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Message-ID: <CA+rthh8smc0fxL5sEhMpKRQgzMi=BCVyxKc=O1JWesHGwYW8XA@mail.gmail.com>
Date: Tue, 29 Sep 2015 20:09:49 +0200
From: Mathias Krause <minipli@...glemail.com>
To: netdev@...r.kernel.org,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: Eric Wong <normalperson@...t.net>,
Eric Dumazet <eric.dumazet@...il.com>,
Rainer Weikusat <rweikusat@...ileactivedefense.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Davide Libenzi <davidel@...ilserver.org>,
Davidlohr Bueso <dave@...olabs.net>,
Olivier Mauras <olivier@...ras.ch>,
PaX Team <pageexec@...email.hu>,
Jason Baron <jbaron@...mai.com>
Subject: Re: List corruption on epoll_ctl(EPOLL_CTL_DEL) an AF_UNIX socket
On 14 September 2015 at 04:39, Eric Wong <normalperson@...t.net> wrote:
> +cc Jason Baron since he might be able to provide more insight into
> epoll.
>
> Mathias Krause <minipli@...glemail.com> wrote:
>> 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;
>> }
Ping. This still triggers on v4.3-rc3.
Any epoll expert around to have a look at this?
Regards,
Mathias
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