[PATCH for 4.19, 5.4] KVM: SVM: Periodically schedule when unregistering regions on destroy

[PATCH for 4.19, 5.4] KVM: SVM: Periodically schedule when unregistering regions on destroy

[Nobuhiro Iwamatsu]

From: David Rientjes <rientjes@google.com>

commit 7be74942f184fdfba34ddd19a0d995deb34d4a03 upstream.

There may be many encrypted regions that need to be unregistered when a
SEV VM is destroyed.  This can lead to soft lockups.  For example, on a
host running 4.15:

watchdog: BUG: soft lockup - CPU#206 stuck for 11s! [t_virtual_machi:194348]
CPU: 206 PID: 194348 Comm: t_virtual_machi
RIP: 0010:free_unref_page_list+0x105/0x170
...
Call Trace:
 [<0>] release_pages+0x159/0x3d0
 [<0>] sev_unpin_memory+0x2c/0x50 [kvm_amd]
 [<0>] __unregister_enc_region_locked+0x2f/0x70 [kvm_amd]
 [<0>] svm_vm_destroy+0xa9/0x200 [kvm_amd]
 [<0>] kvm_arch_destroy_vm+0x47/0x200
 [<0>] kvm_put_kvm+0x1a8/0x2f0
 [<0>] kvm_vm_release+0x25/0x30
 [<0>] do_exit+0x335/0xc10
 [<0>] do_group_exit+0x3f/0xa0
 [<0>] get_signal+0x1bc/0x670
 [<0>] do_signal+0x31/0x130

Although the CLFLUSH is no longer issued on every encrypted region to be
unregistered, there are no other changes that can prevent soft lockups for
very large SEV VMs in the latest kernel.

Periodically schedule if necessary.  This still holds kvm->lock across the
resched, but since this only happens when the VM is destroyed this is
assumed to be acceptable.

Signed-off-by: David Rientjes <rientjes@google.com>
Message-Id: <alpine.DEB.2.23.453.2008251255240.2987727@chino.kir.corp.google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[iwamatsu: adjust filename.]
Reference: CVE-2020-36311
Signed-off-by: Nobuhiro Iwamatsu (CIP) <nobuhiro1.iwamatsu@toshiba.co.jp>
---
 arch/x86/kvm/svm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index c5673bda4b66df..3f776e654e3aec 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1910,6 +1910,7 @@ static void sev_vm_destroy(struct kvm *kvm)
 		list_for_each_safe(pos, q, head) {
 			__unregister_enc_region_locked(kvm,
 				list_entry(pos, struct enc_region, list));
+			cond_resched();
 		}
 	}
 
-- 
2.31.1

Re: [PATCH for 4.19, 5.4] KVM: SVM: Periodically schedule when unregistering regions on destroy

[Paolo Bonzini]

On 08/07/21 07:02, Nobuhiro Iwamatsu wrote:
> From: David Rientjes <rientjes@google.com>
> 
> commit 7be74942f184fdfba34ddd19a0d995deb34d4a03 upstream.

Part of 5.9.

> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index c5673bda4b66df..3f776e654e3aec 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1910,6 +1910,7 @@ static void sev_vm_destroy(struct kvm *kvm)
>   		list_for_each_safe(pos, q, head) {
>   			__unregister_enc_region_locked(kvm,
>   				list_entry(pos, struct enc_region, list));
> +			cond_resched();
>   		}
>   	}
>   
> 

Patch is the same as the upstream commit, except for the name of the 
file.  Thanks!

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo

Re: [PATCH for 4.19, 5.4] KVM: SVM: Periodically schedule when unregistering regions on destroy

[Greg KH]

On Thu, Jul 08, 2021 at 11:17:33AM +0200, Paolo Bonzini wrote:
> On 08/07/21 07:02, Nobuhiro Iwamatsu wrote:
> > From: David Rientjes <rientjes@google.com>
> > 
> > commit 7be74942f184fdfba34ddd19a0d995deb34d4a03 upstream.
> 
> Part of 5.9.
> 
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index c5673bda4b66df..3f776e654e3aec 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -1910,6 +1910,7 @@ static void sev_vm_destroy(struct kvm *kvm)
> >   		list_for_each_safe(pos, q, head) {
> >   			__unregister_enc_region_locked(kvm,
> >   				list_entry(pos, struct enc_region, list));
> > +			cond_resched();
> >   		}
> >   	}
> > 
> 
> Patch is the same as the upstream commit, except for the name of the file.
> Thanks!
> 
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Now queued up, thanks.

greg k-h