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

Andrew Morton
2025-04-15T00:04:16Z

On Mon, 14 Apr 2025 21:36:48 +0000 Mostafa Saleh <smostafa@google.com> wrote:

> 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:

argh.

> 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.

Would it be better to put the above three items into a struct, so we
specify the layout?

> --- 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];
> }