x86/hyperv: Prevent potential NULL pointer dereference

The page allocation in hv_cpu_init() can fail, but the code does not
have a check for that.

Add a check and return -ENOMEM when the allocation fails.

[ tglx: Massaged changelog ]
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Acked-by: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: pakki001@umn.edu
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-hyperv@vger.kernel.org
Link: https://lkml.kernel.org/r/20190314054651.1315-1-kjlu@umn.edu

arch/x86/hyperv/hv_init.c

@@ -103,9 +103,13 @@ static int hv_cpu_init(unsigned int cpu)
 	u64 msr_vp_index;
 	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
 	void **input_arg;
+	struct page *pg;
 
 	input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
-	*input_arg = page_address(alloc_page(GFP_KERNEL));
+	pg = alloc_page(GFP_KERNEL);
+	if (unlikely(!pg))
+		return -ENOMEM;
+	*input_arg = page_address(pg);
 
 	hv_get_vp_index(msr_vp_index);