syzbot had reported another race around ep->rdllist. ep_poll() calls list_empty_careful() locklessly to check if the list is empty or not by testing rdllist->prev == rdllist->next. When the list does not have any nodes, the next and prev arguments of __list_add() is the same head pointer. Thus the write to head->prev there is racy with lockless list_empty_careful() and needs WRITE_ONCE() to avoid store-tearing. Note that the reader side is already fixed in the patch [0]. [0]: https://lore.kernel.org/mm-commits/20220326031647.DD24EC004DD@xxxxxxxxxxxxxxx/ BUG: KCSAN: data-race in do_epoll_ctl / do_epoll_wait write to 0xffff888103e43058 of 8 bytes by task 1799 on cpu 0: __list_add include/linux/list.h:72 [inline] list_add_tail include/linux/list.h:102 [inline] ep_insert fs/eventpoll.c:1542 [inline] do_epoll_ctl+0x1331/0x1880 fs/eventpoll.c:2141 __do_sys_epoll_ctl fs/eventpoll.c:2192 [inline] __se_sys_epoll_ctl fs/eventpoll.c:2183 [inline] __x64_sys_epoll_ctl+0xc2/0xf0 fs/eventpoll.c:2183 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae read to 0xffff888103e43058 of 8 bytes by task 1802 on cpu 1: list_empty_careful include/linux/list.h:329 [inline] ep_events_available fs/eventpoll.c:381 [inline] ep_poll fs/eventpoll.c:1797 [inline] do_epoll_wait+0x279/0xf40 fs/eventpoll.c:2234 do_epoll_pwait fs/eventpoll.c:2268 [inline] __do_sys_epoll_pwait fs/eventpoll.c:2281 [inline] __se_sys_epoll_pwait+0x12b/0x240 fs/eventpoll.c:2275 __x64_sys_epoll_pwait+0x74/0x80 fs/eventpoll.c:2275 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae value changed: 0xffff888103e43050 -> 0xffff88812d515498 Reported by Kernel Concurrency Sanitizer on: CPU: 1 PID: 1802 Comm: syz-fuzzer Not tainted 5.17.0-rc8-syzkaller-00003-g56e337f2cf13-dirty #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Fixes: e59d3c64cba6 ("epoll: eliminate unnecessary lock for zero timeout") Fixes: c5a282e9635e ("fs/epoll: reduce the scope of wq lock in epoll_wait()") Fixes: bf3b9f6372c4 ("epoll: Add busy poll support to epoll with socket fds.") Reported-by: syzbot+19480160ef25c9ffa29d@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx> --- CC: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx> CC: Davidlohr Bueso <dave@xxxxxxxxxxxx> CC: Sridhar Samudrala <sridhar.samudrala@xxxxxxxxx> CC: Alexander Duyck <alexander.h.duyck@xxxxxxxxx> --- include/linux/list.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/list.h b/include/linux/list.h index dd6c2041d..2eaadc84a 100644 --- a/include/linux/list.h +++ b/include/linux/list.h @@ -69,10 +69,10 @@ static inline void __list_add(struct list_head *new, if (!__list_add_valid(new, prev, next)) return; - next->prev = new; new->next = next; new->prev = prev; WRITE_ONCE(prev->next, new); + WRITE_ONCE(next->prev, new); } /** -- 2.30.2
