Re: Memory leak in ath9k_hif_usb_dealloc_tx_urbs()

[Atul Gopinathan]

On Tue, Apr 27, 2021 at 03:29:28PM +0300, Pavel Skripkin wrote:
> On Tue, 27 Apr 2021 15:04:29 +0300
> Pavel Skripkin <paskripkin@xxxxxxxxx> wrote:
> 
> > Hi!
> > 
> > On Tue, 2021-04-27 at 17:05 +0530, Atul Gopinathan wrote:
> > > On Wed, Mar 31, 2021 at 08:28:15AM +0200, Greg KH wrote:
> > > > On Tue, Mar 30, 2021 at 10:36:52PM +0300, Pavel Skripkin wrote:
> > > > > Hi!
> > > > > 
> > > > > I did some debugging on this
> > > > > https://syzkaller.appspot.com/bug?id=3ea507fb3c47426497b52bd82b8ef0dd5b6cc7ee
> > > > > and, I believe, I recognized the problem. The problem appears in
> > > > > case of
> > > > > ath9k_htc_hw_init() fail. In case of this fail all tx_buf->urb
> > > > > krefs will be
> > > > > initialized to 1, but in free function:
> > > > > 
> > > > > static void ath9k_hif_usb_dealloc_tx_urbs(struct hif_device_usb
> > > > > *hif_dev)
> > > > > 
> > > > > ....
> > > > > 
> > > > > static void ath9k_hif_usb_dealloc_tx_urbs(struct hif_device_usb
> > > > > *hif_dev)
> > > > > {
> > > > >     ...
> > > > >         list_for_each_entry_safe(tx_buf, tx_buf_tmp,
> > > > >                                  &hif_dev->tx.tx_buf, list) {
> > > > >                 usb_get_urb(tx_buf->urb);
> > > > >                 ...
> > > > >                 usb_free_urb(tx_buf->urb);
> > > > >                 ...
> > > > >                 }
> > > > > 
> > > > > Krefs are incremented and then decremented, that means urbs
> > > > > won't be freed.
> > > > > I found your patch and I can't properly understand why You added
> > > > > usb_get_urb(tx_buf->urb).
> > > > > Can You explain please, I believe this will help me or somebody
> > > > > to fix this ussue :)
> > > > 
> > > > I think almost everyone who has looked into this has given up due
> > > > to the
> > > > mess of twisty-passages here with almost no real-world benefits
> > > > for unwinding them :)
> > > 
> > > Just wanted to confirm, what is the status of this bug then, as in
> > > is it
> > > invalid (not sure if that's the correct word)? I happened to stumble
> > > across the same syzkaller bug report Pavel posted above, in the
> > > morning.
> > > Saw that there has been no patch tests/fixes on this yet according
> > > to syzkaller. Spent a couple of hours going through it before
> > > sending a test patch to syzbot which returned an "OK" (and the
> > > patch is exactly what Pavel pointed out, I simply removed the
> > > `usb_get_urb()`). Before sending anything to the mailing list, I
> > > made sure to search all the relavant networking lists to see if
> > > this topic had been brought up (learnt
> > > to do this from my preious mistakes of sending already accepted
> > > patches) and
> > > luckily I found this.
> > > 
> > > Syzbot has had 380 crashes caused by this bug, with the latest being
> > > today. So I wanted to confirm what should be done be about this
> > > bug. 
> > > 
> > 
> > I saw on dashboard, that Dmitry tested latest upstream commit and
> > syzbot returned "OK", but usb_get_urb(tx_buf->urb); is still there.
> > 
> 
> I am sorry, I clicked wrong link on dashboard :( My bad.

Oh right, I forgot to mention. Just want to make it clear that the test
patch was mine. There was a bug in syzkaller, so when I sent the patches
for testing they returned a weird error. Dmitry later pointed out that
it was a syzkaller bug and was kind enough to re-send my patch on a
fixed commit of syzkaller.

https://groups.google.com/g/syzkaller-bugs/c/cBQP4fKjhFQ

> 
> I believe, You can test your patch on this
> https://syzkaller.appspot.com/bug?id=cabffad18eb74197f84871802fd2c5117b61febf.
> 
> usb_get_urb(tx_buf->urb) was introduced in patch related to this bug
> 
> > I think, this usb_get_urb prevents race condition, but I'm not sure
> > about it, that's why I sent an email to patch author. As You can see,
> > he has not responded yet :)

Ah that's how it is. Well not sure we could do much here. Also thanks
for clarifying things, I thought that no one had been looking into this
bug especially when it had so many crash counts which suprised me, but I
guess I was wrong.

Thank you!
Atul