[patch 120/166] percpu_counter: fix a data race at vm_committed_as

Andrew Morton
2020-04-07T03:10:28Z

From: Qian Cai <cai@lca.pw>
Subject: percpu_counter: fix a data race at vm_committed_as

"vm_committed_as.count" could be accessed concurrently as reported by
KCSAN,

BUG: KCSAN: data-race in __vm_enough_memory / percpu_counter_add_batch

write to 0xffffffff9451c538 of 8 bytes by task 65879 on cpu 35:
percpu_counter_add_batch+0x83/0xd0
percpu_counter_add_batch at lib/percpu_counter.c:91
__vm_enough_memory+0xb9/0x260
dup_mm+0x3a4/0x8f0
copy_process+0x2458/0x3240
_do_fork+0xaa/0x9f0
__do_sys_clone+0x125/0x160
__x64_sys_clone+0x70/0x90
do_syscall_64+0x91/0xb05
entry_SYSCALL_64_after_hwframe+0x49/0xbe

read to 0xffffffff9451c538 of 8 bytes by task 66773 on cpu 19:
__vm_enough_memory+0x199/0x260
percpu_counter_read_positive at include/linux/percpu_counter.h:81
(inlined by) __vm_enough_memory at mm/util.c:839
mmap_region+0x1b2/0xa10
do_mmap+0x45c/0x700
vm_mmap_pgoff+0xc0/0x130
ksys_mmap_pgoff+0x6e/0x300
__x64_sys_mmap+0x33/0x40
do_syscall_64+0x91/0xb05
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The read is outside percpu_counter::lock critical section which results in
a data race. Fix it by adding a READ_ONCE() in
percpu_counter_read_positive() which could also service as the existing
compiler memory barrier.

Link: http://lkml.kernel.org/r/1582302724-2804-1-git-send-email-cai@lca.pw
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

include/linux/percpu_counter.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/percpu_counter.h~percpu_counter-fix-a-data-race-at-vm_committed_as
+++ a/include/linux/percpu_counter.h
@@ -78,9 +78,9 @@ static inline s64 percpu_counter_read(st
*/
static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
{
- s64 ret = fbc->count;
+ /* Prevent reloads of fbc->count */
+ s64 ret = READ_ONCE(fbc->count);

- barrier(); /* Prevent reloads of fbc->count */
if (ret >= 0)
return ret;
return 0;
_