Subject: Re: [Linux-kernel-mentees] [PATCH net] qrtr: Fix ZERO_SIZE_PTR deref in qrtr_tun_write_iter()

From: Dan Carpenter <[email-masked]>
Date: Mon, 13 Jul 2020 12:13:29 +0300

On Sun, Jul 12, 2020 at 02:36:31PM -0700, Eric Biggers wrote:
> On Sun, Jul 12, 2020 at 05:03:00PM -0400, Peilin Ye wrote:
> > qrtr_tun_write_iter() is dereferencing `ZERO_SIZE_PTR`s when `from->count`
> > equals to zero. Fix it by rejecting zero-length kzalloc() requests.
> > 
> > This patch fixes the following syzbot bug:
> > 
> >     https://syzkaller.appspot.com/bug?id=f56bbe6668873ee245986bbd23312b895fa5a50a
> > 
> > Reported-by: [email-masked]
> > Signed-off-by: Peilin Ye <[email-masked]>
> > ---
> >  net/qrtr/tun.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/net/qrtr/tun.c b/net/qrtr/tun.c
> > index 15ce9b642b25..5465e94ba8e5 100644
> > --- a/net/qrtr/tun.c
> > +++ b/net/qrtr/tun.c
> > @@ -80,6 +80,9 @@ static ssize_t qrtr_tun_write_iter(struct kiocb *iocb, struct iov_iter *from)
> >  	ssize_t ret;
> >  	void *kbuf;
> >  
> > +	if (!len)
> > +		return -EINVAL;
> > +
> >  	kbuf = kzalloc(len, GFP_KERNEL);
> >  	if (!kbuf)
> >  		return -ENOMEM;
> 
> Wasn't this already fixed by:
> 
>     commit 8ff41cc21714704ef0158a546c3c4d07fae2c952
>     Author: Dan Carpenter <[email-masked]>
>     Date:   Tue Jun 30 14:46:15 2020 +0300
> 
>         net: qrtr: Fix an out of bounds read qrtr_endpoint_post()

Yep.  If you're using kmalloc() you can allocate a zero byte buffer but
you just can't access the array.  for (i = 0; i < 0; i++) works.

It's interesting because at the time, I wrote the patch I thought "len"
probably couldn't be zero but I just checked it for completeness and
readability.

regards,
dan carpenter