Message-ID: <1354111409.6276.124.camel@gandalf.local.home>
Date:	Wed, 28 Nov 2012 09:03:29 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lars Ellenberg <lars.ellenberg@...bit.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] [PATCH] fix infinite loop; increase robustness of
 debugfs_remove_recursive [attached test module]

On Wed, 2012-11-28 at 10:37 +0100, Lars Ellenberg wrote:
> On Tue, Nov 27, 2012 at 10:16:28PM -0500, Steven Rostedt wrote:
> > On Fri, 2012-11-23 at 18:15 +0100, Lars Ellenberg wrote:
> > > When toying around with debugfs, intentionally trying to break things,
> > > I managed to get it into a reproducible endless loop when cleaning up.
> > > 
> > > debugfs_remove_recursive() completely ignores that entries found
> > > on ->d_subdirs may already be d_delete()d, but not yet d_kill()ed.
> > > 
> > > In this case, the first two entries found have both been such dentries
> > > (d_iname = ".", btw), while later in the list there was still a real,
> > > not yet deleted entry.
> > > 
> > > That way, the "goto next_sibling;" did not catch this case,
> > > the "break the infinit loop if we fail to remove something"
> > > did not catch it either.
> > > 
> > > 
> > > Disclaimer: I'm not a dentries and inodes guy...
> > 
> > I'm not a dentries or inodes guy either, so I wont comment on the actual
> > merits of this patch.
> 
> Though you submitted the "next_sibling" code back in 2009, right?
> That's why you ended up on Cc.

2009! Egad, I was another person back then ;-)

Yes, I was digging into the bowels of debugfs and it's use of dentries
back then and understood a lot more of it back then too.

> I suspect that even back then, the real problem was
> "already dead but still linked" dentries, as below:

Could be.

> 
> > > Still, I think the "is this child a non-empty subdir" check
> > > was just wrong. This is my fix:
> > > -	if (list_empty(&child->d_subdirs)) 
> > > +	if (!simple_emty(child))
> > 
> > "simple_empty"
> 
> Of course. I mistyped both lines in the email body :-/
> In the patch below it was correct:
> -               if (!list_empty(&child->d_subdirs)) {
> +               if (!simple_empty(child)) {
> 
> > > Also, always trying to __debugfs_remove() the first entry found from
> > > parent->d_subdirs.next is wrong, we need to skip over any already
> > > deleted children. I introduced the debugfs_find_next_positive_child()
> > > helper for this.
> > > 
> > > If I understand it correctly, if we do it this way, it can never fail.
> > > That is, as long as we can be sure that no new dentries will be created
> > > while we are in debugfs_remove_recursive().
> > > So the 
> > > 		if (debugfs_positive(child)) {
> > >  			/*
> > >  			 * Avoid infinite loop if we fail to remove
> > >  			 * one dentry.
> > > is probably dead code now?
> > > 
> > > 
> > > As an additional fix, to prevent an access after free and resulting Oops,
> > > I serialize dereferencing of attr->get/set and attr->data with d_delete(),
> > > using the file->f_dentry->d_parent->d_inode->i_mutex.
> > > 
> > > If this file->f_dentry meanwhile has been deleted, simple_attr_read()
> > > and simple_attr_write() now returns -ESTALE. (Any other error code preferences?)
> > > 
> > > 
> > > With this patch, I was able to
> > > cd /sys/debugfs/test-module/some/dir
> > > exec 7< some-file
> > > rmmod test-module
> > > cat <&7
> > 
> > I saw this and thought "hmm, I wonder if the trace_events have issues,
> > as they create debugfs directories and files via modules too". I went
> > and tried to reproduce but couldn't get passed the rmmod, as the module
> > count gets incremented for any open files that the module creates.
> 
> Is that so.
> 
> > I take it that you didn't add that feature to your test module.
> 
> If you use the "debugfs_create_u32" (and similar "_<simple attribute>")
> wrappers, you don't get a chance to do so from the attribute callbacks,
> all callbacks are predefined, you just submit a pointer.
> 
> You mean something like this, or am I missing something?
>   __module_get();
>   debugfs_create_u32();
> And later
>   debugfs_remove_recursive();
>   module_put();
> 
> You still have the race between someone opening some attribute file,
> you removing the attribute and releasing your refcount, then the other
> guy dereferencing that pointer in the read() call.
> 
> I think you can only deal with that race,
> if you provide your own file_operations for all your attributes.
> 
> I'm about to add some such debugfs entries for our driver, and would
> like to avoid re-implementing all of the simple_attribute fops.

I went through a lot of hassle to get this to work. If you look at
kernel/trace/trace_events.c:trace_create_file_ops() you'll see that I
dynamically create a structure that has the specific file_operations in
it. I then set the "owner" field for each of them to the given module. 

I haven't looked at the implementation of the owner field in the file
operations, but it does seem to keep me from removing any module that
created any of these files.


> 
> > > without any infinite loops, hangs, oopses or other problems,
> > > and as expected get an ESTALE for the cat.
> > > 
> > > Without the patch, I'll get either an infinite loop and rmmod never
> > > terminates, or cat oopses.
> > > 
> > > 
> > > If you think this is correct, please comment on the FIXME
> > > below, and help me write a nice commit message.
> > > 
> > > If you think this is still wrong or even makes things worse,
> > > please help me with a proper fix ;-)
> > > 
> > > 
> > > Patch is against upstream as of yesterday,
> > > but looks like it still applies way back into 2009, 2.6.3x,
> > > so if it is correct, it may even qualify for the stable branches?
> > > 
> > 
> > Now, is there any current user of debugfs that is susceptible for this
> > bug?
> 
> I'm unsure. Grepping for debugfs_create_u32, I suspect that some of the
> crypto or wifi modules may be affected.

Yeah, looking at some of the drivers here, it does seem that they have
issues.

> 
> > I'm not saying that these issues shouldn't be fixed. But I'm also
> > concerned about exploits and other things that just a root user may
> > accidentally cause harm.
> 
> I'm concerned about monitoring/statistic gathering stuff (running as any
> user) may hold some debugfs file open,
> and race with removal of dynamic debugfs entries.

I agree.

> 
> > If there's current problem then maybe this
> > isn't needed for stable. But should probably be fixed for the future.
> 
> I attach my "hello-debugfs" module.
> # make -C /lib/modules/`uname -r`/build M=$PWD modules
> 
> Test case1:
> 	cd /sys/kernel/debug/hello-debugfs/dir1/dir3/
> 	rmmod
> will unload the module, but give up on removing
> /sys/kernel/debug/hello-debugfs/dir1
> You won't be able to insmod a second time.
> 
> Test case 2:
> 	cd /sys/kernel/debug/hello-debugfs/dir1/dir3/
> 	exec 7< file7
> 	rmmod
> 	cat <&7
> As described above, depending on I don't know what exactly,
> the outcome was either some infinite loop, or cat will oops/panic.
> 
> Note that all but rmmod may be done as normal user,
> e.g. some monitoring or statistics gathering tool,
> and the rmmod is just a placeholder for
> "any event that triggers removal of dynamic debugfs entries".

Thanks for the test case. This is something that Greg should look into
(I love volunteering others :-)

-- Steve


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