Re: possible deadlock in seq_read

[Dmitry Vyukov]

On Mon, Aug 27, 2018 at 11:20 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, Dec 12, 2017 at 2:06 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
>> On Fri, Dec 01, 2017 at 03:29:01AM -0800, syzbot wrote:
>>> Hello,
>>>
>>> syzkaller hit the following crash on
>>> df8ba95c572a187ed2aa7403e97a7a7f58c01f00
>>> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
>>> compiler: gcc (GCC) 7.1.1 20170620
>>> .config is attached
>>> Raw console output is attached.
>>>
>>> Unfortunately, I don't have any reproducer for this bug yet.
>>>
>>>
>>>
>>> ======================================================
>>> WARNING: possible circular locking dependency detected
>>> 4.15.0-rc1+ #202 Not tainted
>>> ------------------------------------------------------
>>> syz-executor4/26476 is trying to acquire lock:
>>>  (&p->lock){+.+.}, at: [<0000000040185b66>] seq_read+0xd5/0x13d0
>>> fs/seq_file.c:165
>>>
>>> but task is already holding lock:
>>>  (&pipe->mutex/1){+.+.}, at: [<00000000c644bcdc>] pipe_lock_nested
>>> fs/pipe.c:67 [inline]
>>>  (&pipe->mutex/1){+.+.}, at: [<00000000c644bcdc>]
>>> pipe_lock+0x56/0x70 fs/pipe.c:75
>>>
>>> which lock already depends on the new lock.
>>>
>>>
>>> the existing dependency chain (in reverse order) is:
>>>
>>> -> #2 (&pipe->mutex/1){+.+.}:
>>>        lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:4004
>>>        __mutex_lock_common kernel/locking/mutex.c:756 [inline]
>>>        __mutex_lock+0x16f/0x1a80 kernel/locking/mutex.c:893
>>>        mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
>>>        __pipe_lock fs/pipe.c:88 [inline]
>>>        fifo_open+0x15c/0xa40 fs/pipe.c:916
>>>        do_dentry_open+0x682/0xd70 fs/open.c:752
>>>        vfs_open+0x107/0x230 fs/open.c:866
>>>        do_last fs/namei.c:3379 [inline]
>>>        path_openat+0x1157/0x3530 fs/namei.c:3519
>>>        do_filp_open+0x25b/0x3b0 fs/namei.c:3554
>>>        do_open_execat+0x1b9/0x5c0 fs/exec.c:849
>>>        do_execveat_common.isra.30+0x90c/0x23c0 fs/exec.c:1741
>>>        do_execveat fs/exec.c:1859 [inline]
>>>        SYSC_execveat fs/exec.c:1940 [inline]
>>>        SyS_execveat+0x4f/0x60 fs/exec.c:1932
>>>        do_syscall_64+0x26c/0x920 arch/x86/entry/common.c:285
>>>        return_from_SYSCALL_64+0x0/0x75
>>>
>>> -> #1 (&sig->cred_guard_mutex){+.+.}:
>>>        lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:4004
>>>        __mutex_lock_common kernel/locking/mutex.c:756 [inline]
>>>        __mutex_lock+0x16f/0x1a80 kernel/locking/mutex.c:893
>>>        mutex_lock_killable_nested+0x16/0x20 kernel/locking/mutex.c:923
>>>        do_io_accounting+0x1c2/0xf50 fs/proc/base.c:2682
>>>        proc_tid_io_accounting+0x1f/0x30 fs/proc/base.c:2725
>>>        proc_single_show+0xf8/0x170 fs/proc/base.c:744
>>>        seq_read+0x385/0x13d0 fs/seq_file.c:234
>>>        __vfs_read+0xef/0xa00 fs/read_write.c:411
>>>        vfs_read+0x124/0x360 fs/read_write.c:447
>>>        SYSC_read fs/read_write.c:573 [inline]
>>>        SyS_read+0xef/0x220 fs/read_write.c:566
>>>        entry_SYSCALL_64_fastpath+0x1f/0x96
>>>
>>
>> So the problem with all these deadlocks involving pipe->mutex and
>> sig->cred_guard_mutex is that execve() ranks pipe->mutex below
>> sig->cred_guard_mutex when it tries to open a fifo, whereas reading or writing
>> some of the /proc files result in ->cred_guard_mutex being taken which may be
>> underneath pipe->mutex from splice().  Here's a program which causes an actual
>> deadlock using this bug (in addition to reproducing the lockdep report):
>>
>>         #define _GNU_SOURCE
>>         #include <fcntl.h>
>>         #include <pthread.h>
>>         #include <sys/stat.h>
>>         #include <unistd.h>
>>
>>         static void *exec_thread(void *_arg)
>>         {
>>                 for (;;)
>>                         execl("fifo", "fifo", NULL);
>>         }
>>
>>         int main()
>>         {
>>                 int readend, writeend;
>>                 int syscallfd;
>>                 pthread_t t;
>>
>>                 mknod("fifo", 0777|S_IFIFO, 0);
>>                 readend = open("fifo", O_RDONLY|O_NONBLOCK);
>>                 writeend = open("fifo", O_WRONLY);
>>                 syscallfd = open("/proc/self/syscall", O_RDONLY);
>>
>>                 pthread_create(&t, NULL, exec_thread, NULL);
>>
>>                 for (;;) {
>>                         char buffer[16];
>>                         loff_t off_in = 0;
>>                         splice(syscallfd, &off_in, writeend, NULL, 16, 0);
>>                         read(readend, buffer, 16);
>>                 }
>>         }
>>
>> I'm not sure what the fix will be.  Maybe the proc handlers should take a
>> different lock instead of cred_guard_mutex.  Or perhaps execve should check that
>> the file is a regular file before it attempts to open it.
>
> This cleaner reproducer still generates the lockdep warning (but I can
> ctrl-C out of it without leaving behind a zombie), but I see that
> syzbot isn't seeing this any more. Why did it stop? (And can we feed a
> reproducer in to syzbot?)
>
> Was this creating an uninterruptible deadlock before? (Perhaps
> something did change here?)


Hi,

This never was an uninterruptible deadlock. This is a lockdep
"potential deadlock". For the deadlock to actually happen you need
very precise thread interleaving when 2 threads lock 2 different locks
first and then try to lock them the other way around.