mshv: Fix infinite fault loop on permission-denied GPA intercepts

Prevent infinite fault loops when guests access memory regions without
proper permissions. Currently, mshv_handle_gpa_intercept() attempts to
remap pages for all faults on movable memory regions, regardless of
whether the access type is permitted. When a guest writes to a read-only
region, the remap succeeds but the region remains read-only, causing
immediate re-fault and spinning the vCPU indefinitely.

Validate intercept access type against region permissions before
attempting remaps. Reject writes to non-writable regions and executes to
non-executable regions early, returning false to let the VMM handle the
intercept appropriately.

This also closes a potential DoS vector where malicious guests could
intentionally trigger these fault loops to consume host resources.

Fixes: b9a66cd ("mshv: Add support for movable memory regions")
Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Reviewed-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

Author: Stanislav Kinsburskii

drivers/hv/mshv_root_main.c

@@ -630,7 +630,7 @@ static bool mshv_handle_gpa_intercept(struct mshv_vp *vp)
 {
 	struct mshv_partition *p = vp->vp_partition;
 	struct mshv_mem_region *region;
-	bool ret;
+	bool ret = false;
 	u64 gfn;
 #if defined(CONFIG_X86_64)
 	struct hv_x64_memory_intercept_message *msg =
@@ -641,19 +641,28 @@ static bool mshv_handle_gpa_intercept(struct mshv_vp *vp)
 		(struct hv_arm64_memory_intercept_message *)
 		vp->vp_intercept_msg_page->u.payload;
 #endif
+	enum hv_intercept_access_type access_type =
+		msg->header.intercept_access_type;
 
 	gfn = HVPFN_DOWN(msg->guest_physical_address);
 
 	region = mshv_partition_region_by_gfn_get(p, gfn);
 	if (!region)
 		return false;
 
+	if (access_type == HV_INTERCEPT_ACCESS_WRITE &&
+	    !(region->hv_map_flags & HV_MAP_GPA_WRITABLE))
+		goto put_region;
+
+	if (access_type == HV_INTERCEPT_ACCESS_EXECUTE &&
+	    !(region->hv_map_flags & HV_MAP_GPA_EXECUTABLE))
+		goto put_region;
+
 	/* Only movable memory ranges are supported for GPA intercepts */
 	if (region->mreg_type == MSHV_REGION_TYPE_MEM_MOVABLE)
 		ret = mshv_region_handle_gfn_fault(region, gfn);
-	else
-		ret = false;
 
+put_region:
 	mshv_region_put(region);
 
 	return ret;

include/hyperv/hvgdk_mini.h

@@ -1533,4 +1533,10 @@ struct hv_mmio_write_input {
 	u8 data[HV_HYPERCALL_MMIO_MAX_DATA_LENGTH];
 } __packed;
 
+enum hv_intercept_access_type {
+	HV_INTERCEPT_ACCESS_READ	= 0,
+	HV_INTERCEPT_ACCESS_WRITE	= 1,
+	HV_INTERCEPT_ACCESS_EXECUTE	= 2
+};
+
 #endif /* _HV_HVGDK_MINI_H */

include/hyperv/hvhdk.h

@@ -779,7 +779,7 @@ struct hv_x64_intercept_message_header {
 	u32 vp_index;
 	u8 instruction_length:4;
 	u8 cr8:4; /* Only set for exo partitions */
-	u8 intercept_access_type;
+	u8 intercept_access_type; /* enum hv_intercept_access_type */
 	union hv_x64_vp_execution_state execution_state;
 	struct hv_x64_segment_register cs_segment;
 	u64 rip;
@@ -825,7 +825,7 @@ union hv_arm64_vp_execution_state {
 struct hv_arm64_intercept_message_header {
 	u32 vp_index;
 	u8 instruction_length;
-	u8 intercept_access_type;
+	u8 intercept_access_type; /* enum hv_intercept_access_type */
 	union hv_arm64_vp_execution_state execution_state;
 	u64 pc;
 	u64 cpsr;