Re: [PATCH] Fix the race between the fget() and close()

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2013-08-27 at 00:12 +0800, Chuansheng Liu wrote:
> When one thread is calling sys_ioctl(), and another thread is calling
> sys_close(), current code has protected most cases.
> 
> But for the below case, it will cause issue:
> T1                                T2                             T3
> sys_close(oldfile)                sys_open(newfile)              sys_ioctl(oldfile)
>  -> __close_fd()
>    lock file_lock
>     assign NULL file
>     put fd to be unused
>    unlock file_lock
> 				   get new fd is same as old
> 				   assign newfile to same fd
> 								   fget_flight()
>                                                                     get the newfile!!!
>     decrease file->f_count
>      file->f_count == 0
>       --> try to release file
> 
> The race is when T1 try to close one oldFD, T3 is trying to ioctl the oldFD,
> if currently the new T2 is trying to open a newfile, it maybe get the newFD is
> same as oldFD.
> 
> And normal case T3 should get NULL file pointer due to released by T1, but T3
> get the newfile pointer, and continue the ioctl accessing.
> 
> It maybe causes unexpectable error, we hit one system panic at do_vfs_ioctl().
> 

Not clear if the bug is not elsewhere.

What panic did you have exactly ?

> Here we can fix it that putting "put_unused_fd()" after filp_close(),
> it can avoid this case.
> 

Three threads doing this kind of stuff cannot expect T3 gets the old or
new file anyway. Its totally unspecified.