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(); } }