[PATCH v3 1/2] MIPS: Don't crash in stack_top() for tasks without ABI or vDSO

[Thomas Weißschuh]

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.

Also move the randomization adjustment into the same conditional.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
---
 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