[PATCH] list: Fix another data-race around ep->rdllist.

[Kuniyuki Iwashima <kuniyu@amazon.co.jp>]

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@smtp.kernel.org/

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@syzkaller.appspotmail.com
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
---
CC: Soheil Hassas Yeganeh <soheil@google.com>
CC: Davidlohr Bueso <dave@stgolabs.net>
CC: Sridhar Samudrala <sridhar.samudrala@intel.com>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
---
 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