kernel/kmod: fix use-after-free of the sub_info structure

kernel/kmod: fix use-after-free of the sub_info structure

Found this in the message log on a s390 system:

    BUG kmalloc-192 (Not tainted): Poison overwritten
    Disabling lock debugging due to kernel taint
    INFO: 0x00000000684761f4-0x00000000684761f7. First byte 0xff instead of 0x6b
    INFO: Allocated in call_usermodehelper_setup+0x70/0x128 age=71 cpu=2 pid=648
     __slab_alloc.isra.47.constprop.56+0x5f6/0x658
     kmem_cache_alloc_trace+0x106/0x408
     call_usermodehelper_setup+0x70/0x128
     call_usermodehelper+0x62/0x90
     cgroup_release_agent+0x178/0x1c0
     process_one_work+0x36e/0x680
     worker_thread+0x2f0/0x4f8
     kthread+0x10a/0x120
     kernel_thread_starter+0x6/0xc
     kernel_thread_starter+0x0/0xc
    INFO: Freed in call_usermodehelper_exec+0x110/0x1b8 age=71 cpu=2 pid=648
     __slab_free+0x94/0x560
     kfree+0x364/0x3e0
     call_usermodehelper_exec+0x110/0x1b8
     cgroup_release_agent+0x178/0x1c0
     process_one_work+0x36e/0x680
     worker_thread+0x2f0/0x4f8
     kthread+0x10a/0x120
     kernel_thread_starter+0x6/0xc
     kernel_thread_starter+0x0/0xc

There is a use-after-free bug on the subprocess_info structure allocated
by the user mode helper.  In case do_execve() returns with an error
____call_usermodehelper() stores the error code to sub_info->retval, but
sub_info can already have been freed.

Regarding UMH_NO_WAIT, the sub_info structure can be freed by
__call_usermodehelper() before the worker thread returns from
do_execve(), allowing memory corruption when do_execve() failed after
exec_mmap() is called.

Regarding UMH_WAIT_EXEC, the call to umh_complete() allows
call_usermodehelper_exec() to continue which then frees sub_info.

To fix this race the code needs to make sure that the call to
call_usermodehelper_freeinfo() is always done after the last store to
sub_info->retval.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>