Date:	Fri, 20 Nov 2015 16:01:30 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	fuse-devel@...ts.sourceforge.net,
	Miklos Szeredi <miklos@...redi.hu>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: deadlock during fuseblk shutdown

Hello,

I've hit the following deadlock on
8005c49d9aea74d382f474ce11afbbc7d7130bec (Nov 15).
I know that fuse docs warn about deadlocks and this can happen only
under root because of the mount call, but maybe there is still
something to fix. The first suspicious thing is that do_exit in daemon
sends a fuse request to daemon, which it cannot answer obviously. The
second thing is that the hanged processes are unkillable and
/sys/fs/fuse/connections/ is empty, so I don't see any way to repair
it.

The program is:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <syscall.h>
#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <signal.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mount.h>
#include <linux/fuse.h>
#include <sched.h>

#define CLONE_NEWNS     0x00020000

int unshare(int flags);

struct msg {
        struct fuse_out_header hdr;
        struct fuse_poll_out data;
};

void work(const char *bklname)
{
        unshare(CLONE_NEWNS);
        int fd = open("/dev/fuse", O_RDWR);
        if (fd == -1)
                exit(printf("open /dev/fuse failed: %d\n", errno));
        if (mknod(bklname, S_IFBLK, makedev(7, 199)))
                exit(printf("mknod failed: %d\n", errno));
        char buf[4<<10];
        sprintf(buf, "fd=%d,user_id=%d,group_id=%d,rootmode=0%o", fd,
getuid(), getgid(), 0xc000);
        if (mount(bklname, bklname, "fuseblk", 0x1000080, buf))
                exit(printf("mount failed: %d\n", errno));
        read(fd, buf, sizeof(buf));
        struct msg m;
        memset(&m, 0, sizeof(m));
        m.hdr.len = sizeof(m);
        m.hdr.error = 0;
        m.hdr.unique = 1;
        m.data.revents = 7;
        write(fd, &m, sizeof(m));
        exit(1);
}

int main()
{
        int pid1 = fork();
        if (pid1 == 0)
                work("./fuseblk1");
        sleep(1);
        kill(pid1, SIGKILL);
        int pid2 = fork();
        if (pid2 == 0)
                work("./fuseblk2");
        sleep(1);
        kill(pid2, SIGKILL);
        return 0;
}

It results in two hanged processes:

root# cat /proc/2769/stack
[<ffffffff815399a8>] request_wait_answer+0x308/0x4c0 fs/fuse/dev.c:436
[<ffffffff8153a36a>] __fuse_request_send+0xaa/0x100 fs/fuse/dev.c:496
[<ffffffff8153a40b>] fuse_request_send+0x4b/0x50 fs/fuse/dev.c:509
[<     inline     >] fuse_send_destroy fs/fuse/inode.c:367
[<ffffffff815525b9>] fuse_put_super+0xa9/0x180 fs/fuse/inode.c:382
[<ffffffff812daf8b>] generic_shutdown_super+0xcb/0x1d0 fs/super.c:427
[<ffffffff812db532>] kill_block_super+0x52/0xb0 fs/super.c:1047
[<ffffffff8155229b>] fuse_kill_sb_blk+0x6b/0x80 fs/fuse/inode.c:1214
[<ffffffff812db7e0>] deactivate_locked_super+0x60/0xa0 fs/super.c:301
[<ffffffff812dbe64>] deactivate_super+0x94/0xb0 fs/super.c:332
[<ffffffff8131490b>] cleanup_mnt+0x6b/0xd0 fs/namespace.c:1067
[<ffffffff813149d6>] __cleanup_mnt+0x16/0x20 fs/namespace.c:1074
[<ffffffff810d19b1>] task_work_run+0xe1/0x110 kernel/task_work.c:115
[<     inline     >] exit_task_work include/linux/task_work.h:21
[<ffffffff8109c9ef>] do_exit+0x55f/0x1690 kernel/exit.c:748
[<ffffffff810a0057>] do_group_exit+0xa7/0x190 kernel/exit.c:878
[<     inline     >] SYSC_exit_group kernel/exit.c:889
[<ffffffff810a015d>] SyS_exit_group+0x1d/0x20 kernel/exit.c:887
[<ffffffff821d6311>] entry_SYSCALL_64_fastpath+0x31/0x9a
arch/x86/entry/entry_64.S:187

root# cat /proc/2772/stack
[<ffffffff81676783>] call_rwsem_down_write_failed+0x13/0x20
arch/x86/lib/rwsem.S:99
[<ffffffff812dbb60>] grab_super+0x40/0xf0 fs/super.c:355
[<ffffffff812dc782>] sget+0x492/0x630 fs/super.c:468
[<ffffffff812dcc3a>] mount_bdev+0x15a/0x340 fs/super.c:991
[<ffffffff815522e4>] fuse_mount_blk+0x34/0x40 fs/fuse/inode.c:1201
[<ffffffff812ddc39>] mount_fs+0x69/0x200 fs/super.c:1123
[<ffffffff8131517a>] vfs_kern_mount+0x7a/0x200 fs/namespace.c:948
[<     inline     >] do_new_mount fs/namespace.c:2409
[<ffffffff81319d5b>] do_mount+0x40b/0x1a80 fs/namespace.c:2725
[<     inline     >] SYSC_mount fs/namespace.c:2915
[<ffffffff8131ba4a>] SyS_mount+0x10a/0x1a0 fs/namespace.c:2893
[<ffffffff821d6311>] entry_SYSCALL_64_fastpath+0x31/0x9a
arch/x86/entry/entry_64.S:187


The first process holds a superblock mutex, so the whole system
becomes unstable. For example, sync invocations also hang in D state.

Is this intentional? Or there is something to fix?

Thank you
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