netdev - Linux netfilter IPT_SO_SET_REPLACE memory corruption

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Date:	Wed, 9 Mar 2016 09:25:05 -0800
From:	Ben Hawkes <hawkes@...gle.com>
To:	netdev@...r.kernel.org
Subject: Linux netfilter IPT_SO_SET_REPLACE memory corruption

N.B. I was redirected to this list by security@...nel.org. I'm told
that networking related issues such as this are handled on the public
netdev mailing list.

A memory corruption vulnerability exists in the IPT_SO_SET_REPLACE
ioctl in the netfilter code for iptables support. This ioctl is can be
triggered by an unprivileged user on PF_INET sockets when unprivileged
user namespaces are available (CONFIG_USER_NS=y). Android does not
enable this option, but desktop/server distributions and Chrome OS
will commonly enable this to allow for containers support or
sandboxing.

In the mark_source_chains function (net/ipv4/netfilter/ip_tables.c) it
is possible for a user-supplied ipt_entry structure to have a large
next_offset field. This field is not bounds checked prior to writing a
counter value at the supplied offset:

newpos = pos + e->next_offset;
...
e = (struct ipt_entry *) (entry0 + newpos);
e->counters.pcnt = pos;

This means that an out of bounds 32-bit write can occur in a 64kb
range from the allocated heap entry, with a controlled offset and a
partially controlled write value ("pos") or zero. The attached
proof-of-concept (netfilter_setsockopt_v3.c) triggers the corruption
multiple times to set adjacent heap structures to zero.

This issue affects (at least) kernel versions 3.10, 3.18 and 4.4, and
could be used for local privilege escalation. It appears that a
similar codepath is accessible via arp_tables.c/ARPT_SO_SET_REPLACE as
well.

Furthermore, a recent refactoring of this codepath
(https://github.com/torvalds/linux/commit/2e4e6a17af35be359cc8f1c924f8f198fbd478cc)
introduced an integer overflow in xt_alloc_table_info, which on 32-bit
systems can lead to small structure allocation and a copy_from_user
based heap corruption. The attached proof-of-concept
(netfilter_setsockopt_v4.c) triggers this issue on 4.4.

View attachment "netfilter_setsockopt_v3.c" of type "text/x-csrc" (2096 bytes)

View attachment "netfilter_setsockopt_v4.c" of type "text/x-csrc" (1297 bytes)