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

[Liu, Chuansheng]

> -----Original Message-----
> From: Al Viro [mailto:viro@xxxxxxxxxxxxxxxx] On Behalf Of Al Viro
> Sent: Saturday, August 31, 2013 3:36 PM
> To: Liu, Chuansheng
> Cc: Eric Dumazet; linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] Fix the race between the fget() and close()
> 
> On Sat, Aug 31, 2013 at 07:01:33AM +0000, Liu, Chuansheng wrote:
> 
> > My scenario is:
> > P1 files_struct refcount is 1, P2's is 1 also.
> > P1 get_files_struct(P2)
> > P1 install one file into P2's files_struct
> > P1 put_files_struct(P2)
> >
> > Then P1 and P2's files_struct refcount are 1, then when P1 is doing ioctl() and
> P2 is exiting
> > with put_files_struct(P2), the race will occur, my understanding is wrong?
> 
> First of all, this wouldn't have been a problem (so you get a new reference
> to file inserted in P2's files_struct; file refcount had been bumped, so
> destruction of P2's files_struct will undo that increment of file refcount
> and we are still fine).  _Removal_ in a similar scenario would have been
> a problem, with P2 doing fdget() while its table isn't shared, then P1
> removing a reference from it and dropping a file - the last one, at that,
> since fdget() assumed that the reference would've stayed in P2's descriptor
> table.  HOWEVER, P1 does not do get_files_struct(P2) at all - it's only
> done by P2 in binder_mmap().
Got it, thanks. In other process, the fget() + _fd_install() should be the same
as the process call open() directly, and the file reference count will be at least 2.

> 
> Again, the invariant to look for is this:
> 	* if current->files had not been shared at fdget() time, it won't
> be shared at matching fdput() and no entries will have been removed in
> between.
> 
> task_fd_install()/task_close_fd() are done on proc->files, which contributes
> to descriptor table refcount.  All other modifications are done to
> current->files, which also contributes to refcount.  If at fdget() time
> current->files had refcount 1, we had no other processes with task->files
> pointing to this descriptor table *and* no binder_proc had their ->files
> pointint to it.  No new ones may appear, since new process could get
> such a reference only from do_fork() called by us and new binder_proc could
> get such a reference only from binder_mmap() called by us.  Neither is
> called between fdget() and fdput().  So in that case the only reference
> to this descriptor table will remain current->files and all removals
> would have to be done by ourselves (and not via task_close_fd(), at that).
> 
> And AFAICS, binder_lock() prevents proc->files being dropped under
> task_close_fd() and task_fd_install().  Hell knows...
> 
> How reproducible it is?  Do you have any more instances, or had that
> been a one-off panic?
Just meet once yet.


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