> On Mar 17, 2020, at 9:31 AM, Marco Elver <elver@xxxxxxxxxx> wrote: > > On Tue, 17 Mar 2020 at 14:22, Qian Cai <cai@xxxxxx> wrote: >> >> Even if KCSAN is disabled for kmemleak, update_checksum() could still >> call crc32() (which is outside of kmemleak.c) to dereference >> object->pointer. Thus, the value of object->pointer could be accessed >> concurrently as noticed by KCSAN, >> >> BUG: KCSAN: data-race in crc32_le_base / do_raw_spin_lock >> >> write to 0xffffb0ea683a7d50 of 4 bytes by task 23575 on cpu 12: >> do_raw_spin_lock+0x114/0x200 >> debug_spin_lock_after at kernel/locking/spinlock_debug.c:91 >> (inlined by) do_raw_spin_lock at kernel/locking/spinlock_debug.c:115 >> _raw_spin_lock+0x40/0x50 >> __handle_mm_fault+0xa9e/0xd00 >> handle_mm_fault+0xfc/0x2f0 >> do_page_fault+0x263/0x6f9 >> page_fault+0x34/0x40 >> >> read to 0xffffb0ea683a7d50 of 4 bytes by task 839 on cpu 60: >> crc32_le_base+0x67/0x350 >> crc32_le_base+0x67/0x350: >> crc32_body at lib/crc32.c:106 >> (inlined by) crc32_le_generic at lib/crc32.c:179 >> (inlined by) crc32_le at lib/crc32.c:197 >> kmemleak_scan+0x528/0xd90 >> update_checksum at mm/kmemleak.c:1172 >> (inlined by) kmemleak_scan at mm/kmemleak.c:1497 >> kmemleak_scan_thread+0xcc/0xfa >> kthread+0x1e0/0x200 >> ret_from_fork+0x27/0x50 >> >> If a shattered value was returned due to a data race, it will be >> corrected in the next scan. Thus, annotate it as an intentional data >> race using the data_race() macro. >> >> Signed-off-by: Qian Cai <cai@xxxxxx> >> --- >> mm/kmemleak.c | 7 ++++++- >> 1 file changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/mm/kmemleak.c b/mm/kmemleak.c >> index e362dc3d2028..d3327756c3a4 100644 >> --- a/mm/kmemleak.c >> +++ b/mm/kmemleak.c >> @@ -1169,7 +1169,12 @@ static bool update_checksum(struct kmemleak_object *object) >> u32 old_csum = object->checksum; >> >> kasan_disable_current(); > > Suggested: > + kcsan_disable_current(); > >> - object->checksum = crc32(0, (void *)object->pointer, object->size); >> + /* >> + * crc32() will dereference object->pointer. If an unstable value was >> + * returned due to a data race, it will be corrected in the next scan. >> + */ >> + object->checksum = data_race(crc32(0, (void *)object->pointer, >> + object->size)); > > This will work with the default config, because for word-sized-aligned > writes no marking is enforced. But this will still cause a data race > if the write is e.g. due to a memcpy. I saw this spla atmt but just decided to reuse an old one to save some time. Looks like that "head->func = func;” not aligned. [77392.095571][ T839] BUG: KCSAN: data-race in call_rcu / crc32_le_base [77392.102066][ T839] [77392.104297][ T839] write to 0xffff898ea73a8748 of 8 bytes by task 114682 on cpu 79: [77392.112111][ T839] call_rcu+0xe8/0x4b0 __call_rcu at kernel/rcu/tree.c:2701 (inlined by) call_rcu at kernel/rcu/tree.c:2777 [77392.116084][ T839] __fput+0x23a/0x3d0 [77392.119970][ T839] ____fput+0x1e/0x30 [77392.123852][ T839] task_work_run+0xba/0x120 [77392.128257][ T839] do_syscall_64+0x7d7/0xb05 [77392.132753][ T839] entry_SYSCALL_64_after_hwframe+0x49/0xb3 [77392.138544][ T839] [77392.140760][ T839] INFO: lockdep is turned off. [77392.145478][ T839] irq event stamp: 0 [77392.149270][ T839] hardirqs last enabled at (0): [<0000000000000000>] 0x0 [77392.156307][ T839] hardirqs last disabled at (0): [<ffffffffb0ab4d42>] copy_process+0x1122/0x3240 [77392.165348][ T839] softirqs last enabled at (0): [<ffffffffb0ab4d42>] copy_process+0x1122/0x3240 [77392.174384][ T839] softirqs last disabled at (0): [<0000000000000000>] 0x0 [77392.181405][ T839] [77392.183625][ T839] read to 0xffff898ea73a8748 of 4 bytes by task 839 on cpu 46: [77392.191088][ T839] crc32_le_base+0x67/0x350 [77392.195498][ T839] kmemleak_scan+0x3ee/0x9f0 [77392.199992][ T839] kmemleak_scan_thread+0x9f/0xc4 [77392.204921][ T839] kthread+0x1cd/0x1f0 [77392.208894][ T839] ret_from_fork+0x27/0x50 > > There are already markers for KASAN around, so the most reliable thing > is to just disable KCSAN in this region. OK, I’ll test that a bit first. > >> kasan_enable_current(); > > Suggested: > + kcsan_enable_current(); > > Thanks, > -- Marco > >> return object->checksum != old_csum; >> -- >> 2.21.0 (Apple Git-122.2)
