linux-kernel - Re: [PATCH MANUALSEL 5.18 5/6] entry/kvm: Exit to user mode when TIF_NOTIFY

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Message-ID: <87edzrzwoi.fsf@email.froward.int.ebiederm.org>
Date:   Mon, 13 Jun 2022 23:38:37 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Seth Forshee <sforshee@...italocean.com>,
        Petr Mladek <pmladek@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tglx@...utronix.de,
        peterz@...radead.org, luto@...nel.org
Subject: Re: [PATCH MANUALSEL 5.18 5/6] entry/kvm: Exit to user mode when
 TIF_NOTIFY_SIGNAL is set


Starting with v5.18 TIF_NOTIFY_SIGNAL and task_work_run are separated,
so backporting this to v5.18 looks safe.

For anyone considering backporting this any farther please be aware
that TIF_NOTIFY_SIGNAL and task_work were all mixed together and
to backport this also likely requires backporting my cleanups that
separated them.

Eric


Sasha Levin <sashal@...nel.org> writes:

> From: Seth Forshee <sforshee@...italocean.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@...ssion.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Seth Forshee <sforshee@...italocean.com>
> Message-Id: <20220504180840.2907296-1-sforshee@...italocean.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  kernel/entry/kvm.c | 6 ------
>  1 file changed, 6 deletions(-)
>
> diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
> index 9d09f489b60e..2e0f75bcb7fd 100644
> --- a/kernel/entry/kvm.c
> +++ b/kernel/entry/kvm.c
> @@ -9,12 +9,6 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
>  		int ret;
>  
>  		if (ti_work & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL)) {
> -			clear_notify_signal();
> -			if (task_work_pending(current))
> -				task_work_run();
> -		}
> -
> -		if (ti_work & _TIF_SIGPENDING) {
>  			kvm_handle_signal_exit(vcpu);
>  			return -EINTR;
>  		}