Re: [PATCH 3.4 022/214] filter: prevent nla extensions to peek beyond the end of the message

[Luis Henriques]

On Wed, Jun 04, 2014 at 09:16:25PM -0700, Greg Kroah-Hartman wrote:
> 3.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> 
> [ Upstream commit 05ab8f2647e4221cbdb3856dd7d32bd5407316b3 ]
> 
> 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: 4738c1db15 ("[SKFILTER]: Add SKF_ADF_NLATTR instruction")
> Fixes: d214c7537b ("filter: add SKF_AD_NLATTR_NEST to look for nested..")
> Cc: Patrick McHardy <kaber@xxxxxxxxx>
> Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> Acked-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
>  net/core/filter.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -338,11 +338,15 @@ load_b:
>  
>  			if (skb_is_nonlinear(skb))
>  				return 0;
> +			if (skb->len < sizeof(struct nlattr))
> +				return 0;
> +			if (skb->len < sizeof(struct nlattr))
> +				return 0;

There above code is duplicated this backport.  The same comment
applies to the 3.2.y backport (I've added Ben to the CC list).

Cheers,
--
Luís

>  			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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