[PATCH v4 1/2] MIPS: Don't crash in stack_top() for tasks without ABI or vDSO

["Thomas Weißschuh" <thomas.weissschuh@linutronix.de>]

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

This can for example happen when using kunit:

    mips_stack_top+0x28/0xc0
    arch_pick_mmap_layout+0x190/0x220
    kunit_vm_mmap_init+0xf8/0x138
    __kunit_add_resource+0x40/0xa8
    kunit_vm_mmap+0x88/0xd8
    usercopy_test_init+0xb8/0x240
    kunit_try_run_case+0x5c/0x1a8
    kunit_generic_run_threadfn_adapter+0x28/0x50
    kthread+0x118/0x240
    ret_from_kernel_thread+0x14/0x1c

Only dereference the ABI point if it is set.

The GIC page is also included as it is specific to the vDSO.
Also move the randomization adjustment into the same conditional.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: David Gow <davidgow@google.com>
---
 arch/mips/kernel/process.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index b630604c577f9ff3f2493b0f254363e499c8318c..02aa6a04a21da437909eeac4f149155cc298f5b5 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -690,18 +690,20 @@ unsigned long mips_stack_top(void)
 	}
 
 	/* Space for the VDSO, data page & GIC user page */
-	top -= PAGE_ALIGN(current->thread.abi->vdso->size);
-	top -= PAGE_SIZE;
-	top -= mips_gic_present() ? PAGE_SIZE : 0;
+	if (current->thread.abi) {
+		top -= PAGE_ALIGN(current->thread.abi->vdso->size);
+		top -= PAGE_SIZE;
+		top -= mips_gic_present() ? PAGE_SIZE : 0;
+
+		/* Space to randomize the VDSO base */
+		if (current->flags & PF_RANDOMIZE)
+			top -= VDSO_RANDOMIZE_SIZE;
+	}
 
 	/* Space for cache colour alignment */
 	if (cpu_has_dc_aliases)
 		top -= shm_align_mask + 1;
 
-	/* Space to randomize the VDSO base */
-	if (current->flags & PF_RANDOMIZE)
-		top -= VDSO_RANDOMIZE_SIZE;
-
 	return top;
 }
 

-- 
2.49.0