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filter: prevent nla extensions to peek beyond the end of the message

CVE-2014-3145, CVE-2014-3144

The BPF_S_ANC_NLATTR and BPF_S_ANC_NLATTR_NEST extensions fail to check
for a minimal message length before testing the supplied offset to be
within the bounds of the message. This allows the subtraction of the nla
header to underflow and therefore -- as the data type is unsigned --
allowing far to big offset and length values for the search of the
netlink attribute.

The remainder calculation for the BPF_S_ANC_NLATTR_NEST extension is
also wrong. It has the minuend and subtrahend mixed up, therefore
calculates a huge length value, allowing to overrun the end of the
message while looking for the netlink attribute.

The following three BPF snippets will trigger the bugs when attached to
a UNIX datagram socket and parsing a message with length 1, 2 or 3.

 ,-[ PoC for missing size check in BPF_S_ANC_NLATTR ]--
 | ld	#0x87654321
 | ldx	#42
 | ld	#nla
 | ret	a
 `---

 ,-[ PoC for the same bug in BPF_S_ANC_NLATTR_NEST ]--
 | ld	#0x87654321
 | ldx	#42
 | ld	#nlan
 | ret	a
 `---

 ,-[ PoC for wrong remainder calculation in BPF_S_ANC_NLATTR_NEST ]--
 | ; (needs a fake netlink header at offset 0)
 | ld	#0
 | ldx	#42
 | ld	#nlan
 | ret	a
 `---

Fix the first issue by ensuring the message length fulfills the minimal
size constrains of a nla header. Fix the second bug by getting the math
for the remainder calculation right.

Fixes: 4738c1d ("[SKFILTER]: Add SKF_ADF_NLATTR instruction")
Fixes: d214c75 ("filter: add SKF_AD_NLATTR_NEST to look for nested..")
Cc: Patrick McHardy <kaber@trash.net>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Change-Id: I3740d1d5825ff7f8c28ee28e9f310d4b271b84b6
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minipli authored and andi34 committed Apr 13, 2014
1 parent 5388f75 commit ba426e66a261be53d2c5cc26c85e2899e657b9cb
Showing with 9 additions and 1 deletion.
  1. +9 −1 net/core/filter.c
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@@ -320,6 +320,10 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
if (skb_is_nonlinear(skb))
return 0;
if (skb->len < sizeof(struct nlattr))
return 0;
if (A > skb->len - sizeof(struct nlattr))
return 0;
@@ -336,11 +340,15 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
if (skb_is_nonlinear(skb))
return 0;
if (skb->len < sizeof(struct nlattr))
return 0;
if (A > skb->len - sizeof(struct nlattr))
return 0;
nla = (struct nlattr *)&skb->data[A];
if (nla->nla_len > A - skb->len)
if (nla->nla_len > skb->len - A)
return 0;
nla = nla_find_nested(nla, X);

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