[Xen-devel] [PATCH for-4.12] x86/altp2m: fix HVMOP_altp2m_set_domain_state race

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

HVMOP_altp2m_set_domain_state does not domain_pause(), presumably
on purpose (as it was originally supposed to cater to a in-guest
agent, and a domain pausing itself is not a good idea).

This can lead to domain crashes in the vmx_vmexit_handler() code
that checks if the guest has the ability to switch EPTP without an
exit. That code can __vmread() the host p2m's EPT_POINTER
(before HVMOP_altp2m_set_domain_state "for_each_vcpu()" has a
chance to run altp2m_vcpu_initialise(), but after
d->arch.altp2m_active is set).

While the in-guest scenario continues to pose problems, this
patch fixes the "external" case.

Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
---
 xen/arch/x86/hvm/hvm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 21944e9..33038dc 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4533,6 +4533,9 @@ static int do_altp2m_op(
             break;
         }
 
+        if ( d != current->domain )
+            domain_pause(d);
+
         ostate = d->arch.altp2m_active;
         d->arch.altp2m_active = !!a.u.domain_state.state;
 
@@ -4551,6 +4554,10 @@ static int do_altp2m_op(
             if ( ostate )
                 p2m_flush_altp2m(d);
         }
+
+        if ( d != current->domain )
+            domain_unpause(d);
+
         break;
     }
 
-- 
2.7.4


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