net: uninitialized loopback addr leaks to userspace

From: Vegard Nossum
Date: Sat May 30 2009 - 16:23:38 EST


It seems that loopback's hardware address is never initialized by the
kernel. So if userspace attempts to read this address before it has
been set, the kernel will return some uninitialized data (only 6
bytes, though). This can be demonstrated by creating a new network
namespace (CLONE_NEWNET), which creates a new loopback device, then
call ioctl() with SIOCGIFHWADDR on "lo". If this is done in a loop,
with some background load, or by running multiple instances, random
data will start to show up in the returned address.

[ 406.750329] WARNING: kmemcheck: Caught 16-bit read from
uninitialized memory (ffff880007220974)
[ 406.753555] 18a2d7060088ffff18a2d7060088ffff00000000010000000100000003000000
[ 406.758862] i i i i i i i i i i i i i i i i i u u u u u u u u u u u u u u u
[ 406.766224] ^
[ 406.768792] Modules linked in:
[ 406.770416] Pid: 757, comm: ifconfig Not tainted
2.6.30-rc7-next-20090529 #404
[ 406.772876] RIP: 0010:[<ffffffff80664789>] [<ffffffff80664789>]
[ 406.804677] [<ffffffff8064ff75>] sock_ioctl+0x95/0x2a0
[ 406.807242] [<ffffffff802c35eb>] vfs_ioctl+0x1b/0x70
[ 406.809348] [<ffffffff802c36fa>] do_vfs_ioctl+0x8a/0x570
[ 406.811419] [<ffffffff802c3c79>] sys_ioctl+0x99/0xa0
[ 406.813400] [<ffffffff802f3941>] dev_ifsioc+0x81/0x2f0
[ 406.815424] [<ffffffff802f454d>] compat_sys_ioctl+0xed/0x3c0
[ 406.817596] [<ffffffff8022d476>] cstar_dispatch+0x7/0x26
[ 406.819978] [<ffffffffffffffff>] 0xffffffffffffffff

This is the code that triggers the warning, in net/core/dev.c, around line 4150:

memcpy(ifr->ifr_hwaddr.sa_data, dev->dev_addr,
min(sizeof ifr->ifr_hwaddr.sa_data, (size_t) dev->addr_len));

So it's dev->dev_addr that is the pointer to the uninitialized data.

I didn't know how to fix it.

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