Dan Carpenter <dan.carpenter@xxxxxxxxxx> [Mon, 2021-08-30 02:04 -0700]:
> Hello Andrey Ignatov,
>
> This is a semi-automatic email about new static checker warnings.
>
> The patch 96a6b93b6988: "rtnetlink: Return correct error on changing
> device netns" from Aug 25, 2021, leads to the following Smatch
> complaint:
>
> net/core/rtnetlink.c:2698 do_setlink()
> error: we previously assumed 'ifname' could be null (see line 2608)
>
> net/core/rtnetlink.c
> 2607 if (tb[IFLA_NET_NS_PID] || tb[IFLA_NET_NS_FD] || tb[IFLA_TARGET_NETNSID]) {
> 2608 const char *pat = ifname && ifname[0] ? ifname : NULL;
> ^^^^^^
> The patch adds a new check for if "ifname" is NULL. Is this required?
Yes. There is one do_setlink() call that passes NULL as ifname:
net/core/rtnetlink.c
static int rtnl_group_changelink(const struct sk_buff *skb,
...
err = do_setlink(skb, dev, ifm, extack, tb, NULL, 0);
It handles IFLA_GROUP that can be used with these netns attributes.
W/o the NULL check something like `ip link set group GROUP netns NETNS`
would cause a panic like this:
# ./ifname.sh
+ ip netns add ns0
+ ip link add g1 group 1 type dummy
+ ip link show group 1
10: g1: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group 1 qlen 1000
link/ether ba:3c:5e:9d:8e:ab brd ff:ff:ff:ff:ff:ff
+ ip link set group 1 netns ns0
[ 7.566918] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 7.567746] #PF: supervisor read access in kernel mode
[ 7.568299] #PF: error_code(0x0000) - not-present page
[ 7.568853] PGD 8000000107758067 P4D 8000000107758067 PUD 107757067 PMD 0
[ 7.569638] Oops: 0000 [#1] SMP PTI
[ 7.570007] CPU: 2 PID: 242 Comm: ip Not tainted 5.14.0-rc7-00093-g1a6436f37512-dirty #533
[ 7.570918] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 7.572126] RIP: 0010:do_setlink+0x69/0x1070
...
> 2609 struct net *net;
> 2610 int new_ifindex;
> 2611
> 2612 net = rtnl_link_get_net_capable(skb, dev_net(dev),
> 2613 tb, CAP_NET_ADMIN);
> 2614 if (IS_ERR(net)) {
> 2615 err = PTR_ERR(net);
> 2616 goto errout;
> 2617 }
> 2618
> 2619 if (tb[IFLA_NEW_IFINDEX])
> 2620 new_ifindex = nla_get_s32(tb[IFLA_NEW_IFINDEX]);
> 2621 else
> 2622 new_ifindex = 0;
> 2623
> 2624 err = __dev_change_net_namespace(dev, net, pat, new_ifindex);
> 2625 put_net(net);
> 2626 if (err)
> 2627 goto errout;
> 2628 status |= DO_SETLINK_MODIFIED;
> 2629 }
...
> 2693 /*
> 2694 * Interface selected by interface index but interface
> 2695 * name provided implies that a name change has been
> 2696 * requested.
> 2697 */
> 2698 if (ifm->ifi_index > 0 && ifname[0]) {
> ^^^^^^^^^
> The existing code does not check.
+Jakub
As Jakub explained in [0] this code can not be called with ifname=NULL
because the only do_setlink() call with ifname=NULL is done only when
`ifm->ifi_index == 0` so it looks like false positive.
[0]
https://lore.kernel.org/netdev/20210830094301.4f6ada72@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
--
Andrey Ignatov
