LoongArch: Don't crash in stack_top() for tasks without vDSO

Not all tasks have a vDSO mapped, for example kthreads never do. If such
a task ever ends up calling stack_top(), it will derefence the NULL vdso
pointer and crash.

This can for example happen when using kunit:

	[<9000000000203874>] stack_top+0x58/0xa8
	[<90000000002956cc>] arch_pick_mmap_layout+0x164/0x220
	[<90000000003c284c>] kunit_vm_mmap_init+0x108/0x12c
	[<90000000003c1fbc>] __kunit_add_resource+0x38/0x8c
	[<90000000003c2704>] kunit_vm_mmap+0x88/0xc8
	[<9000000000410b14>] usercopy_test_init+0xbc/0x25c
	[<90000000003c1db4>] kunit_try_run_case+0x5c/0x184
	[<90000000003c3d54>] kunit_generic_run_threadfn_adapter+0x24/0x48
	[<900000000022e4bc>] kthread+0xc8/0xd4
	[<9000000000200ce8>] ret_from_kernel_thread+0xc/0xa4

Fixes: 803b0fc ("LoongArch: Add process management")
Signed-off-by: Thomas Weißschuh <[email protected]>
Signed-off-by: Huacai Chen <[email protected]>

Author: t-8ch

arch/loongarch/kernel/process.c

@@ -293,13 +293,15 @@ unsigned long stack_top(void)
 {
 	unsigned long top = TASK_SIZE & PAGE_MASK;
 
-	/* Space for the VDSO & data page */
-	top -= PAGE_ALIGN(current->thread.vdso->size);
-	top -= VVAR_SIZE;
-
-	/* Space to randomize the VDSO base */
-	if (current->flags & PF_RANDOMIZE)
-		top -= VDSO_RANDOMIZE_SIZE;
+	if (current->thread.vdso) {
+		/* Space for the VDSO & data page */
+		top -= PAGE_ALIGN(current->thread.vdso->size);
+		top -= VVAR_SIZE;
+
+		/* Space to randomize the VDSO base */
+		if (current->flags & PF_RANDOMIZE)
+			top -= VDSO_RANDOMIZE_SIZE;
+	}
 
 	return top;
 }