[PATCH] fix UBSAN splat in test_scanf
From: Chris von Recklinghausen
Date: Mon Aug 09 2021 - 06:20:30 EST
On a system with CONFIG_UBSAN_SHIFT=y, the following splat was observed:
[ 104.676826] test_scanf: loaded.
[ 104.680862] ================================================================================
[ 104.690304] UBSAN: shift-out-of-bounds in lib/test_scanf.c:274:51
[ 104.697144] shift exponent 32 is too large for 32-bit type 'unsigned int'
[ 104.704747] CPU: 18 PID: 1790 Comm: modprobe Tainted: G S 5.14.0-rc4 #1
[ 104.713592] Hardware name: Intel Corporation S2600CP/S2600CP, BIOS SE5C600.86B.02.01.0002.082220131453 08/22/2013
[ 104.725052] Call Trace:
[ 104.727787] dump_stack_lvl+0x44/0x57
[ 104.731888] ubsan_epilogue+0x5/0x40
[ 104.735882] __ubsan_handle_shift_out_of_bounds.cold.14+0x14/0x98
[ 104.742704] next_test_random+0x3f/0x56 [test_scanf]
[ 104.748253] numbers_list+0x1e91/0x3364 [test_scanf]
[ 104.753807] ? numbers_list_field_width_typemax+0x3859/0x3859 [test_scanf]
[ 104.761501] ? numbers_simple+0x3839/0x388f [test_scanf]
[ 104.767436] ? load_module+0x6ba5/0x7200
[ 104.771822] ? _test+0x152/0x152 [test_scanf]
[ 104.776690] ? hlock_class+0x4e/0x120
[ 104.780784] ? __lock_acquire+0x171d/0x2f80
[ 104.785459] ? lock_is_held_type+0x99/0x100
[ 104.790153] ? find_held_lock+0x3a/0x1c0
[ 104.794557] ? rcu_read_lock_bh_held+0xc0/0xc0
[ 104.799529] ? __kasan_kmalloc+0x7f/0xa0
[ 104.803916] ? kmem_cache_alloc_trace+0x185/0x270
[ 104.809172] ? test_scanf_init+0xe3/0x11cd [test_scanf]
[ 104.815010] ? rcu_read_lock_sched_held+0xaf/0xe0
[ 104.820272] test_scanf_init+0x17e/0x11cd [test_scanf]
[ 104.826014] ? numbers_list+0x3364/0x3364 [test_scanf]
[ 104.831757] ? numbers_list+0x3364/0x3364 [test_scanf]
[ 104.837497] ? rcu_read_lock_sched_held+0xaf/0xe0
[ 104.842760] ? rcu_read_lock_bh_held+0xc0/0xc0
[ 104.847730] ? numbers_list+0x3364/0x3364 [test_scanf]
[ 104.853473] do_one_initcall+0xfb/0x540
[ 104.857776] ? perf_trace_initcall_level+0x490/0x490
[ 104.863328] ? __kasan_kmalloc+0x7f/0xa0
[ 104.867714] ? kmem_cache_alloc_trace+0x185/0x270
[ 104.872969] ? do_init_module+0x4e/0x760
[ 104.877350] ? kasan_unpoison+0x23/0x50
[ 104.881650] do_init_module+0x1f2/0x760
[ 104.885945] load_module+0x6ba5/0x7200
[ 104.890174] ? layout_and_allocate+0x2c00/0x2c00
[ 104.895337] ? ima_read_file+0x150/0x150
[ 104.899734] ? kernel_read_file+0x2f6/0x710
[ 104.904415] ? __x64_sys_fsconfig+0x6a0/0x6a0
[ 104.909300] ? __do_sys_finit_module+0xff/0x180
[ 104.914363] __do_sys_finit_module+0xff/0x180
[ 104.919230] ? __ia32_sys_init_module+0xa0/0xa0
[ 104.924294] ? rcu_read_unlock+0x50/0x50
[ 104.928679] ? kfree+0x2fd/0x3c0
[ 104.932316] do_syscall_64+0x3a/0x80
[ 104.936316] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 104.941971] RIP: 0033:0x7f7d8220e52d
[ 104.945962] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 2b 79 2c 00 f7 d8 64 89 01 48
[ 104.966933] RSP: 002b:00007ffd1a1a9448 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[ 104.975392] RAX: ffffffffffffffda RBX: 00005611e43bde40 RCX: 00007f7d8220e52d
[ 104.983360] RDX: 0000000000000000 RSI: 00005611e401d7d6 RDI: 0000000000000003
[ 104.991327] RBP: 00005611e401d7d6 R08: 0000000000000000 R09: 0000000000000000
[ 104.999294] R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000000
[ 105.007261] R13: 00005611e43bdec0 R14: 0000000000040000 R15: 0000000000000000
[ 105.015285] ================================================================================
[ 105.026917] test_scanf: all 2545 tests passed
[ 105.045896] test_scanf: unloaded.
This is due to n_bits having a value of 0 in next_test_random. Avoid this by
repeatedly calling hweight32(prandom_u32_state(&rnd_state)) % (max_bits + 1)
until n_bits is nonzero.
Signed-off-by: Chris von Recklinghausen <crecklin@xxxxxxxxxx>
---
lib/test_scanf.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/lib/test_scanf.c b/lib/test_scanf.c
index 84fe09eaf55e..4e3754b916f2 100644
--- a/lib/test_scanf.c
+++ b/lib/test_scanf.c
@@ -269,7 +269,13 @@ static void __init numbers_simple(void)
*/
static u32 __init next_test_random(u32 max_bits)
{
- u32 n_bits = hweight32(prandom_u32_state(&rnd_state)) % (max_bits + 1);
+ u32 n_bits;
+
+ /* shifting a 32 bit unsigned int by 32 is undefined behavior */
+ do {
+ n_bits = hweight32(prandom_u32_state(&rnd_state))
+ % (max_bits + 1);
+ } while (n_bits == 0);
return prandom_u32_state(&rnd_state) & (UINT_MAX >> (32 - n_bits));
}
--
2.27.0