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    Subject[PATCH 4.19 25/34] KVM: SVM: Periodically schedule when unregistering regions on destroy
    Date
    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/kvm/svm.c | 1 +
    1 file changed, 1 insertion(+)

    --- a/arch/x86/kvm/svm.c
    +++ b/arch/x86/kvm/svm.c
    @@ -1954,6 +1954,7 @@ static void sev_vm_destroy(struct kvm *k
    list_for_each_safe(pos, q, head) {
    __unregister_enc_region_locked(kvm,
    list_entry(pos, struct enc_region, list));
    + cond_resched();
    }
    }


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