  include/linux/entry-kvm.h | 4 ++--
> >  kernel/entry/kvm.c        | 7 ++++++-
> >  2 files changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
> > index 6813171afccb..bf79e4cbb5a2 100644
> > --- a/include/linux/entry-kvm.h
> > +++ b/include/linux/entry-kvm.h
> > @@ -17,8 +17,8 @@
> >  #endif
> >  
> >  #define XFER_TO_GUEST_MODE_WORK						\
> > -	(_TIF_NEED_RESCHED | _TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL |	\
> > -	 _TIF_NOTIFY_RESUME | ARCH_XFER_TO_GUEST_MODE_WORK)
> > +	(_TIF_NEED_RESCHED | _TIF_SIGPENDING | _TIF_PATCH_PENDING |	\
> > +	 _TIF_NOTIFY_SIGNAL | _TIF_NOTIFY_RESUME | ARCH_XFER_TO_GUEST_MODE_WORK)
> >  
> >  struct kvm_vcpu;
> >  
> > diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
> > index 9d09f489b60e..98439dfaa1a0 100644
> > --- a/kernel/entry/kvm.c
> > +++ b/kernel/entry/kvm.c
> > @@ -14,7 +14,12 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
> >  				task_work_run();
> >  		}
> >  
> > -		if (ti_work & _TIF_SIGPENDING) {
> > +		/*
> > +		 * When a livepatch is pending, force an exit to userspace
> > +		 * as though a signal is pending to allow the task to be
> > +		 * patched.
> > +		 */
> > +		if (ti_work & (_TIF_SIGPENDING | _TIF_PATCH_PENDING)) {
> >  			kvm_handle_signal_exit(vcpu);
> >  			return -EINTR;
> >  		}
> 
> This looks strange:
> 
>   + klp_send_signals() calls set_notify_signal(task) that sets
>     TIF_NOTIFY_SIGNAL
> 
>   + xfer_to_guest_mode_work() handles TIF_NOTIFY_SIGNAL by calling
>     task_work_run().
> 
>   + This patch calls kvm_handle_signal_exit(vcpu) when
>     _TIF_PATCH_PENDING is set. It probably causes the guest
>     to call exit_to_user_mode_loop() because TIF_PATCH_PENDING
>     bit is set. But neither TIF_NOTIFY_SIGNAL not TIF_NOTIFY_SIGNAL
>     is set so that it works different way than on the real hardware.
> 
> 
> Question:
> 
> Does xfer_to_guest_mode_work() interrupts the syscall running
> on the guest?

xfer_to_guest_mode_work() is called as part of a loop to execute kvm
guests (for example, on x86 see vcpu_run() in arch/x86/kvm/x86.c). When
guest execution is interrupted (in the livepatch case it is interrupted
when set_notify_signal() is called for the vCPU task)
xfer_to_guest_mode_work() is called if there is pending work, and if it
returns non-zero the loop does not immediately re-enter guest execution
but instead returns to userspace.

> If "yes" then we do not need to call kvm_handle_signal_exit(vcpu).
> It will be enough to call:
> 
> 		if (ti_work & _TIF_PATCH_PENDING)
> 			klp_update_patch_state(current);

What if the task's call stack contains a function being patched?

> 
> If "no" then I do not understand why TIF_NOTIFY_SIGNAL interrupts
> the syscall on the real hardware and not in kvm.

It does interrupt, but xfer_to_guest_mode_handle_work() concludes it's
not necessary to return to userspace and resumes guest execution.

Thanks,
Seth

> Anyway, we either should make sure that TIF_NOTIFY_SIGNAL has the same
> effect on the real hardware and in kvm. Or we need another interface
> for the fake signal used by livepatching.
> 
> Adding Jens Axboe and Eric into Cc.
> 
> Best Regards,
> Petr