Subject: CVE-2024-26596: net: dsa: fix netdev_priv() dereference before check on non-DSA netdevice events

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: fix netdev_priv() dereference before check on non-DSA netdevice events

After the blamed commit, we started doing this dereference for every
NETDEV_CHANGEUPPER and NETDEV_PRECHANGEUPPER event in the system.

static inline struct dsa_port *dsa_user_to_port(const struct net_device *dev)
{
	struct dsa_user_priv *p = netdev_priv(dev);

	return p->dp;
}

Which is obviously bogus, because not all net_devices have a netdev_priv()
of type struct dsa_user_priv. But struct dsa_user_priv is fairly small,
and p->dp means dereferencing 8 bytes starting with offset 16. Most
drivers allocate that much private memory anyway, making our access not
fault, and we discard the bogus data quickly afterwards, so this wasn't
caught.

But the dummy interface is somewhat special in that it calls
alloc_netdev() with a priv size of 0. So every netdev_priv() dereference
is invalid, and we get this when we emit a NETDEV_PRECHANGEUPPER event
with a VLAN as its new upper:

$ ip link add dummy1 type dummy
$ ip link add link dummy1 name dummy1.100 type vlan id 100
[   43.309174] ==================================================================
[   43.316456] BUG: KASAN: slab-out-of-bounds in dsa_user_prechangeupper+0x30/0xe8
[   43.323835] Read of size 8 at addr ffff3f86481d2990 by task ip/374
[   43.330058]
[   43.342436] Call trace:
[   43.366542]  dsa_user_prechangeupper+0x30/0xe8
[   43.371024]  dsa_user_netdevice_event+0xb38/0xee8
[   43.375768]  notifier_call_chain+0xa4/0x210
[   43.379985]  raw_notifier_call_chain+0x24/0x38
[   43.384464]  __netdev_upper_dev_link+0x3ec/0x5d8
[   43.389120]  netdev_upper_dev_link+0x70/0xa8
[   43.393424]  register_vlan_dev+0x1bc/0x310
[   43.397554]  vlan_newlink+0x210/0x248
[   43.401247]  rtnl_newlink+0x9fc/0xe30
[   43.404942]  rtnetlink_rcv_msg+0x378/0x580

Avoid the kernel oops by dereferencing after the type check, as customary.

The Linux kernel CVE team has assigned CVE-2024-26596 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.1 with commit 4c3f80d22b2e and fixed in 6.6.55 with commit 69a1e2d938db
	Issue introduced in 6.1 with commit 4c3f80d22b2e and fixed in 6.7.2 with commit dbd909c20c11
	Issue introduced in 6.1 with commit 4c3f80d22b2e and fixed in 6.8 with commit 844f104790bd

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26596
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/dsa/user.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/69a1e2d938dbbfcff0e064269adf60ad26dbb102
	https://git.kernel.org/stable/c/dbd909c20c11f0d29c0054d41e0d1f668a60e8c8
	https://git.kernel.org/stable/c/844f104790bd69c2e4dbb9ee3eba46fde1fcea7b