syzbot

random: sshd: uninitialized urandom read (32 bytes read, 98 bits of entropy available)
random: sshd: uninitialized urandom read (32 bytes read, 104 bits of entropy available)
random: sshd: uninitialized urandom read (32 bytes read, 110 bits of entropy available)

======================================================
[ INFO: possible circular locking dependency detected ]
4.4.120-gd63fdf6 #28 Not tainted
-------------------------------------------------------
syzkaller221042/3785 is trying to acquire lock:
 (&mm->mmap_sem){++++++}, at: [<ffffffff81495684>] __might_fault+0xe4/0x1d0 mm/memory.c:3809

but task is already holding lock:
 (ashmem_mutex){+.+.+.}, at: [<ffffffff82c628a7>] ashmem_pin_unpin drivers/staging/android/ashmem.c:701 [inline]
 (ashmem_mutex){+.+.+.}, at: [<ffffffff82c628a7>] ashmem_ioctl+0x367/0xfa0 drivers/staging/android/ashmem.c:778

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       [<ffffffff8123d7ce>] lock_acquire+0x15e/0x460 kernel/locking/lockdep.c:3592
       [<ffffffff8376a39b>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8376a39b>] mutex_lock_nested+0xbb/0x850 kernel/locking/mutex.c:621
       [<ffffffff82c61463>] ashmem_mmap+0x53/0x400 drivers/staging/android/ashmem.c:366
       [<ffffffff814b0e4f>] mmap_region+0x94f/0x1250 mm/mmap.c:1664
       [<ffffffff814b1c4d>] do_mmap+0x4fd/0x9d0 mm/mmap.c:1441
       [<ffffffff814700ce>] do_mmap_pgoff include/linux/mm.h:1915 [inline]
       [<ffffffff814700ce>] vm_mmap_pgoff+0x16e/0x1c0 mm/util.c:296
       [<ffffffff814afe1f>] SYSC_mmap_pgoff mm/mmap.c:1491 [inline]
       [<ffffffff814afe1f>] SyS_mmap_pgoff+0x33f/0x560 mm/mmap.c:1449
       [<ffffffff8101beb6>] SYSC_mmap arch/x86/kernel/sys_x86_64.c:95 [inline]
       [<ffffffff8101beb6>] SyS_mmap+0x16/0x20 arch/x86/kernel/sys_x86_64.c:86
       [<ffffffff8377395f>] entry_SYSCALL_64_fastpath+0x1c/0x98

       [<ffffffff8123ab2f>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff8123ab2f>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff8123ab2f>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff8123ab2f>] __lock_acquire+0x371f/0x4b50 kernel/locking/lockdep.c:3213
       [<ffffffff8123d7ce>] lock_acquire+0x15e/0x460 kernel/locking/lockdep.c:3592
       [<ffffffff814956ea>] __might_fault+0x14a/0x1d0 mm/memory.c:3810
       [<ffffffff82c628f4>] copy_from_user arch/x86/include/asm/uaccess.h:724 [inline]
       [<ffffffff82c628f4>] ashmem_pin_unpin drivers/staging/android/ashmem.c:706 [inline]
       [<ffffffff82c628f4>] ashmem_ioctl+0x3b4/0xfa0 drivers/staging/android/ashmem.c:778
       [<ffffffff81559daa>] vfs_ioctl fs/ioctl.c:43 [inline]
       [<ffffffff81559daa>] do_vfs_ioctl+0x7aa/0xee0 fs/ioctl.c:607
       [<ffffffff8155a56f>] SYSC_ioctl fs/ioctl.c:622 [inline]
       [<ffffffff8155a56f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:613
       [<ffffffff8377395f>] entry_SYSCALL_64_fastpath+0x1c/0x98

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(ashmem_mutex);
                               lock(&mm->mmap_sem);
                               lock(ashmem_mutex);
  lock(&mm->mmap_sem);

 *** DEADLOCK ***

1 lock held by syzkaller221042/3785:
 #0:  (ashmem_mutex){+.+.+.}, at: [<ffffffff82c628a7>] ashmem_pin_unpin drivers/staging/android/ashmem.c:701 [inline]
 #0:  (ashmem_mutex){+.+.+.}, at: [<ffffffff82c628a7>] ashmem_ioctl+0x367/0xfa0 drivers/staging/android/ashmem.c:778

stack backtrace:
CPU: 1 PID: 3785 Comm: syzkaller221042 Not tainted 4.4.120-gd63fdf6 #28
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 0000000000000000 2b4b7b174cad59c5 ffff8801d91bf9b8 ffffffff81d0408d
 ffffffff851a0010 ffffffff851a0010 ffffffff851bda40 ffff8800bbb5a0f8
 ffff8800bbb59800 ffff8801d91bfa00 ffffffff81233ba1 ffff8800bbb5a0f8
Call Trace:
 [<ffffffff81d0408d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81d0408d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff81233ba1>] print_circular_bug+0x271/0x310 kernel/locking/lockdep.c:1226
 [<ffffffff8123ab2f>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
 [<ffffffff8123ab2f>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
 [<ffffffff8123ab2f>] validate_chain kernel/locking/lockdep.c:2144 [inline]
 [<ffffffff8123ab2f>] __lock_acquire+0x371f/0x4b50 kernel/locking/lockdep.c:3213
 [<ffffffff8123d7ce>] lock_acquire+0x15e/0x460 kernel/locking/lockdep.c:3592
 [<ffffffff814956ea>] __might_fault+0x14a/0x1d0 mm/memory.c:3810
 [<ffffffff82c628f4>] copy_from_user arch/x86/include/asm/uaccess.h:724 [inline]
 [<ffffffff82c628f4>] ashmem_pin_unpin drivers/staging/android/ashmem.c:706 [inline]
 [<ffffffff82c628f4>] ashmem_ioctl+0x3b4/0xfa0 drivers/staging/android/ashmem.c:778
 [<ffffffff81559daa>] vfs_ioctl fs/ioctl.c:43 [inline]
 [<ffffffff81559daa>] do_vfs_ioctl+0x7aa/0xee0 fs/ioctl.c:607
 [<ffffffff814afbb8>] ? SyS_mmap_pgoff+0x