[xen master] x86/mem_sharing: block interrupt injection for forks

[patchbot@xxxxxxx]

commit 3371ced37ced359167b5a71abee2062854371323
Author:     Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
AuthorDate: Tue Jun 16 10:29:16 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jun 16 10:29:16 2020 +0200

    x86/mem_sharing: block interrupt injection for forks
    
    When running VM forks without device models (QEMU), it may
    be undesirable for Xen to inject interrupts. When creating such forks from
    Windows VMs we have observed the kernel trying to process interrupts
    immediately after the fork is executed. However without QEMU running such
    interrupt handling may not be possible because it may attempt to interact 
with
    devices that are not emulated by a backend. In the best case scenario such
    interrupt handling would only present a detour in the VM forks' execution
    flow, but in the worst case as we actually observed can completely stall it.
    By disabling interrupt injection a fuzzer can exercise the target code 
without
    interference. For other use-cases this option probably doesn't make sense,
    that's why this is not enabled by default.
    
    Forks & memory sharing are only available on Intel CPUs so this only applies
    to vmx. Note that this is part of the experimental VM forking feature that's
    completely disabled by default and can only be enabled by using
    XEN_CONFIG_EXPERT during compile time.
    
    Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Wei Liu <wl@xxxxxxx>
    Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
    Release-acked-by: Paul Durrant <paul@xxxxxxx>
---
 xen/arch/x86/hvm/vmx/intr.c      | 6 ++++++
 xen/arch/x86/mm/mem_sharing.c    | 6 +++++-
 xen/include/asm-x86/hvm/domain.h | 2 +-
 xen/include/public/memory.h      | 3 +++
 4 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/intr.c b/xen/arch/x86/hvm/vmx/intr.c
index 000e14af49..80bfbb4787 100644
--- a/xen/arch/x86/hvm/vmx/intr.c
+++ b/xen/arch/x86/hvm/vmx/intr.c
@@ -256,6 +256,12 @@ void vmx_intr_assist(void)
     if ( unlikely(v->arch.vm_event) && v->arch.vm_event->sync_event )
         return;
 
+#ifdef CONFIG_MEM_SHARING
+    /* Block event injection for VM fork if requested */
+    if ( unlikely(v->domain->arch.hvm.mem_sharing.block_interrupts) )
+        return;
+#endif
+
     /* 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 19922ab5d1..c428fd16ce 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_BLOCK_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_BLOCK_INTERRUPTS) )
+            d->arch.hvm.mem_sharing.block_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..9d247baf4d 100644
--- a/xen/include/asm-x86/hvm/domain.h
+++ b/xen/include/asm-x86/hvm/domain.h
@@ -67,7 +67,7 @@ struct hvm_ioreq_server {
 #ifdef CONFIG_MEM_SHARING
 struct mem_sharing_domain
 {
-    bool enabled;
+    bool enabled, block_interrupts;
 
     /*
      * When releasing shared gfn's in a preemptible manner, recall where
diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
index dbd35305df..850bd72c52 100644
--- a/xen/include/public/memory.h
+++ b/xen/include/public/memory.h
@@ -536,7 +536,10 @@ struct xen_mem_sharing_op {
         } debug;
         struct mem_sharing_op_fork {      /* OP_FORK */
             domid_t parent_domain;        /* IN: parent's domain id */
+/* Only makes sense for short-lived forks */
 #define XENMEM_FORK_WITH_IOMMU_ALLOWED (1u << 0)
+/* Only makes sense for short-lived forks */
+#define XENMEM_FORK_BLOCK_INTERRUPTS   (1u << 1)
             uint16_t flags;               /* IN: optional settings */
             uint32_t pad;                 /* Must be set to 0 */
         } fork;
--
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