Re: [PATCH nf] netfilter: nfnetlink: correctly validate length of batch messages

[Pablo Neira Ayuso]

On Tue, Feb 02, 2016 at 01:36:45PM -0500, phil.turnbull@xxxxxxxxxx wrote:
> From: Phil Turnbull <phil.turnbull@xxxxxxxxxx>
> 
> If nlh->nlmsg_len is zero then an infinite loop is triggered because
> 'skb_pull(skb, msglen);' pulls zero bytes.
> 
> The calculation in nlmsg_len() underflows if 'nlh->nlmsg_len <
> NLMSG_HDRLEN' which bypasses the length validation and will later
> trigger an out-of-bound read.
> 
> If the length validation does fail then the malformed batch message is
> copied back to userspace. However, we cannot do this because the
> nlh->nlmsg_len can be invalid. This leads to an out-of-bounds read in
> netlink_ack:
> 
>     [   41.455421] ==================================================================
>     [   41.456431] BUG: KASAN: slab-out-of-bounds in memcpy+0x1d/0x40 at addr ffff880119e79340
>     [   41.456431] Read of size 4294967280 by task a.out/987
>     [   41.456431] =============================================================================
>     [   41.456431] BUG kmalloc-512 (Not tainted): kasan: bad access detected
>     [   41.456431] -----------------------------------------------------------------------------
>     ...
>     [   41.456431] Bytes b4 ffff880119e79310: 00 00 00 00 d5 03 00 00 b0 fb fe ff 00 00 00 00  ................
>     [   41.456431] Object ffff880119e79320: 20 00 00 00 10 00 05 00 00 00 00 00 00 00 00 00   ...............
>     [   41.456431] Object ffff880119e79330: 14 00 0a 00 01 03 fc 40 45 56 11 22 33 10 00 05  .......@EV."3...
>     [   41.456431] Object ffff880119e79340: f0 ff ff ff 88 99 aa bb 00 14 00 0a 00 06 fe fb  ................
>                                             ^^ start of batch nlmsg with
>                                                nlmsg_len=4294967280
>     ...
>     [   41.456431] Memory state around the buggy address:
>     [   41.456431]  ffff880119e79400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     [   41.456431]  ffff880119e79480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     [   41.456431] >ffff880119e79500: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
>     [   41.456431]                                ^
>     [   41.456431]  ffff880119e79580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>     [   41.456431]  ffff880119e79600: fc fc fc fc fc fc fc fc fc fc fb fb fb fb fb fb
>     [   41.456431] ==================================================================
> 
> Fix this with better validation of nlh->nlmsg_len and by setting
> NFNL_BATCH_FAILURE if any batch message fails length validation.
> 
> CAP_NET_ADMIN is required to trigger the bugs.

Applied, thanks.