[PATCH 2/2] list: Fix a data-race around ep->rdllist.

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

ep_poll() first calls ep_events_available() with no lock held and
checks if ep->rdllist is empty by list_empty_careful(), which reads
rdllist->prev.  Thus all accesses to it need some protection to avoid
store/load-tearing.

Note INIT_LIST_HEAD_RCU() already has the annotation for both prev
and next.

Commit bf3b9f6372c4 ("epoll: Add busy poll support to epoll with
socket fds.") added the first lockless ep_events_available(), and
commit c5a282e9635e ("fs/epoll: reduce the scope of wq lock in
epoll_wait()") made some ep_events_available() calls lockless and
added single call under a lock, finally commit e59d3c64cba6 ("epoll:
eliminate unnecessary lock for zero timeout") made the last
ep_events_available() lockless.

BUG: KCSAN: data-race in do_epoll_wait / do_epoll_wait

write to 0xffff88810480c7d8 of 8 bytes by task 1802 on cpu 0:
 INIT_LIST_HEAD include/linux/list.h:38 [inline]
 list_splice_init include/linux/list.h:492 [inline]
 ep_start_scan fs/eventpoll.c:622 [inline]
 ep_send_events fs/eventpoll.c:1656 [inline]
 ep_poll fs/eventpoll.c:1806 [inline]
 do_epoll_wait+0x4eb/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

read to 0xffff88810480c7d8 of 8 bytes by task 1799 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: 0xffff88810480c7d0 -> 0xffff888103c15098

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1799 Comm: syz-fuzzer Tainted: G        W         5.17.0-rc7-syzkaller-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+bdd6e38a1ed5ee58d8bd@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 | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index dd6c2041d..d7d2bfa1a 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -35,7 +35,7 @@
 static inline void INIT_LIST_HEAD(struct list_head *list)
 {
 	WRITE_ONCE(list->next, list);
-	list->prev = list;
+	WRITE_ONCE(list->prev, list);
 }
 
 #ifdef CONFIG_DEBUG_LIST
@@ -306,7 +306,7 @@ static inline int list_empty(const struct list_head *head)
 static inline void list_del_init_careful(struct list_head *entry)
 {
 	__list_del_entry(entry);
-	entry->prev = entry;
+	WRITE_ONCE(entry->prev, entry);
 	smp_store_release(&entry->next, entry);
 }
 
@@ -326,7 +326,7 @@ static inline void list_del_init_careful(struct list_head *entry)
 static inline int list_empty_careful(const struct list_head *head)
 {
 	struct list_head *next = smp_load_acquire(&head->next);
-	return list_is_head(next, head) && (next == head->prev);
+	return list_is_head(next, head) && (next == READ_ONCE(head->prev));
 }
 
 /**
-- 
2.30.2