linux-kernel - Re: [PATCH] wifi: mwifiex: Fix two buggy list traversals

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Message-ID: <ZrFFQ-dOmgL99CfF@mozart.vkv.me>
Date: Mon, 5 Aug 2024 14:33:55 -0700
From: Calvin Owens <calvin@...nvd.org>
To: Brian Norris <briannorris@...omium.org>
Cc: linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
	Francesco Dolcini <francesco@...cini.it>,
	Kalle Valo <kvalo@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	David Lin <yu-hao.lin@....com>
Subject: Re: [PATCH] wifi: mwifiex: Fix two buggy list traversals

On Wednesday 07/31 at 13:09 -0700, Brian Norris wrote:
> On Tue, Jul 30, 2024 at 11:05:30AM -0700, Calvin Owens wrote:
> > Both of these list traversals use list_for_each_entry_safe(), yet drop
> > the lock protecting the list during the traversal.
> > 
> > Because the _safe() iterator stores a pointer to the next list node
> > locally so the current node can be deleted, dropping the lock this way
> > means the next "cached" list_head might be freed by another caller,
> > leading the iterator to dereference pointers in freed memory after
> > reacquiring the lock.
> 
> There are lots of unclear and/or unsound locking patterns in this
> driver. You've probably identified one, although I don't think you've
> solved 100% of it.
> 
> Here's another: is it valid for mwifiex_11n_rx_reorder_pkt() ->
> mwifiex_11n_get_rx_reorder_tbl() to retrieve a 'tbl' pointer (without
> removing it from the list), and then continue to operate on that without
> holding any locks? (I think the answer is "no".)
> 
> Side note: you might also refer to this old thread:
> https://lore.kernel.org/all/CAD=FV=VuxFtDdcMndLNzVYDoid8N3jP46j0sOFXG1D4CzX0=Zw@mail.gmail.com/
> I don't think Marvell ever fully resolved all the issues there.

That's all helpful, thank you.

> > Fix by moving to-be-deleted objects to an on-stack list before actually
> > deleting them, so the lock can be held for the entire traversal.
> > 
> > This is a bit ugly, because mwifiex_del_rx_reorder_entry() will still
> > take the rx_reorder_tbl_lock to delete the item from the two on-stack
> > lists introduced in this patch. But that is just ugly, not wrong, and
> > the function has other callers... making the locking conditional seems
> > strictly uglier.
> 
> I noticed this "ugliness", but I agree with your reasoning -- it's as
> good as we can do here for now.
> 
> > I discovered this bug while studying the new "nxpwifi" driver, which was
> > sent to the mailing list about a month ago:
> > 
> > https://lore.kernel.org/lkml/20240621075208.513497-1-yu-hao.lin@nxp.com/
> > 
> > ...but it turns out the new 11n_rxreorder.c in nxpwifi is essentially
> > exactly identical to mwifiex, save for s/mwifiex/nxpwifi/, so I wanted
> > to pass along a bugfix for the original driver as well.
> 
> That's another can of worms. mwifiex is horrible, and so if you were
> asking me, I'd reject any attempt at copy/paste/modify that doesn't make
> significant efforts to refactor and improve -- for instance, better
> documentation about what all the locks mean, and clarity such that
> readers can be confident that the code is doing the right thing. For
> example, I think this mwifiex comment is a lie:
> 
> 	/* spin lock for rx_reorder_tbl_ptr queue */
> 	spinlock_t rx_reorder_tbl_lock;
> 
> I believe it's supposed to protect the elements within the list too --
> but it doesn't do a good job of that.
> 
> But that's a side track...
> 
> > I only have an IW612, so this patch was only tested on "nxpwifi".
> 
> I don't think we can accept an untested patch here. If you're lucky,
> maybe I or someone else on CC can test for you though.
>
> > Signed-off-by: Calvin Owens <calvin@...nvd.org>
> > ---
> >  .../wireless/marvell/mwifiex/11n_rxreorder.c  | 26 +++++++++----------
> >  1 file changed, 12 insertions(+), 14 deletions(-)
> 
> I think the patch looks good enough, but I won't ack it without testing.
> And while you're at it, I'd recommend some further auditing, per the
> above.

Understood. I was honestly a bit hesitant to send this in the first
place without some sort of reproducer, I'll sit on the patch until I'm
able to find one.

Thanks,
Calvin

> Brian