Re: [BUG] signal: sighand unprotected when accessed by /proc

[Steven Rostedt]

On Tue, 3 Jun 2014 19:26:32 +0200
Oleg Nesterov <oleg@xxxxxxxxxx> wrote:

> On 06/03, Steven Rostedt wrote:
> >
> > We were able to trigger this bug in -rt, and by review, I'm thinking
> > that this could very well be a mainline bug too. I had our QA team add
> > a trace patch to the kernel to prove my analysis, and it did.
> >
> > Here's the patch:
> >
> >  http://rostedt.homelinux.com/private/sighand-trace.patch
> >
> > Let me try to explain the bug:
> >
> >
> > 	CPU0				CPU1
> > 	----				----
> >  [ read of /proc/<pid>/stat ]
> >   get_task_struct();
> >   [...]
> > 				  [ <pid> exits ]
> > 				  [ parent does wait on <pid> ]
> > 				  wait_task_zombie()
> > 				    release_task()
> > 				      proc_flush_task()
> > 				      /* the above removes new access
> > 				         to the /proc system */
> > 				      __exit_signal()
> > 				        __cleanup_sighand(sighand);
> > 					  atomic_dec_and_test(sighand->count);
> >   do_task_stat()
> >     lock_task_sighand(task);
> >       sighand = rcu_dereference(tsk->sighand);
> >
> > 					    kmem_cache_free(sighand);
> >
> >       if (sighand != NULL)
> >         spin_lock(sighand->siglock);
> >
> >        ** BOOM! use after free **
> 
> Yes, ->sighand can be already freed at this point, but this should be
> fine because sighand_cachep is SLAB_DESTROY_BY_RCU.

Ah, I didn't notice that. This makes this even more bazaar.

You know, this code could use some comments. I may send you a patch,
because that __lock_task_sighand() is doing a lot of subtle things and
there's not a single comment explaining it :-(



> 
> That is why lock_task_sighand() does rcu_read_lock() and re-checks
> sighand == tsk->sighand after it takes ->siglock. It is fine if it was
> already freed or even reallocated via kmem_cache_alloc(sighand_cachep).
> We only need to ensure that (SLAB_DESTROY_BY_RCU should ensure this)
> this memory won't be returned to system, so this peace of memory must
> be "struct sighand" with the properly initialized ->siglock until
> rcu_read_unlock().

OK, this makes __lock_task_sighand() make some more sense.

> 
> > Seems there is no protection between reading the sighand from proc and
> > freeing it. The sighand->count is not updated, and the sighand is not
> > freed via rcu.
> 
> See above.
> 
> > One, the spinlock in -rt is an rtmutex. The list_del_entry() bug is the
> > task trying to remove itself from sighand->lock->wait_list. As the lock
> > has been freed, the list head of the rtmutex is corrupted.
> 
> looks like, SLAB_DESTROY_BY_RCU logic is broken?

Could be. I'll look to see if we didn't break something.

Thanks!

-- Steve
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