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: 803b0fc5c3f2 ("LoongArch: Add process management")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

diff --git a/arch/loongarch/kernel/process.c b/arch/loongarch/kernel/process.c
index f2ff8b5d591e4fd638109d2c98d75543c01a112c..6e58f65455c7ca3eae2e88ed852c8655a6701e5c 100644 (file)
--- a/arch/loongarch/kernel/process.c
+++ b/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;
 }