linux-kernel - Re: [PATCH] Change judgment len position

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Message-ID: <20181024163230.GA25382@1wt.eu>
Date:   Wed, 24 Oct 2018 18:32:30 +0200
From:   Willy Tarreau <w@....eu>
To:     Joe Perches <joe@...ches.com>
Cc:     Wang Hai <wanghaifine@...il.com>, 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, Oct 24, 2018 at 09:23:19AM -0700, Joe Perches wrote:
> 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

It could indeed, or we could also have min_t() done on an int instead
of an unsigned int and this would avoid the need to shuffle the code
around and open a huge hole like this one.

Willy