list: fix a data-race around ep->rdllist

In the Linux kernel, the following vulnerability has been resolved:

list: fix a data-race around ep->rdllist

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