As the documentation for kthread_stop() says, "if threadfn() may call
do_exit() itself, the caller must ensure task_struct can't go away".
dm-crypt does not ensure this and therefore crashes when crypt_dtr()
calls kthread_stop(). The crash is trivially reproducible by adding a
delay before the call to kthread_stop() and just opening and closing a
dm-crypt device.
general protection fault: 0000 [#1] PREEMPT SMP
CPU: 0 PID: 533 Comm: cryptsetup Not tainted 4.8.0-rc7+ #7
task: ffff88003bd0df40 task.stack: ffff8800375b4000
RIP: 0010: kthread_stop+0x52/0x300
Call Trace:
crypt_dtr+0x77/0x120
dm_table_destroy+0x6f/0x120
__dm_destroy+0x130/0x250
dm_destroy+0x13/0x20
dev_remove+0xe6/0x120
? dev_suspend+0x250/0x250
ctl_ioctl+0x1fc/0x530
? __lock_acquire+0x24f/0x1b10
dm_ctl_ioctl+0x13/0x20
do_vfs_ioctl+0x91/0x6a0
? ____fput+0xe/0x10
? entry_SYSCALL_64_fastpath+0x5/0xbd
? trace_hardirqs_on_caller+0x151/0x1e0
SyS_ioctl+0x41/0x70
entry_SYSCALL_64_fastpath+0x1f/0xbd
This problem was introduced by bcbd94ff48 ("dm crypt: fix a possible
hang due to race condition on exit").
Looking at the description of that patch (excerpted below), it seems
like the problem it addresses can be solved by just using
set_current_state instead of __set_current_state, since we obviously
need the memory barrier.
| dm crypt: fix a possible hang due to race condition on exit
|
| A kernel thread executes __set_current_state(TASK_INTERRUPTIBLE),
| __add_wait_queue, spin_unlock_irq and then tests kthread_should_stop().
| It is possible that the processor reorders memory accesses so that
| kthread_should_stop() is executed before __set_current_state(). If
| such reordering happens, there is a possible race on thread
| termination: [...]
So this patch just reverts the aforementioned patch and changes the
__set_current_state(TASK_INTERRUPTIBLE) to set_current_state(...). This
fixes the crash and should also fix the potential hang.
Fixes: bcbd94ff48 ("dm crypt: fix a possible hang due to race condition on exit")
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org # v4.0+
Signed-off-by: Rabin Vincent <rabinv@axis.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
f659b10087