linux-kernel - Re: [PATCH] cxgb4: cm: fix a incorrect NULL check on list iterator

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Date:   Wed, 30 Mar 2022 15:53:38 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     bharat@...lsio.com, jgg@...pe.ca, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, roland@...estorage.com,
        stable@...r.kernel.org, vipul@...lsio.com
Subject: Re: [PATCH] cxgb4: cm: fix a incorrect NULL check on list iterator

On Wed, Mar 30, 2022 at 08:30:27PM +0800, Xiaomeng Tong wrote:
> On Sun, 27 Mar 2022 19:38:31 +0300, Leon Romanovsky wrote:
> > 
> > On Sun, Mar 27, 2022 at 03:35:42PM +0800, Xiaomeng Tong wrote:
> > > The bug is here:
> > > 	if (!pdev) {
> > > 
> > > The list iterator value 'pdev' will *always* be set and non-NULL
> > > by for_each_netdev(), so it is incorrect to assume that the
> > > iterator value will be NULL if the list is empty or no element
> > > found (in this case, the check 'if (!pdev)' can be bypassed as
> > > it always be false unexpectly).
> > > 
> > > To fix the bug, use a new variable 'iter' as the list iterator,
> > > while use the original variable 'pdev' as a dedicated pointer to
> > > point to the found element.
> > 
> > I don't think that the description is correct.
> > We are talking about loopback interface which received packet, the pdev will always exist.
> 
> Do the both conditions impossible?
> 1. the list is empty or
> 2. we can not found a pdev due to this check
> 	if (ipv6_chk_addr(&init_net,
>   			  (struct in6_addr *)peer_ip,
> 			  pdev, 1))
> 			  iter, 1))

Yes, both are impossible.

Thanks

> 
> > Most likely. the check of "if (!pdev)" is to catch impossible situation where IPV6 packet
> > was sent over loopback, but IPV6 is not enabled.
> 
> --
> Xiaomeng Tong