Re: [BUG] kvm: dereference srcu-protected pointer withoutsrcu_read_lock() held

[Paul E. McKenney]

On Tue, Apr 20, 2010 at 02:29:29PM +0800, Lai Jiangshan wrote:
> Marcelo Tosatti wrote:
> > On Mon, Apr 19, 2010 at 01:08:29PM +0300, Avi Kivity wrote:
> >> On 04/19/2010 12:58 PM, Lai Jiangshan wrote:
> >>> Applied the patch I just sent and let CONFIG_PROVE_RCU=y,
> >>> we can got the following dmesg. And we found that it is
> >>> because some codes in KVM dereferences srcu-protected pointer without
> >>> srcu_read_lock() held or update-side lock held.
> >>>
> >>> It is not hard to fix, the problem is that:
> >>> Where is the most proper place to put a srcu_read_lock()?
> >>>
> >>> I can not determine the answer, so I report this bug
> >>> instead of fixing it.
> >>>
> >> I think the else branch in complete_pio() should work.  Marcelo?
> >>
> >> Longer term I'd like to see the lock taken at the high levels
> >> (ioctls, in virt/kvm) and dropped only for guest entry and when we
> >> explicitly sleep (hlt emulation).
> >>
> >> Note: complete_pio() is gone in the current code.
> > 
> > Yes, this was fixed by 7fb2ea1e6.
> > 
> > 
> 
> Applied the patch I sent yesterday and let CONFIG_PROVE_RCU=y
> I can get the following dmesg.
> 
> Under very simple test, these is no complaint from PROVE_RCU
> after this patch applied.
> 
> More test or reviewing of code are need in future.
> 
> ----------
> Subject: [PATCH] kvm: add missing srcu_read_lock()
> 
> I got this dmesg due to srcu_read_lock() is missing in
> kvm_mmu_notifier_release().
> 
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> arch/x86/kvm/x86.h:72 invoked rcu_dereference_check() without protection!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 0
> 2 locks held by qemu-system-x86/3100:
>  #0:  (rcu_read_lock){.+.+..}, at: [<ffffffff810d73dc>] __mmu_notifier_release+0x38/0xdf
>  #1:  (&(&kvm->mmu_lock)->rlock){+.+...}, at: [<ffffffffa0130a6a>] kvm_mmu_zap_all+0x21/0x5e [kvm]
> 
> stack backtrace:
> Pid: 3100, comm: qemu-system-x86 Not tainted 2.6.34-rc3-22949-gbc8a97a-dirty #2
> Call Trace:
>  [<ffffffff8106afd9>] lockdep_rcu_dereference+0xaa/0xb3
>  [<ffffffffa0123a89>] unalias_gfn+0x56/0xab [kvm]
>  [<ffffffffa0119600>] gfn_to_memslot+0x16/0x25 [kvm]
>  [<ffffffffa012ffca>] gfn_to_rmap+0x17/0x6e [kvm]
>  [<ffffffffa01300c1>] rmap_remove+0xa0/0x19d [kvm]
>  [<ffffffffa0130649>] kvm_mmu_zap_page+0x109/0x34d [kvm]
>  [<ffffffffa0130a7e>] kvm_mmu_zap_all+0x35/0x5e [kvm]
>  [<ffffffffa0122870>] kvm_arch_flush_shadow+0x16/0x22 [kvm]
>  [<ffffffffa01189e0>] kvm_mmu_notifier_release+0x15/0x17 [kvm]
>  [<ffffffff810d742c>] __mmu_notifier_release+0x88/0xdf
>  [<ffffffff810d73dc>] ? __mmu_notifier_release+0x38/0xdf
>  [<ffffffff81040848>] ? exit_mm+0xe0/0x115
>  [<ffffffff810c2cb0>] exit_mmap+0x2c/0x17e
>  [<ffffffff8103c472>] mmput+0x2d/0xd4
>  [<ffffffff81040870>] exit_mm+0x108/0x115
> [...]

Queued, thank you, Lai!

							Thanx, Paul

> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> ---
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a5dfea1..a6d639d 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -341,7 +341,11 @@ static void kvm_mmu_notifier_release(struct mmu_notifier *mn,
>  				     struct mm_struct *mm)
>  {
>  	struct kvm *kvm = mmu_notifier_to_kvm(mn);
> +	int idx;
> +
> +	idx = srcu_read_lock(&kvm->srcu);
>  	kvm_arch_flush_shadow(kvm);
> +	srcu_read_unlock(&kvm->srcu, idx);
>  }
> 
>  static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
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