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SubjectRe: possible deadlock in seq_read
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

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