Re: [PATCH v2] lib/test_ubsan.c: Fix panic from test_ubsan_out_of_bounds

[Kees Cook <kees@kernel.org>]

On Tue, Apr 15, 2025 at 08:33:54PM +0000, Mostafa Saleh 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:
>         volatile char above[4] = { }; /* Protect surrounding memory. */
>         volatile int arr[4];
>         volatile char below[4] = { }; /* Protect surrounding memory. */
> 
> There is another problem where the out-of-bound index is 5 which is larger
> than the extra surrounding memory for protection.
> 
> So, use a struct to enforce the ordering, and fix the index to be 4.
> Also, remove some of the volatiles and rely on OPTIMIZER_HIDE_VAR()
> 
> Signed-off-by: Mostafa Saleh <smostafa@google.com>

Looks good; thanks!

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook