linux-kernel - Re: [PATCH v7 2/2] firmware_loader: fix use-after-free in firmware_fallback

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Message-ID: <YQACopP/tRsLFQVJ@anirudhrb.com>
Date:   Tue, 27 Jul 2021 18:27:06 +0530
From:   Anirudh Rayabharam <mail@...rudhrb.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     gregkh@...uxfoundation.org, rafael@...nel.org,
        skhan@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+de271708674e2093097b@...kaller.appspotmail.com
Subject: Re: [PATCH v7 2/2] firmware_loader: fix use-after-free in
 firmware_fallback_sysfs

On Mon, Jul 26, 2021 at 11:27:21AM -0700, Luis Chamberlain wrote:
> On Sat, Jul 24, 2021 at 05:41:34PM +0530, Anirudh Rayabharam wrote:
> > This use-after-free happens when a fw_priv object has been freed but
> > hasn't been removed from the pending list (pending_fw_head). The next
> > time fw_load_sysfs_fallback tries to insert into the list, it ends up
> > accessing the pending_list member of the previoiusly freed fw_priv.
> > 
> > The root cause here is that all code paths that abort the fw load
> > don't delete it from the pending list. For example:
> > 
> > 	_request_firmware()
> > 	  -> fw_abort_batch_reqs()
> > 	      -> fw_state_aborted()
> > 
> > To fix this, delete the fw_priv from the list in __fw_set_state() if
> > the new state is DONE or ABORTED. This way, all aborts will remove
> > the fw_priv from the list. Accordingly, remove calls to list_del_init
> > that were being made before calling fw_state_(aborted|done).
> > 
> > Also, in fw_load_sysfs_fallback, don't add the fw_priv to the pending
> > list if it is already aborted. Instead, just jump out and return early.
> > 
> > Fixes: bcfbd3523f3c ("firmware: fix a double abort case with fw_load_sysfs_fallback")
> > Reported-by: syzbot+de271708674e2093097b@...kaller.appspotmail.com
> > Tested-by: syzbot+de271708674e2093097b@...kaller.appspotmail.com
> 
> Curious, how do you get syzbot to test this, I mean your custom tree?

Don't need a custom tree. You can send syzbot the git url of an existing
tree (such as linux-next or Linus' tree) and a patch to apply on that
tree before testing. This is documented here:

https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches

Of course, using a custom tree is also possible.

> 
> > Signed-off-by: Anirudh Rayabharam <mail@...rudhrb.com>
> 
> With the changes Shua requested being made:

I'll implement Shua's suggestions and send a new version.

Thanks!

	- Anirudh.

> 
> Acked-by: Luis Chamberlain <mcgrof@...nel.org>
> 
>   Luis