syzbot

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)

======================================================
WARNING: possible circular locking dependency detected
4.14.67+ #1 Not tainted
------------------------------------------------------
syz-executor947/1975 is trying to acquire lock:
 (&sb->s_type->i_mutex_key#11){++++}, at: [<ffffffffb688a759>] inode_lock include/linux/fs.h:713 [inline]
 (&sb->s_type->i_mutex_key#11){++++}, at: [<ffffffffb688a759>] shmem_fallocate+0x149/0xb20 mm/shmem.c:2850

but task is already holding lock:
 (ashmem_mutex){+.+.}, at: [<ffffffffb75ec9b2>] ashmem_shrink_scan+0x52/0x4e0 drivers/staging/android/ashmem.c:440

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+0xf5/0x1480 kernel/locking/mutex.c:893
       ashmem_mmap+0x4c/0x3b0 drivers/staging/android/ashmem.c:369
       call_mmap include/linux/fs.h:1787 [inline]
       mmap_region+0x836/0xfb0 mm/mmap.c:1731
       do_mmap+0x551/0xb80 mm/mmap.c:1509
       do_mmap_pgoff include/linux/mm.h:2167 [inline]
       vm_mmap_pgoff+0x180/0x1d0 mm/util.c:333
       SYSC_mmap_pgoff mm/mmap.c:1559 [inline]
       SyS_mmap_pgoff+0xf8/0x1a0 mm/mmap.c:1517
       do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

-> #1 (&mm->mmap_sem){++++}:
       __might_fault+0x137/0x1b0 mm/memory.c:4519
       _copy_to_user+0x27/0xc0 lib/usercopy.c:25
       copy_to_user include/linux/uaccess.h:155 [inline]
       filldir+0x192/0x340 fs/readdir.c:197
       dir_emit_dot include/linux/fs.h:3380 [inline]
       dir_emit_dots include/linux/fs.h:3391 [inline]
       dcache_readdir+0x12f/0x5d0 fs/libfs.c:192
       iterate_dir+0x19f/0x5e0 fs/readdir.c:52
       SYSC_getdents fs/readdir.c:232 [inline]
       SyS_getdents+0x146/0x270 fs/readdir.c:213
       do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

-> #0 (&sb->s_type->i_mutex_key#11){++++}:
       lock_acquire+0x10f/0x380 kernel/locking/lockdep.c:3991
       down_write+0x34/0x90 kernel/locking/rwsem.c:54
       inode_lock include/linux/fs.h:713 [inline]
       shmem_fallocate+0x149/0xb20 mm/shmem.c:2850
       ashmem_shrink_scan+0x1b6/0x4e0 drivers/staging/android/ashmem.c:447
       ashmem_ioctl+0x2cc/0xe20 drivers/staging/android/ashmem.c:789
       vfs_ioctl fs/ioctl.c:46 [inline]
       file_ioctl fs/ioctl.c:500 [inline]
       do_vfs_ioctl+0x1a0/0x1030 fs/ioctl.c:684
       SYSC_ioctl fs/ioctl.c:701 [inline]
       SyS_ioctl+0x7e/0xb0 fs/ioctl.c:692
       do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
       entry_SYSCALL_64_after_hwframe+0x42/0xb7

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 syz-executor947/1975:
 #0:  (ashmem_mutex){+.+.}, at: [<ffffffffb75ec9b2>] ashmem_shrink_scan+0x52/0x4e0 drivers/staging/android/ashmem.c:440

stack backtrace:
CPU: 0 PID: 1975 Comm: syz-executor947 Not tainted 4.14.67+ #1
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xb9/0x11b lib/dump_stack.c:53
 print_circular_bug.isra.18.cold.43+0x2d3/0x40c kernel/locking/lockdep.c:1258
 check_prev_add kernel/locking/lockdep.c:1901 [inline]
 check_prevs_add kernel/locking/lockdep.c:2018 [inline]
 validate_chain kernel/locking/lockdep.c:2460 [inline]
 __lock_acquire+0x2ff9/0x4320 kernel/locking/lockdep.c:3487
 lock_acquire+0x10f/0x380 kernel/locking/lockdep.c:3991
 down_write+0x34/0x90 kernel/locking/rwsem.c:54
 inode_lock include/linux/fs.h:713 [inline]
 shmem_fallocate+0x149/0xb20 mm/shmem.c:2850
 ashmem_shrink_scan+0x1b6/0x4e0 drivers/staging/android/ashmem.c:447
 ashmem_ioctl+0x2cc/0xe20 drivers/staging/android/ashmem.c:789
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x1a0/0x1030 fs/ioctl.c:684
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x7e/0xb0 fs/ioctl.c:692
 do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
 entry_SYSCALL_64_after_hwframe+0