x86/hyperv: Fix error pointer dereference

[ Upstream commit 705d01c ]

The function idle_thread_get() can return an error pointer and is not
checked for it. Add check for error pointer.

Detected by Smatch:
arch/x86/hyperv/hv_vtl.c:126 hv_vtl_bringup_vcpu() error:
'idle' dereferencing possible ERR_PTR()

Fixes: 2b4b90e ("x86/hyperv: Use per cpu initial stack for vtl context")
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Ethan Tidmore

arch/x86/hyperv/hv_vtl.c

@@ -110,7 +110,7 @@ static void hv_vtl_ap_entry(void)
 
 static int hv_vtl_bringup_vcpu(u32 target_vp_index, int cpu, u64 eip_ignored)
 {
-	u64 status;
+	u64 status, rsp, rip;
 	int ret = 0;
 	struct hv_enable_vp_vtl *input;
 	unsigned long irq_flags;
@@ -123,9 +123,11 @@ static int hv_vtl_bringup_vcpu(u32 target_vp_index, int cpu, u64 eip_ignored)
 	struct desc_struct *gdt;
 
 	struct task_struct *idle = idle_thread_get(cpu);
-	u64 rsp = (unsigned long)idle->thread.sp;
+	if (IS_ERR(idle))
+		return PTR_ERR(idle);
 
-	u64 rip = (u64)&hv_vtl_ap_entry;
+	rsp = (unsigned long)idle->thread.sp;
+	rip = (u64)&hv_vtl_ap_entry;
 
 	native_store_gdt(&gdt_ptr);
 	store_idt(&idt_ptr);