Re: [PATCH v2 1/1] KVM: x86/xen: add support for 32-bit guests in SCHEDOP_poll

["Kaya, Metin" <metikaya@amazon.com>]

From: Metin Kaya <metikaya@amazon.com>

This patch introduces compat version of struct sched_poll for
SCHEDOP_poll sub-operation of sched_op hypercall, reads correct amount
of data (16 bytes in 32-bit case, 24 bytes otherwise) by using new
compat_sched_poll struct, copies it to sched_poll properly, and lets
rest of the code run as is.

Signed-off-by: Metin Kaya <metikaya@amazon.com>
---
 arch/x86/kvm/xen.c | 31 +++++++++++++++++++++++++++----
 arch/x86/kvm/xen.h |  7 +++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 7d01983d1087..2d0a5d2ca6f1 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -998,20 +998,43 @@ static bool kvm_xen_schedop_poll(struct kvm_vcpu *vcpu, bool longmode,
 	evtchn_port_t port, *ports;
 	gpa_t gpa;
 
-	if (!longmode || !lapic_in_kernel(vcpu) ||
+	if (!lapic_in_kernel(vcpu) ||
 	    !(vcpu->kvm->arch.xen_hvm_config.flags & KVM_XEN_HVM_CONFIG_EVTCHN_SEND))
 		return false;
 
 	idx = srcu_read_lock(&vcpu->kvm->srcu);
 	gpa = kvm_mmu_gva_to_gpa_system(vcpu, param, NULL);
 	srcu_read_unlock(&vcpu->kvm->srcu, idx);
-
-	if (!gpa || kvm_vcpu_read_guest(vcpu, gpa, &sched_poll,
-					sizeof(sched_poll))) {
+	if (!gpa) {
 		*r = -EFAULT;
 		return true;
 	}
 
+	if (IS_ENABLED(CONFIG_64BIT) && longmode) {
+		if (kvm_vcpu_read_guest(vcpu, gpa, &sched_poll,
+					sizeof(sched_poll))) {
+			*r = -EFAULT;
+			return true;
+		}
+	} else {
+		struct compat_sched_poll sp;
+
+		/*
+		 * Sanity check that __packed trick works fine and size of
+		 * compat_sched_poll is 16 bytes just like in the real Xen
+		 * 32-bit case.
+		 */
+		BUILD_BUG_ON(sizeof(struct compat_sched_poll) != 16);
+
+		if (kvm_vcpu_read_guest(vcpu, gpa, &sp, sizeof(sp))) {
+			*r = -EFAULT;
+			return true;
+		}
+		sched_poll.ports = (evtchn_port_t *)(unsigned long)(sp.ports);
+		sched_poll.nr_ports = sp.nr_ports;
+		sched_poll.timeout = sp.timeout;
+	}
+
 	if (unlikely(sched_poll.nr_ports > 1)) {
 		/* Xen (unofficially) limits number of pollers to 128 */
 		if (sched_poll.nr_ports > 128) {
diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index ee5c4ae0755c..8b36d346fc9c 100644
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -196,6 +196,13 @@ struct compat_shared_info {
 	struct compat_arch_shared_info arch;
 };
 
+struct compat_sched_poll {
+	/* This is actually a guest virtual address which points to ports. */
+	uint32_t ports;
+	unsigned int nr_ports;
+	uint64_t timeout;
+} __packed;
+
 #define COMPAT_EVTCHN_2L_NR_CHANNELS (8 *				\
 				      sizeof_field(struct compat_shared_info, \
 						   evtchn_pending))
-- 
2.32.0