[PATCH] lib/test_ubsan.c: Fix panic from test_ubsan_out_of_bounds

Mostafa Saleh
2025-04-14T21:37:00Z

Running lib_ubsan.ko on arm64 (without CONFIG_UBSAN_TRAP) panics the
kernel

[ 31.616546] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: test_ubsan_out_of_bounds+0x158/0x158 [test_ubsan]
[ 31.646817] CPU: 3 UID: 0 PID: 179 Comm: insmod Not tainted 6.15.0-rc2 #1 PREEMPT
[ 31.648153] Hardware name: linux,dummy-virt (DT)
[ 31.648970] Call trace:
[ 31.649345] show_stack+0x18/0x24 (C)
[ 31.650960] dump_stack_lvl+0x40/0x84
[ 31.651559] dump_stack+0x18/0x24
[ 31.652264] panic+0x138/0x3b4
[ 31.652812] __ktime_get_real_seconds+0x0/0x10
[ 31.653540] test_ubsan_load_invalid_value+0x0/0xa8 [test_ubsan]
[ 31.654388] init_module+0x24/0xff4 [test_ubsan]
[ 31.655077] do_one_initcall+0xd4/0x280
[ 31.655680] do_init_module+0x58/0x2b4

That happens because the test corrupts other data in the stack:
400: d5384108 mrs x8, sp_el0
404: f9426d08 ldr x8, [x8, #1240]
408: f85f83a9 ldur x9, [x29, #-8]
40c: eb09011f cmp x8, x9
410: 54000301 b.ne 470 <test_ubsan_out_of_bounds+0x154> // b.any

As there is no guarantee the compiler will order the local variables
as declared in the module:
volatile char above[4] = { }; /* Protect surrounding memory. */
volatile int arr[4];
volatile char below[4] = { }; /* Protect surrounding memory. */

So, instead of writing out-of-bound, we can read out-of-bound which
still triggers UBSAN but doesn't corrupt the stack.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
---
lib/test_ubsan.c | 11 ++++-------
1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/lib/test_ubsan.c b/lib/test_ubsan.c
index 8772e5edaa4f..0e5c18b32b2d 100644
--- a/lib/test_ubsan.c
+++ b/lib/test_ubsan.c
@@ -77,18 +77,15 @@ static void test_ubsan_shift_out_of_bounds(void)

static void test_ubsan_out_of_bounds(void)
{
- volatile int i = 4, j = 5, k = -1;
- volatile char above[4] = { }; /* Protect surrounding memory. */
+ volatile int j = 5, k = -1;
+ volatile int scratch[4] = { };
volatile int arr[4];
- volatile char below[4] = { }; /* Protect surrounding memory. */
-
- above[0] = below[0];

UBSAN_TEST(CONFIG_UBSAN_BOUNDS, "above");
- arr[j] = i;
+ scratch[1] = arr[j];

UBSAN_TEST(CONFIG_UBSAN_BOUNDS, "below");
- arr[k] = i;
+ scratch[2] = arr[k];
}

enum ubsan_test_enum {
--
2.49.0.604.gff1f9ca942-goog