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From: Kees Cook <keescook@chromium.org>
To: Eric Biggers <ebiggers3@gmail.com>
Cc: syzbot
	<bot+e93a80c1bb7c5c56e522461c149f8bf55eab1b2b@syzkaller.appspotmail.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	syzkaller-bugs@googlegroups.com,
	Al Viro <viro@zeniv.linux.org.uk>,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: possible deadlock in seq_read
Date: Mon, 27 Aug 2018 11:20:12 -0700
Message-ID: <CAGXu5jLcuE-h9V04fTtoSfz9mZdVeH1zfJOpRncK2NZkv2gsnQ@mail.gmail.com> (raw)
In-Reply-To: <20171212220647.GJ185376@gmail.com>

On Tue, Dec 12, 2017 at 2:06 PM, Eric Biggers <ebiggers3@gmail.com> 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?)

-Kees

-- 
Kees Cook
Pixel Security

  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <001a113f711ae2110c055f45acb8@google.com>
2017-12-02  8:12 ` syzbot
2017-12-03 22:02 ` syzbot
2017-12-12 22:06 ` Eric Biggers
2017-12-27 18:30   ` Dmitry Vyukov
2018-08-27 18:20   ` Kees Cook [this message]
2018-08-27 20:57     ` Dmitry Vyukov
2019-01-21 10:14       ` [PATCH] fs: Allow opening only regular files during execve() Tetsuo Handa
2019-01-21 20:24         ` Kees Cook
2019-01-21 21:18         ` Al Viro
2019-01-22  0:50           ` [PATCH v2] " Tetsuo Handa
2019-01-28 23:42             ` Andrew Morton
2019-02-12  2:01               ` Tetsuo Handa
2019-02-12  5:26               ` Tetsuo Handa
2019-02-19  9:51                 ` Tetsuo Handa
2019-01-22  0:51           ` [PATCH] " Kees Cook

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