Re: [PATCH] KVM: x86/mmu: Fix an inverted list_empty() check when zapping sptes

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: "Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Fix an inverted list_empty() check when zapping sptes
Date: Mon, 15 Apr 2019 07:44:10 -0700	[thread overview]
Message-ID: <20190415144410.GA24010@linux.intel.com> (raw)
In-Reply-To: <20190413025541.20303-1-sean.j.christopherson@intel.com>

On Fri, Apr 12, 2019 at 07:55:41PM -0700, Sean Christopherson wrote:
> A recently introduced helper for handling zap vs. remote flush
> incorrectly bails early, effectively leaking defunct shadow pages.
> Manifests as a slab BUG when exiting KVM due to the shadow pages
> being alive when their associated cache is destroyed.
> 
> ==========================================================================
> BUG kvm_mmu_page_header: Objects remaining in kvm_mmu_page_header on ...
> --------------------------------------------------------------------------
> Disabling lock debugging due to kernel taint
> INFO: Slab 0x00000000fc436387 objects=26 used=23 fp=0x00000000d023caee ...
> CPU: 6 PID: 4315 Comm: rmmod Tainted: G    B             5.1.0-rc2+ #19
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> Call Trace:
>  dump_stack+0x46/0x5b
>  slab_err+0xad/0xd0
>  ? on_each_cpu_mask+0x3c/0x50
>  ? ksm_migrate_page+0x60/0x60
>  ? on_each_cpu_cond_mask+0x7c/0xa0
>  ? __kmalloc+0x1ca/0x1e0
>  __kmem_cache_shutdown+0x13a/0x310
>  shutdown_cache+0xf/0x130
>  kmem_cache_destroy+0x1d5/0x200
>  kvm_mmu_module_exit+0xa/0x30 [kvm]
>  kvm_arch_exit+0x45/0x60 [kvm]
>  kvm_exit+0x6f/0x80 [kvm]
>  vmx_exit+0x1a/0x50 [kvm_intel]
>  __x64_sys_delete_module+0x153/0x1f0
>  ? exit_to_usermode_loop+0x88/0xc0
>  do_syscall_64+0x4f/0x100
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Fixes: a21136345cb6f ("KVM: x86/mmu: Split remote_flush+zap case out of kvm_mmu_flush_or_zap()")
> Cc: stable@vger.kernel.org

Stable doesn't need to be Cc'd.  I got my kernel versions mixed up, this
bug only exists in 5.1.

> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/x86/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index bd0727019674..e10962dfc203 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2238,7 +2238,7 @@ static bool kvm_mmu_remote_flush_or_zap(struct kvm *kvm,
>  					struct list_head *invalid_list,
>  					bool remote_flush)
>  {
> -	if (!remote_flush && !list_empty(invalid_list))
> +	if (!remote_flush && list_empty(invalid_list))
>  		return false;
>  
>  	if (!list_empty(invalid_list))
> -- 
> 2.21.0
> 