Message-ID: <001a1144d6e854b3c90562668d74@google.com>
Date:   Tue, 09 Jan 2018 22:58:00 -0800
From:   syzbot <syzbot+8ec30bb7bf1a981a2012@...kaller.appspotmail.com>
To:     hughd@...gle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: possible deadlock in shmem_file_llseek

Hello,

syzkaller hit the following crash on  
d476c5334f1dee122534b29639f8d46a85ecbb9d
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.
C reproducer is attached
syzkaller reproducer is attached. See https://goo.gl/kgGztJ
for information about syzkaller reproducers


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Reported-by: syzbot+8ec30bb7bf1a981a2012@...kaller.appspotmail.com
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======================================================
audit: type=1400 audit(1515535006.781:8): avc:  denied  { map } for   
pid=3497 comm="syzkaller303685" path="/dev/ashmem" dev="devtmpfs" ino=178  
scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023  
tcontext=system_u:object_r:device_t:s0 tclass=chr_file permissive=1
WARNING: possible circular locking dependency detected
4.15.0-rc7+ #255 Not tainted
------------------------------------------------------
syzkaller303685/3497 is trying to acquire lock:
  (&sb->s_type->i_mutex_key#11){++++}, at: [<000000006886b3fb>] inode_lock  
include/linux/fs.h:713 [inline]
  (&sb->s_type->i_mutex_key#11){++++}, at: [<000000006886b3fb>]  
shmem_file_llseek+0xef/0x240 mm/shmem.c:2579

but task is already holding lock:
  (ashmem_mutex){+.+.}, at: [<00000000a44304a8>] ashmem_llseek+0x56/0x1f0  
drivers/staging/android/ashmem.c:334

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (ashmem_mutex){+.+.}:
        __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
        ashmem_mmap+0x53/0x410 drivers/staging/android/ashmem.c:370
        call_mmap include/linux/fs.h:1777 [inline]
        mmap_region+0xa99/0x15a0 mm/mmap.c:1705
        do_mmap+0x6c0/0xe00 mm/mmap.c:1483
        do_mmap_pgoff include/linux/mm.h:2217 [inline]
        vm_mmap_pgoff+0x1de/0x280 mm/util.c:333
        SYSC_mmap_pgoff mm/mmap.c:1533 [inline]
        SyS_mmap_pgoff+0x462/0x5f0 mm/mmap.c:1491
        SYSC_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
        SyS_mmap+0x16/0x20 arch/x86/kernel/sys_x86_64.c:91
        entry_SYSCALL_64_fastpath+0x23/0x9a

-> #1 (&mm->mmap_sem){++++}:
        __might_fault+0x13a/0x1d0 mm/memory.c:4529
        _copy_to_user+0x2c/0xc0 lib/usercopy.c:25
        copy_to_user include/linux/uaccess.h:155 [inline]
        filldir+0x1a7/0x320 fs/readdir.c:196
        dir_emit_dot include/linux/fs.h:3374 [inline]
        dir_emit_dots include/linux/fs.h:3385 [inline]
        dcache_readdir+0x12d/0x5e0 fs/libfs.c:192
        iterate_dir+0x1ca/0x530 fs/readdir.c:51
        SYSC_getdents fs/readdir.c:231 [inline]
        SyS_getdents+0x225/0x450 fs/readdir.c:212
        entry_SYSCALL_64_fastpath+0x23/0x9a

-> #0 (&sb->s_type->i_mutex_key#11){++++}:
        lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3914
        down_write+0x87/0x120 kernel/locking/rwsem.c:70
        inode_lock include/linux/fs.h:713 [inline]
        shmem_file_llseek+0xef/0x240 mm/shmem.c:2579
        vfs_llseek+0xa2/0xd0 fs/read_write.c:300
        ashmem_llseek+0xe7/0x1f0 drivers/staging/android/ashmem.c:346
        vfs_llseek fs/read_write.c:300 [inline]
        SYSC_lseek fs/read_write.c:313 [inline]
        SyS_lseek+0xeb/0x170 fs/read_write.c:304
        entry_SYSCALL_64_fastpath+0x23/0x9a

other info that might help us debug this:

Chain exists of:
   &sb->s_type->i_mutex_key#11 --> &mm->mmap_sem --> ashmem_mutex

  Possible unsafe locking scenario:

        CPU0                    CPU1
        ----                    ----
   lock(ashmem_mutex);
                                lock(&mm->mmap_sem);
                                lock(ashmem_mutex);
   lock(&sb->s_type->i_mutex_key#11);

  *** DEADLOCK ***

1 lock held by syzkaller303685/3497:
  #0:  (ashmem_mutex){+.+.}, at: [<00000000a44304a8>]  
ashmem_llseek+0x56/0x1f0 drivers/staging/android/ashmem.c:334

stack backtrace:
CPU: 0 PID: 3497 Comm: syzkaller303685 Not tainted 4.15.0-rc7+ #255
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x257 lib/dump_stack.c:53
  print_circular_bug.isra.37+0x2cd/0x2dc kernel/locking/lockdep.c:1218
  check_prev_add kernel/locking/lockdep.c:1858 [inline]
  check_prevs_add kernel/locking/lockdep.c:1971 [inline]
  validate_chain kernel/locking/lockdep.c:2412 [inline]
  __lock_acquire+0x30a8/0x3e00 kernel/locking/lockdep.c:3426
  lock_acquire+0x1d5/0x580 kernel/locking/lockdep.c:3914
  down_write+0x87/0x120 kernel/locking/rwsem.c:70
  inode_lock include/linux/fs.h:713 [inline]
  shmem_file_llseek+0xef/0x240 mm/shmem.c:2579
  vfs_llseek+0xa2/0xd0 fs/read_write.c:300
  ashmem_llseek+0xe7/0x1f0 drivers/staging/android/ashmem.c:346
  vfs_llseek fs/read_write.c:300 [inline]
  SYSC_lseek fs/read_write.c:313 [inline]
  SyS_lseek+0xeb/0x170 fs/read_write.c:304
  entry_SYSCALL_64_fastpath+0x23/0x9a
RIP: 0033:0x444aa9
RSP: 002b:00007ffd2bec96f8 EFLAGS: 00000217 ORIG_RAX: 0000000000000008
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000444aa9
RDX: 0000000000000003 RSI: fffffffffffffffc RDI: 0000000000000004
RBP: 00000000


---
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