netdev - Re: [B.A.T.M.A.N.] [PATCH] batman-adv: fix null pointer dereference in batadv_gw

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Date:   Fri, 30 Nov 2018 08:41:35 +0100
From:   Sven Eckelmann <sven@...fation.org>
To:     b.a.t.m.a.n@...ts.open-mesh.org
Cc:     Wen Yang <wen.yang99@....com.cn>, mareklindner@...mailbox.ch,
        sw@...onwunderlich.de, a@...table.cc, davem@...emloft.net,
        netdev@...r.kernel.org, zhong.weidong@....com.cn,
        linux-kernel@...r.kernel.org
Subject: Re: [B.A.T.M.A.N.] [PATCH] batman-adv: fix null pointer dereference in batadv_gw_election

On Friday, 30 November 2018 15:00:02 CET Wen Yang wrote:
> This patch fixes a possible null pointer dereference in
> batadv_gw_election, detected by the semantic patch
> deref_null.cocci, with the following warning:
> 
> ./net/batman-adv/gateway_client.c:289:15-24: ERROR: next_gw is NULL but dereferenced.
> ./net/batman-adv/gateway_client.c:290:15-29: ERROR: next_gw is NULL but dereferenced.
> ./net/batman-adv/gateway_client.c:291:15-29: ERROR: next_gw is NULL but dereferenced.
> ./net/batman-adv/gateway_client.c:292:15-27: ERROR: next_gw is NULL but dereferenced.
> ./net/batman-adv/gateway_client.c:293:15-27: ERROR: next_gw is NULL but dereferenced.

This patch is seems to be nonsensical. next_gw cannot be NULL at this point 
(let us call this location in the code "4."). Let us go through the code

    // 1. when both are NULL then it would jump out of the the function.
	if (curr_gw == next_gw)
		goto out;

    [...]

    if (curr_gw && !next_gw) {
        [...]
        // 2. this handles the only valid case when next_gw is NULL
    }  else if (!curr_gw && next_gw) {
        // 3. here we know that next_gw is not NULL and curr_gw is NULL
        // we can therefore infer that 
        [...]
    } else {
        // 4. here you try to add an ugly patch to handle a non-existing 
        // next_gw == NULL case
        [...]
    }

Let us go through all possible combinations:

        curr_gw  next_gw
    I   0        0
    II  x        0
    III 0        y
    IV  x        y

For I:   we would leave the function even at 1. and never reach 4.
For II:  would be handled by 2. and thus never reach 4.
For III: would be handled by 3. and thus never reach 4.
For IV:  This can be handled by 1. (when x == y). Or otherwise it would be
         handled by 4. but is not the next_gw == NULL case.

Please correct me (in case I missed something) but it looks to me that this 
patch should be rejected.

Kind regards,
	Sven
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