netdev - Re: [PATCH] Change judgment len position

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Date:   Wed, 24 Oct 2018 09:23:19 -0700
From:   Joe Perches <joe@...ches.com>
To:     Willy Tarreau <w@....eu>, Wang Hai <wanghaifine@...il.com>
Cc:     edumazet@...gle.com, davem@...emloft.net, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Change judgment len position

On Wed, 2018-10-24 at 17:57 +0200, Willy Tarreau wrote:
> On Wed, Oct 24, 2018 at 11:47:29PM +0800, Wang Hai wrote:
> > To determine whether len is less than zero, it should be put before 
> > the function min_t, because the return value of min_t is not likely 
> > to be less than zero.
> 
> Huh? First, the <0 test is made on "len", not "min_t", so it still
> is signed. Second, you're in fact completely removing the test here,
> look :
> 
> >  	struct net *net = sock_net(sk);
> >  	int val, len;
> >  
> > +	len = min_t(unsigned int, len, sizeof(int));
> > +
> 
> len is used uninitialized here, so the result is undefined.
> 
> >  	if (get_user(len, optlen))
> >  		return -EFAULT;
> 
> Then it gets overridden by get_user()
> 
> > -	len = min_t(unsigned int, len, sizeof(int));
> > -
> 
> Then its positive values are not bounded anymore since you moved the test.

Not quite.

Problem here is negative values are tested as
large positive values and limited to 4

ie:
	ien len = -1,
	len = min_t(unsigned int, len, sizeof(int));

len is now 4
	
> >  	if (len < 0)
> >  		return -EINVAL;

So this test len < 0 could be moved up above min_t

> Then only negative values are dropped. So unless I'm missing something
> obvious, you're just allowing len to be as large as 2GB-1 based on the
> user's fed optlen.
> 
> Am I wrong ?
> 
> Willychee