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Date:   Tue, 29 Dec 2020 17:01:27 +0100
From:   Florian Westphal <fw@strlen.de>
To:     Visa Hankala <visa@hankala.org>
Cc:     Florian Westphal <fw@strlen.de>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>, netdev@vger.kernel.org
Subject: Re: [PATCH] xfrm: Fix wraparound in xfrm_policy_addr_delta()
Message-ID: <20201229160127.GA30823@breakpoint.cc>
References: <20201229145009.cGOUak0JdKIIgGAv@hankala.org>
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Visa Hankala <visa@hankala.org> wrote:
> Use three-way comparison for address elements to avoid integer
> wraparound in the result of xfrm_policy_addr_delta().
> 
> This ensures that the search trees are built and traversed correctly
> when the difference between compared address elements is larger
> than INT_MAX.

Please provide an update to tools/testing/selftests/net/xfrm_policy.sh
that shows that this is a problem.

>  	switch (family) {
>  	case AF_INET:
> -		if (sizeof(long) == 4 && prefixlen == 0)
> -			return ntohl(a->a4) - ntohl(b->a4);
> -		return (ntohl(a->a4) & ((~0UL << (32 - prefixlen)))) -
> -		       (ntohl(b->a4) & ((~0UL << (32 - prefixlen))));
> +		mask = ~0U << (32 - prefixlen);
> +		ma = ntohl(a->a4) & mask;
> +		mb = ntohl(b->a4) & mask;

This is suspicious.  Is prefixlen == 0 impossible?

If not, then after patch
mask = ~0U << 32;

... and function returns 0.