Re: possible deadlock in seq_read

[Eric Biggers]

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.

Eric