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SubjectList corruption on epoll_ctl(EPOLL_CTL_DEL) an AF_UNIX socket
Hi,

this is an attempt to resurrect the thread initially started here:

http://thread.gmane.org/gmane.linux.network/353003

As that patch fixed the issue for the mentioned reproducer, it did not
fix the bug for the production code Olivier is using. :(

Changing the reproducer only slightly allows me to trigger the following
list debug splat (CONFIG_DEBUG_LIST=y) reliable within seconds -- even
with the above linked patch applied:

[ 50.264249] ------------[ cut here ]------------
[ 50.264249] WARNING: CPU: 0 PID: 214 at lib/list_debug.c:59 __list_del_entry+0xa4/0xd0()
[ 50.264249] list_del corruption. prev->next should be ffff88003c2c1bb8, but was ffff88003f07bbb8
[ 50.264249] Modules linked in: ipv6 pcspkr serio_raw microcode virtio_net virtio_pci virtio_ring virtio sr_mod cdrom
[ 50.264249] CPU: 0 PID: 214 Comm: epoll_bug Not tainted 4.2.0 #75
[ 50.264249] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[ 50.264249] ffffffff817e902e ffff880000087d08 ffffffff8155593c 0000000000000007
[ 50.264249] ffff880000087d58 ffff880000087d48 ffffffff8105202a 0000000000000001
[ 50.264249] ffff88003c2c1bb8 ffff88003f07bb80 0000000000000286 ffff88003f736640
[ 50.264249] Call Trace:
[ 50.264249] [<ffffffff8155593c>] dump_stack+0x4c/0x65
[ 50.264249] [<ffffffff8105202a>] warn_slowpath_common+0x8a/0xc0
[ 50.264249] [<ffffffff810520a6>] warn_slowpath_fmt+0x46/0x50
[ 50.264249] [<ffffffff81322354>] __list_del_entry+0xa4/0xd0
[ 50.264249] [<ffffffff81322391>] list_del+0x11/0x40
[ 50.264249] [<ffffffff81094d39>] remove_wait_queue+0x29/0x40
[ 50.264249] [<ffffffff811bc898>] ep_unregister_pollwait.isra.6+0x58/0x1a0
[ 50.264249] [<ffffffff811bc8e9>] ? ep_unregister_pollwait.isra.6+0xa9/0x1a0
[ 50.264249] [<ffffffff811bca02>] ep_remove+0x22/0x110
[ 50.264249] [<ffffffff811be28b>] SyS_epoll_ctl+0x62b/0xf70
[ 50.264249] [<ffffffff81000f44>] ? lockdep_sys_exit_thunk+0x12/0x14
[ 50.264249] [<ffffffff8155cd97>] entry_SYSCALL_64_fastpath+0x12/0x6f
[ 50.264249] ---[ end trace d9af9b915df9667e ]---
[ 50.572100] ------------[ cut here ]------------
[ 50.572100] WARNING: CPU: 1 PID: 212 at lib/list_debug.c:62 __list_del_entry+0xc3/0xd0()
[ 50.584263] list_del corruption. next->prev should be ffff88003f664c90, but was ffff88003f0cb5b8
[ 50.584263] Modules linked in: ipv6 pcspkr serio_raw microcode virtio_net virtio_pci virtio_ring virtio sr_mod cdrom
[ 50.584263] CPU: 1 PID: 212 Comm: epoll_bug Tainted: G W 4.2.0 #75
[ 50.584263] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[ 50.584263] ffffffff817e902e ffff88003d37fce8 ffffffff8155593c 0000000000000007
[ 50.584263] ffff88003d37fd38 ffff88003d37fd28 ffffffff8105202a 0000000000000001
[ 50.584263] ffff88003f664c90 ffff88003f0cb580 0000000000000282 ffff88003f731740
[ 50.584263] Call Trace:
[ 50.584263] [<ffffffff8155593c>] dump_stack+0x4c/0x65
[ 50.584263] [<ffffffff8105202a>] warn_slowpath_common+0x8a/0xc0
[ 50.584263] [<ffffffff810520a6>] warn_slowpath_fmt+0x46/0x50
[ 50.584263] [<ffffffff81322373>] __list_del_entry+0xc3/0xd0
[ 50.584263] [<ffffffff81322391>] list_del+0x11/0x40
[ 50.584263] [<ffffffff81094d39>] remove_wait_queue+0x29/0x40
[ 50.584263] [<ffffffff811bc898>] ep_unregister_pollwait.isra.6+0x58/0x1a0
[ 50.584263] [<ffffffff811bc8e9>] ? ep_unregister_pollwait.isra.6+0xa9/0x1a0
[ 50.584263] [<ffffffff811bca02>] ep_remove+0x22/0x110
[ 50.584263] [<ffffffff811bda62>] eventpoll_release_file+0x62/0xa0
[ 50.584263] [<ffffffff8117704f>] __fput+0x1af/0x200
[ 50.584263] [<ffffffff8155cf20>] ? int_very_careful+0x5/0x3f
[ 50.584263] [<ffffffff811770ee>] ____fput+0xe/0x10
[ 50.584263] [<ffffffff8107271d>] task_work_run+0x8d/0xc0
[ 50.584263] [<ffffffff8100390f>] do_notify_resume+0x4f/0x60
[ 50.584263] [<ffffffff8155cf6c>] int_signal+0x12/0x17
[ 50.584263] ---[ end trace d9af9b915df9667f ]---
[ 50.584263] BUG: spinlock already unlocked on CPU#1, epoll_bug/212
[ 50.584263] lock: 0xffff88003f0cb580, .magic: dead4ead, .owner: <none>/-1, .owner_cpu: -1
[ 50.584263] CPU: 1 PID: 212 Comm: epoll_bug Tainted: G W 4.2.0 #75
[ 50.584263] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[ 50.584263] ffff88003f0cb580 ffff88003d37fd38 ffffffff8155593c 0000000000000007
[ 50.584263] ffffffffffffffff ffff88003d37fd58 ffffffff810a3375 ffff88003f0cb580
[ 50.584263] ffffffff817b9cc8 ffff88003d37fd78 ffffffff810a33f6 ffff88003f0cb580
[ 50.584263] Call Trace:
[ 50.584263] [<ffffffff8155593c>] dump_stack+0x4c/0x65
[ 50.584263] [<ffffffff810a3375>] spin_dump+0x85/0xe0
[ 50.584263] [<ffffffff810a33f6>] spin_bug+0x26/0x30
[ 50.584263] [<ffffffff810a3645>] do_raw_spin_unlock+0x75/0xa0
[ 50.584263] [<ffffffff8155c4ec>] _raw_spin_unlock_irqrestore+0x2c/0x50
[ 50.584263] [<ffffffff81094d44>] remove_wait_queue+0x34/0x40
[ 50.584263] [<ffffffff811bc898>] ep_unregister_pollwait.isra.6+0x58/0x1a0
[ 50.584263] [<ffffffff811bc8e9>] ? ep_unregister_pollwait.isra.6+0xa9/0x1a0
[ 50.584263] [<ffffffff811bca02>] ep_remove+0x22/0x110
[ 50.584263] [<ffffffff811bda62>] eventpoll_release_file+0x62/0xa0
[ 50.584263] [<ffffffff8117704f>] __fput+0x1af/0x200
[ 50.584263] [<ffffffff8155cf20>] ? int_very_careful+0x5/0x3f
[ 50.584263] [<ffffffff811770ee>] ____fput+0xe/0x10
[ 50.584263] [<ffffffff8107271d>] task_work_run+0x8d/0xc0
[ 50.584263] [<ffffffff8100390f>] do_notify_resume+0x4f/0x60
[ 50.584263] [<ffffffff8155cf6c>] int_signal+0x12/0x17
[...]

That 'spinlock already unlocked' message is also interesting. But even
better, enabling slab debugging (CONFIG_SLUB_DEBUG_ON=y) makes that
list_del corruption warning a GPF:

[ 21.124241] general protection fault: 0000 [#1] SMP
[ 21.128193] Modules linked in: ipv6 pcspkr serio_raw microcode virtio_net virtio_pci virtio_ring virtio sr_mod cdrom
[ 21.144249] CPU: 1 PID: 221 Comm: epoll_bug Not tainted 4.2.0 #75
[ 21.144249] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[ 21.144249] task: ffff88001fa82b80 ti: ffff880018894000 task.ti: ffff880018894000
[ 21.144249] RIP: 0010:[<ffffffff8109def0>] [<ffffffff8109def0>] __lock_acquire+0x240/0x1800
[ 21.144249] RSP: 0018:ffff880018897c98 EFLAGS: 00010002
[ 21.144249] RAX: 0000000000000000 RBX: 6b6b6b6b6b6b6b6b RCX: 0000000000000000
[ 21.144249] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88001f8b1c18
[ 21.144249] RBP: ffff880018897d28 R08: 0000000000000001 R09: 0000000000000001
[ 21.144249] R10: 0000000000000000 R11: ffff88001f8b1c18 R12: 0000000000000000
[ 21.144249] R13: 0000000000000000 R14: 0000000000000000 R15: ffff88001fa82b80
[ 21.144249] FS: 00007f0c87e5d700(0000) GS:ffff88001eb00000(0000) knlGS:0000000000000000
[ 21.144249] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 21.144249] CR2: 00007f0c87e5cff8 CR3: 000000001899c000 CR4: 00000000001406e0
[ 21.144249] Stack:
[ 21.144249] ffff880018897cb8 ffffffff8109d94b ffff880018b6fdb0 ffff88001e3d3b80
[ 21.144249] ffff880018897cc8 ffff88001fa83250 0000000000000002 0000000000000000
[ 21.144249] 0000000000000001 ffff88001fa82b80 ffff880018897d88 ffffffff8109e0f7
[ 21.144249] Call Trace:
[ 21.144249] [<ffffffff8109d94b>] ? trace_hardirqs_on_caller+0x14b/0x1e0
[ 21.144249] [<ffffffff8109e0f7>] ? __lock_acquire+0x447/0x1800
[ 21.144249] [<ffffffff8109fdd7>] lock_acquire+0xc7/0x260
[ 21.144249] [<ffffffff81094d2d>] ? remove_wait_queue+0x1d/0x40
[ 21.144249] [<ffffffff8155c373>] _raw_spin_lock_irqsave+0x43/0x60
[ 21.144249] [<ffffffff81094d2d>] ? remove_wait_queue+0x1d/0x40
[ 21.144249] [<ffffffff81094d2d>] remove_wait_queue+0x1d/0x40
[ 21.144249] [<ffffffff811bc898>] ep_unregister_pollwait.isra.6+0x58/0x1a0
[ 21.144249] [<ffffffff811bc8e9>] ? ep_unregister_pollwait.isra.6+0xa9/0x1a0
[ 21.144249] [<ffffffff811bca02>] ep_remove+0x22/0x110
[ 21.144249] [<ffffffff811be28b>] SyS_epoll_ctl+0x62b/0xf70
[ 21.144249] [<ffffffff81000f44>] ? lockdep_sys_exit_thunk+0x12/0x14
[ 21.144249] [<ffffffff8155cd97>] entry_SYSCALL_64_fastpath+0x12/0x6f
[ 21.144249] Code: 49 81 3b c0 9e c4 81 b8 00 00 00 00 44 0f 44 c0 41 83 fe 01 0f 87 39 fe ff ff 44 89 f0 49 8b 5c c3 08 48 85 db 0f 84 28 fe ff ff <f0> ff 83 98 01 00 00 45 8b b7 a0 06 00 00 41 83 fe 2f 76 10 44
[ 21.144249] RIP [<ffffffff8109def0>] __lock_acquire+0x240/0x1800
[ 21.144249] RSP <ffff880018897c98>
[ 21.144249] ---[ end trace 7136cfe3b6480f34 ]---

The slab poisoning hits, as can be seen by the pattern in RBX which the
faulting instruction is using as a memory operand. So this is probably a
use-after-free bug -- or, more likely, an object that better had been
freed via rcu semantics.

I looked at the epoll code really hard and concluded I don't understand
it at all. I've added a few calls to synchroize_rcu() and changed direct
pointer assignments to rcu_assign_pointer() in places I thought would
need them. But that only made the race happen less often, not cured it.

This is what I did:
- add a call to synchroize_rcu() in eventpoll_release_file() after
taking the epmutex. It's traversing an rcu list, after all.
- make the NULL pointer assignment of whead in ep_poll_callback() an
rcu_assign_pointer() assignment and call synchronize_rcu() afterwards.
It's also an rcu pointer that should be assigned that way, no?

But, apparently, all irrelevant.

I had the same luck while staring at the af_unix code. I've added a few
unix_state_lock() / -unlock() calls to places I though would need them
to be able to reliably test / set socket flags and the peer member of
struct unix_sock. But again, that only made the bug happen less often.

What I did:
- take other's unix_state_lock() in unix_dgram_disconnected() for
testing the flags and signaling the error
- moving the 'unix_peer(sk) = NULL' assignment to the section that holds
the unix_state_lock() in unix_release_sock(). This ensures others will
see changes to the peer pointer atomically -- when themselves making
use of the lock only, of course.
- in unix_dgram_poll() avoid calling sock_poll_wait() if the peer is
already SOCK_DEAD or RCV_SHUTDOWN. For testing the flags, other's
unix_state_lock() is taken.

The last one is, in fact, the old patch, extended by the
'(other->sk_shutdown & RCV_SHUTDOWN)' test. But Eric already noted back
then, it might be an expensive lock to take here.

Anyways, none of the above changes fixed the issue. I suspect it's
related to the double usage of the peer_wq waitQ in unix_dgram_sendmsg()
(via unix_wait_for_peer()) and unix_dgram_poll() (via sock_poll_wait()).
But I might be totally wrong, here.

However, it's definitely the second sock_poll_wait() call in
unix_dgram_poll() that triggers the issue. Commenting out the call --
thereby obviously breaking its functionality -- gets me rid of the list
debug splat and the GFP. But that's not a fix either. So I'm asking for
help.


Regards,
Mathias
/* use-after-free in poll routine of AF_UNIX sockets, triggerable using epoll
*
* ..intruduced in 3c73419c09 "af_unix: fix 'poll for write'/ connected DGRAM
* sockets" (v2.6.26-rc7)
*
* $ gcc -pthread -o epoll_bug epoll_bug.c
*
* - minipli
*/
#include <sys/socket.h>
#include <sys/epoll.h>
#include <sys/un.h>
#include <pthread.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>


static long fd_max;
static int ep = -1;


static int get_fd(void) {
int fd;

for (;;) {
fd = rand() % fd_max;

if (fd > 2 && fd != ep)
break;
}

return fd;
}


static void *opener(void *ptr) {
sleep(1);

for (;;) {
if (rand() % 2) {
struct sockaddr_un sa = {
.sun_family = AF_UNIX,
.sun_path = "\0epool_bug-",
};
int sock = socket(AF_UNIX, SOCK_DGRAM, 0);
int err;

/* take a short nap when there are no more fds left so closer() can
* catch up */
if (sock < 0) {
usleep(1);

continue;
}

/* ensure the write won't block */
fcntl(sock, F_SETFL, fcntl(sock, F_GETFL, 0) | O_NONBLOCK);

sa.sun_path[11] = rand() % 26 + 'A';
if (rand() % 2)
err = connect(sock, (struct sockaddr *) &sa, sizeof(sa));
else
err = bind(sock, (struct sockaddr *) &sa, sizeof(sa));

if (err)
close(sock);
} else {
static const char dot[] = { [0 ... 1023] = '.' };

write(get_fd(), dot, rand() % 2 ? 1 : sizeof(dot));
}
}

return ptr;
}


static void *closer(void *ptr) {
int miss = 0;

sleep(1);

for (;;) {
errno = 0;
close(get_fd());

/* take a short nap when we're hitting invalid fds 5 times in a row so
* opener() can catch up */
if (errno == EBADF && ++miss >= 5) {
usleep(10);
miss = 0;
} else if (errno == 0) {
miss = 0;
}
}

return ptr;
}


static void *ep_add(void *ptr) {
sleep(1);

for (;;) {
int fd = get_fd();
struct epoll_event ev = {
.events = EPOLLIN | EPOLLOUT,
.data.fd = fd,
};

if (epoll_ctl(ep, EPOLL_CTL_ADD, fd, &ev) < 0 && errno == ENOSPC)
usleep(1);
}

return ptr;
}


static void *ep_del(void *ptr) {
sleep(1);

for (;;)
epoll_ctl(ep, EPOLL_CTL_DEL, get_fd(), NULL);

return ptr;
}


int main(void) {
pthread_t thread[4];
int i;

signal(SIGPIPE, SIG_IGN);

ep = epoll_create(42); /* use epoll_create() for older kernels */
if (ep < 0) {
fprintf(stderr, "err: epoll_create1() failed (%s)\n", strerror(errno));

return 1;
}

fd_max = sysconf(_SC_OPEN_MAX);
if (pthread_create(&thread[0], NULL, opener, NULL) ||
pthread_create(&thread[1], NULL, closer, NULL) ||
pthread_create(&thread[2], NULL, ep_add, NULL) ||
pthread_create(&thread[3], NULL, ep_del, NULL))
{
fprintf(stderr, "err: failed to start all threads!\n");

return 1;
}

/* XXX: pthread_cancel() all threads on termination request */

for (i = 0; i < 4; i++)
pthread_join(thread[i], NULL);

return 0;
}
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