Add a vhost_task_wake_safe() variant to handle the case where a vhost task has exited due to a signal, i.e. before being explicitly stopped by the owner of the task, and use the "safe" API in KVM when waking NX hugepage recovery tasks. This fixes a bug where KVM will attempt to wake a task that has exited, which ultimately results in all manner of badness, e.g.
Oops: general protection fault, probably for non-canonical address 0xff0e899fa1566052: 0000 [#1] SMP CPU: 51 UID: 0 PID: 53807 Comm: tee Tainted: G S O 6.17.0-smp--38183c31756a-next #826 NONE Tainted: [S]=CPU_OUT_OF_SPEC, [O]=OOT_MODULE Hardware name: Google LLC Indus/Indus_QC_03, BIOS 30.110.0 09/13/2024 RIP: 0010:queued_spin_lock_slowpath+0x123/0x250 Code: ... <48> 89 8c 02 c0 da 47 a2 83 79 08 00 75 08 f3 90 83 79 08 00 74 f8 RSP: 0018:ffffbf55cffe7cf8 EFLAGS: 00010006 RAX: ff0e899fff0e8562 RBX: 0000000000d00000 RCX: ffffa39b40aefac0 RDX: 0000000000000030 RSI: fffffffffffffff8 RDI: ffffa39d0592e68c RBP: 0000000000d00000 R08: 00000000ffffff80 R09: 0000000400000000 R10: ffffa36cce4fe401 R11: 0000000000000800 R12: 0000000000000003 R13: 0000000000000000 R14: ffffa39d0592e68c R15: ffffa39b9e672000 FS: 00007f233b2e9740(0000) GS:ffffa39b9e672000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f233b39fda0 CR3: 00000004d031f002 CR4: 00000000007726f0 PKRU: 55555554 Call Trace: <TASK> _raw_spin_lock_irqsave+0x50/0x60 try_to_wake_up+0x4f/0x5d0 set_nx_huge_pages+0xe4/0x1c0 [kvm] param_attr_store+0x89/0xf0 module_attr_store+0x1e/0x30 kernfs_fop_write_iter+0xe4/0x160 vfs_write+0x2cb/0x420 ksys_write+0x7f/0xf0 do_syscall_64+0x6f/0x1f0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f233b4178b3 R13: 0000000000000002 R14: 00000000226ff3d0 R15: 0000000000000002 </TASK> Provide an API in vhost task instead of forcing KVM to solve the problem, as KVM would literally just add an equivalent to VHOST_TASK_FLAGS_KILLED, along with a new lock to protect said flag. In general, forcing simple usage of vhost task to care about signals _and_ take non-trivial action to do the right thing isn't developer friendly, and is likely to lead to similar bugs in the future. Debugged-by: Sebastian Andrzej Siewior <bige...@linutronix.de> Link: https://lore.kernel.org/all/akkletodxkxaa...@google.com Link: https://lore.kernel.org/all/aj_vep2ehj6l0...@google.com Suggested-by: Sebastian Andrzej Siewior <bige...@linutronix.de> Fixes: d96c77bd4eeb ("KVM: x86: switch hugepage recovery thread to vhost_task") Cc: sta...@vger.kernel.org Signed-off-by: Sean Christopherson <sea...@google.com> --- arch/x86/kvm/mmu/mmu.c | 2 +- include/linux/sched/vhost_task.h | 1 + kernel/vhost_task.c | 42 +++++++++++++++++++++++++++++--- 3 files changed, 41 insertions(+), 4 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 6e838cb6c9e1..d11730467fd4 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -7376,7 +7376,7 @@ static void kvm_wake_nx_recovery_thread(struct kvm *kvm) struct vhost_task *nx_thread = READ_ONCE(kvm->arch.nx_huge_page_recovery_thread); if (nx_thread) - vhost_task_wake(nx_thread); + vhost_task_wake_safe(nx_thread); } static int get_nx_huge_pages(char *buffer, const struct kernel_param *kp) diff --git a/include/linux/sched/vhost_task.h b/include/linux/sched/vhost_task.h index 25446c5d3508..5d5c187088f7 100644 --- a/include/linux/sched/vhost_task.h +++ b/include/linux/sched/vhost_task.h @@ -10,5 +10,6 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *), void vhost_task_start(struct vhost_task *vtsk); void vhost_task_stop(struct vhost_task *vtsk); void vhost_task_wake(struct vhost_task *vtsk); +void vhost_task_wake_safe(struct vhost_task *vtsk); #endif /* _LINUX_SCHED_VHOST_TASK_H */ diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c index bc738fa90c1d..5aa8ddf88d01 100644 --- a/kernel/vhost_task.c +++ b/kernel/vhost_task.c @@ -67,18 +67,54 @@ static int vhost_task_fn(void *data) do_exit(0); } +static void __vhost_task_wake(struct vhost_task *vtsk) +{ + wake_up_process(vtsk->task); +} + /** * vhost_task_wake - wakeup the vhost_task * @vtsk: vhost_task to wake * - * wake up the vhost_task worker thread + * Wake up the vhost_task worker thread. The caller is responsible for ensuring + * that the task hasn't exited. */ void vhost_task_wake(struct vhost_task *vtsk) { - wake_up_process(vtsk->task); + /* + * Checking VHOST_TASK_FLAGS_KILLED can race with signal delivery, but + * a race can only result in false negatives and this is just a sanity + * check, i.e. if KILLED is set, the caller is buggy no matter what. + */ + if (WARN_ON_ONCE(test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags))) + return; + + __vhost_task_wake(vtsk); } EXPORT_SYMBOL_GPL(vhost_task_wake); +/** + * vhost_task_wake_safe - wakeup the vhost_task if it hasn't been killed + * @vtsk: vhost_task to wake + * + * Wake up the vhost_task worker thread if the task hasn't exited, e.g. due to + * a signal. + */ +void vhost_task_wake_safe(struct vhost_task *vtsk) +{ + guard(mutex)(&vtsk->exit_mutex); + + /* Attempting to wake a task that has been explicitly stopped is a bug. */ + if (WARN_ON_ONCE(test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags))) + return; + + if (test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags)) + return; + + __vhost_task_wake(vtsk); +} +EXPORT_SYMBOL_GPL(vhost_task_wake_safe); + /** * vhost_task_stop - stop a vhost_task * @vtsk: vhost_task to stop @@ -91,7 +127,7 @@ void vhost_task_stop(struct vhost_task *vtsk) mutex_lock(&vtsk->exit_mutex); if (!test_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags)) { set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags); - vhost_task_wake(vtsk); + __vhost_task_wake(vtsk); } mutex_unlock(&vtsk->exit_mutex); -- 2.51.0.261.g7ce5a0a67e-goog
