Re: [PATCH] kvm/x86: Handle async PF in RCU read-side critical sections

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/09/2017 13:01, Boqun Feng wrote:
> Sasha Levin reported a WARNING:
> 
> | WARNING: CPU: 0 PID: 6974 at kernel/rcu/tree_plugin.h:329
> | rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline]
> | WARNING: CPU: 0 PID: 6974 at kernel/rcu/tree_plugin.h:329
> | rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458
> ...
> | CPU: 0 PID: 6974 Comm: syz-fuzzer Not tainted 4.13.0-next-20170908+ #246
> | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> | 1.10.1-1ubuntu1 04/01/2014
> | Call Trace:
> ...
> | RIP: 0010:rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline]
> | RIP: 0010:rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458
> | RSP: 0018:ffff88003b2debc8 EFLAGS: 00010002
> | RAX: 0000000000000001 RBX: 1ffff1000765bd85 RCX: 0000000000000000
> | RDX: 1ffff100075d7882 RSI: ffffffffb5c7da20 RDI: ffff88003aebc410
> | RBP: ffff88003b2def30 R08: dffffc0000000000 R09: 0000000000000001
> | R10: 0000000000000000 R11: 0000000000000000 R12: ffff88003b2def08
> | R13: 0000000000000000 R14: ffff88003aebc040 R15: ffff88003aebc040
> | __schedule+0x201/0x2240 kernel/sched/core.c:3292
> | schedule+0x113/0x460 kernel/sched/core.c:3421
> | kvm_async_pf_task_wait+0x43f/0x940 arch/x86/kernel/kvm.c:158
> | do_async_page_fault+0x72/0x90 arch/x86/kernel/kvm.c:271
> | async_page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1069
> | RIP: 0010:format_decode+0x240/0x830 lib/vsprintf.c:1996
> | RSP: 0018:ffff88003b2df520 EFLAGS: 00010283
> | RAX: 000000000000003f RBX: ffffffffb5d1e141 RCX: ffff88003b2df670
> | RDX: 0000000000000001 RSI: dffffc0000000000 RDI: ffffffffb5d1e140
> | RBP: ffff88003b2df560 R08: dffffc0000000000 R09: 0000000000000000
> | R10: ffff88003b2df718 R11: 0000000000000000 R12: ffff88003b2df5d8
> | R13: 0000000000000064 R14: ffffffffb5d1e140 R15: 0000000000000000
> | vsnprintf+0x173/0x1700 lib/vsprintf.c:2136
> | sprintf+0xbe/0xf0 lib/vsprintf.c:2386
> | proc_self_get_link+0xfb/0x1c0 fs/proc/self.c:23
> | get_link fs/namei.c:1047 [inline]
> | link_path_walk+0x1041/0x1490 fs/namei.c:2127
> ...
> 
> And this happened when we hit a page fault in an RCU read-side critical
> section and then we tried to reschedule in kvm_async_pf_task_wait(),
> this reschedule would hit the WARN in rcu_preempt_note_context_switch(),
> and be treated as a sleep in RCU read-side critical section, which is
> not allowed(even in preemptible RCU).

Just a small fix to the commit message:

This happened when the host hit a page fault, and delivered it as in an
async page fault, while the guest was in an RCU read-side critical
section.  The guest then tries to reschedule in kvm_async_pf_task_wait(),
but rcu_preempt_note_context_switch() would treat the reschedule as a
sleep in RCU read-side critical section, which is not allowed (even in
preemptible RCU).  Thus the WARN.

Queued with that change, thanks.

Paolo

> To cure this, make kvm_async_pf_task_wait() go to the halt path if the
> PF happens in a RCU read-side critical section.
> 
> Reported-by: Sasha Levin <levinsasha928@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  arch/x86/kernel/kvm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index aa60a08b65b1..e675704fa6f7 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -140,7 +140,8 @@ void kvm_async_pf_task_wait(u32 token)
>  
>  	n.token = token;
>  	n.cpu = smp_processor_id();
> -	n.halted = is_idle_task(current) || preempt_count() > 1;
> +	n.halted = is_idle_task(current) || preempt_count() > 1 ||
> +		   rcu_preempt_depth();
>  	init_swait_queue_head(&n.wq);
>  	hlist_add_head(&n.link, &b->list);
>  	raw_spin_unlock(&b->lock);
>