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Subjectpossible deadlock in shmem_fallocate (2)
From
Hello,

syzbot found the following crash on:

HEAD commit: 4110b42356f3 Add linux-next specific files for 20180810
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1411d6e2400000
kernel config: https://syzkaller.appspot.com/x/.config?x=1d80606e3795a4f5
dashboard link: https://syzkaller.appspot.com/bug?extid=4b8b031b89e6b96c4b2e
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=175052f8400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11873622400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+4b8b031b89e6b96c4b2e@syzkaller.appspotmail.com

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
4.18.0-rc8-next-20180810+ #36 Not tainted
------------------------------------------------------
syz-executor900/4483 is trying to acquire lock:
00000000d2bfc8fe (&sb->s_type->i_mutex_key#9){++++}, at: inode_lock
include/linux/fs.h:765 [inline]
00000000d2bfc8fe (&sb->s_type->i_mutex_key#9){++++}, at:
shmem_fallocate+0x18b/0x12e0 mm/shmem.c:2602

but task is already holding lock:
0000000025208078 (ashmem_mutex){+.+.}, at: ashmem_shrink_scan+0xb4/0x630
drivers/staging/android/ashmem.c:448

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:925 [inline]
__mutex_lock+0x171/0x1700 kernel/locking/mutex.c:1073
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1088
ashmem_mmap+0x55/0x520 drivers/staging/android/ashmem.c:361
call_mmap include/linux/fs.h:1844 [inline]
mmap_region+0xf27/0x1c50 mm/mmap.c:1762
do_mmap+0xa10/0x1220 mm/mmap.c:1535
do_mmap_pgoff include/linux/mm.h:2298 [inline]
vm_mmap_pgoff+0x213/0x2c0 mm/util.c:357
ksys_mmap_pgoff+0x4da/0x660 mm/mmap.c:1585
__do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
__se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
__x64_sys_mmap+0xe9/0x1b0 arch/x86/kernel/sys_x86_64.c:91
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #1 (&mm->mmap_sem){++++}:
__might_fault+0x155/0x1e0 mm/memory.c:4568
_copy_to_user+0x30/0x110 lib/usercopy.c:25
copy_to_user include/linux/uaccess.h:155 [inline]
filldir+0x1ea/0x3a0 fs/readdir.c:196
dir_emit_dot include/linux/fs.h:3464 [inline]
dir_emit_dots include/linux/fs.h:3475 [inline]
dcache_readdir+0x13a/0x620 fs/libfs.c:193
iterate_dir+0x48b/0x5d0 fs/readdir.c:51
__do_sys_getdents fs/readdir.c:231 [inline]
__se_sys_getdents fs/readdir.c:212 [inline]
__x64_sys_getdents+0x29f/0x510 fs/readdir.c:212
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&sb->s_type->i_mutex_key#9){++++}:
lock_acquire+0x1e4/0x540 kernel/locking/lockdep.c:3924
down_write+0x8f/0x130 kernel/locking/rwsem.c:70
inode_lock include/linux/fs.h:765 [inline]
shmem_fallocate+0x18b/0x12e0 mm/shmem.c:2602
ashmem_shrink_scan+0x236/0x630 drivers/staging/android/ashmem.c:455
ashmem_ioctl+0x3ae/0x13a0 drivers/staging/android/ashmem.c:797
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:501 [inline]
do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685
ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702
__do_sys_ioctl fs/ioctl.c:709 [inline]
__se_sys_ioctl fs/ioctl.c:707 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
&sb->s_type->i_mutex_key#9 --> &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#9);

*** DEADLOCK ***

1 lock held by syz-executor900/4483:
#0: 0000000025208078 (ashmem_mutex){+.+.}, at:
ashmem_shrink_scan+0xb4/0x630 drivers/staging/android/ashmem.c:448

stack backtrace:
CPU: 1 PID: 4483 Comm: syz-executor900 Not tainted
4.18.0-rc8-next-20180810+ #36
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
print_circular_bug.isra.37.cold.58+0x1bd/0x27d
kernel/locking/lockdep.c:1227
check_prev_add kernel/locking/lockdep.c:1867 [inline]
check_prevs_add kernel/locking/lockdep.c:1980 [inline]
validate_chain kernel/locking/lockdep.c:2421 [inline]
__lock_acquire+0x3449/0x5020 kernel/locking/lockdep.c:3435
lock_acquire+0x1e4/0x540 kernel/locking/lockdep.c:3924
down_write+0x8f/0x130 kernel/locking/rwsem.c:70
inode_lock include/linux/fs.h:765 [inline]
shmem_fallocate+0x18b/0x12e0 mm/shmem.c:2602
ashmem_shrink_scan+0x236/0x630 drivers/staging/android/ashmem.c:455
ashmem_ioctl+0x3ae/0x13a0 drivers/staging/android/ashmem.c:797
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:501 [inline]
do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685
ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702
__do_sys_ioctl fs/ioctl.c:709 [inline]
__se_sys_ioctl fs/ioctl.c:707 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440099
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff3613dbf8 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440099
RDX: 00000


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
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