KVM: interrupt kvm_gmem_populate() on signals

kvm_gmem_populate() is a potentially lengthy operation that can involve
multiple calls to the firmware.  Interrupt it if a signal arrives.

Fixes: 1f6c06b1 ("KVM: guest_memfd: Add interface for populating gmem pages with user data")
Cc: Isaku Yamahata <isaku.yamahata@intel.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/guest_memfd.c

@@ -629,6 +629,11 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
 		gfn_t gfn = start_gfn + i;
 		kvm_pfn_t pfn;
 
+		if (signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
+
 		ret = __kvm_gmem_get_pfn(file, slot, gfn, &pfn, &max_order, false);
 		if (ret)
 			break;