On Sat, Mar 26, 2022 at 2:36 AM Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx> wrote: > > 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> Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx> Thank you for the fix! > --- > 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 >
