[PATCH for-4.14 1/2] x86/mem_sharing: Prohibit interrupt injection for forks

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>]

When running shallow forks without device models it may be undesirable for Xen
to inject interrupts. With Windows forks we have observed the kernel going into
infinite loops when trying to process such interrupts. By disabling interrupt
injection the fuzzer can exercise the target code without interference.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
---
 xen/arch/x86/hvm/vmx/intr.c      | 4 ++++
 xen/arch/x86/mm/mem_sharing.c    | 6 +++++-
 xen/include/asm-x86/hvm/domain.h | 2 ++
 xen/include/public/memory.h      | 1 +
 4 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/vmx/intr.c b/xen/arch/x86/hvm/vmx/intr.c
index 000e14af49..3814795e3f 100644
--- a/xen/arch/x86/hvm/vmx/intr.c
+++ b/xen/arch/x86/hvm/vmx/intr.c
@@ -256,6 +256,10 @@ void vmx_intr_assist(void)
     if ( unlikely(v->arch.vm_event) && v->arch.vm_event->sync_event )
         return;
 
+    /* Block event injection for VM fork if requested */
+    if ( unlikely(v->domain->arch.hvm.mem_sharing.prohibit_interrupts) )
+        return;
+
     /* Crank the handle on interrupt state. */
     pt_vector = pt_update_irq(v);
 
diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 7271e5c90b..7352fce866 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -2106,7 +2106,8 @@ int 
mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
         rc = -EINVAL;
         if ( mso.u.fork.pad )
             goto out;
-        if ( mso.u.fork.flags & ~XENMEM_FORK_WITH_IOMMU_ALLOWED )
+        if ( mso.u.fork.flags & ~(XENMEM_FORK_WITH_IOMMU_ALLOWED |
+                                  XENMEM_FORK_PROHIBIT_INTERRUPTS) )
             goto out;
 
         rc = rcu_lock_live_remote_domain_by_id(mso.u.fork.parent_domain,
@@ -2134,6 +2135,9 @@ int 
mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
             rc = hypercall_create_continuation(__HYPERVISOR_memory_op,
                                                "lh", XENMEM_sharing_op,
                                                arg);
+        else if ( !rc && (mso.u.fork.flags & XENMEM_FORK_PROHIBIT_INTERRUPTS) )
+            d->arch.hvm.mem_sharing.prohibit_interrupts = true;
+
         rcu_unlock_domain(pd);
         break;
     }
diff --git a/xen/include/asm-x86/hvm/domain.h b/xen/include/asm-x86/hvm/domain.h
index 95fe18cddc..e114f818d3 100644
--- a/xen/include/asm-x86/hvm/domain.h
+++ b/xen/include/asm-x86/hvm/domain.h
@@ -74,6 +74,8 @@ struct mem_sharing_domain
      * to resume the search.
      */
     unsigned long next_shared_gfn_to_relinquish;
+
+    bool prohibit_interrupts;
 };
 #endif
 
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index dbd35305df..fe2e6caa68 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -537,6 +537,7 @@ struct xen_mem_sharing_op {
         struct mem_sharing_op_fork {      /* OP_FORK */
             domid_t parent_domain;        /* IN: parent's domain id */
 #define XENMEM_FORK_WITH_IOMMU_ALLOWED (1u << 0)
+#define XENMEM_FORK_PROHIBIT_INTERRUPTS (1u << 1)
             uint16_t flags;               /* IN: optional settings */
             uint32_t pad;                 /* Must be set to 0 */
         } fork;
-- 
2.25.1