[PATCH] ppp: fix BUG on non-linear SKB (multilink receive)

[PATCH] ppp: fix BUG on non-linear SKB (multilink receive)

[Ben McKeegan]

PPP does not correctly call pskb_may_pull() on all necessary receive paths
before reading the PPP protocol, thus causing PPP to report seemingly
random 'unsupported protocols' and eventually trigger BUG_ON(skb->len <
skb->data_len) in skb_pull_rcsum() when receiving multilink protocol in
non-linear skbs.

ppp_receive_nonmp_frame() does not call pskb_may_pull() before reading the
protocol number.  For the non-mp receive path this is not a problem, as
this check is done in ppp_receive_frame().  For the mp receive path,
ppp_mp_reconstruct() usually copies the data into a new linear skb.
However, in the case where the frame is made up of a single mp fragment,
the mp header is pulled and the existing skb used.  This skb was then
passed to ppp_receive_nonmp_frame() without checking if the encapsulated
protocol header can safely be read.

Signed-off-by: Ben McKeegan <ben@netservers.co.uk>
---
This a long standing bug we have observed with PPP over L2TP received on
an e1000e interface, although it may arise with other PPP encapsulations
or devices.

diff -uprN linux-2.6.31.6/drivers/net/ppp_generic.c linux-2.6.31.6-ppp-non-linear-skb/drivers/net/ppp_generic.c
--- linux-2.6.31.6/drivers/net/ppp_generic.c	2009-11-10 00:32:31.000000000 +0000
+++ linux-2.6.31.6-ppp-non-linear-skb/drivers/net/ppp_generic.c	2009-11-12 11:58:40.000000000 +0000
@@ -1944,7 +1944,13 @@ ppp_receive_mp_frame(struct ppp *ppp, st

 	/* Pull completed packets off the queue and receive them. */
 	while ((skb = ppp_mp_reconstruct(ppp)))
-		ppp_receive_nonmp_frame(ppp, skb);
+	  	if (pskb_may_pull(skb, 2))
+			ppp_receive_nonmp_frame(ppp, skb);
+		else {
+			++ppp->dev->stats.rx_length_errors;
+			kfree_skb(skb);
+			ppp_receive_error(ppp);
+		}

 	return;

Re: [PATCH] ppp: fix BUG on non-linear SKB (multilink receive)

[David Miller]

From: Ben McKeegan <ben@netservers.co.uk>
Date: Thu, 12 Nov 2009 13:09:57 +0000 (GMT)

> @@ -1944,7 +1944,13 @@ ppp_receive_mp_frame(struct ppp *ppp, st
> 
>  	/* Pull completed packets off the queue and receive them. */
>  	while ((skb = ppp_mp_reconstruct(ppp)))
> -		ppp_receive_nonmp_frame(ppp, skb);
> +	  	if (pskb_may_pull(skb, 2))
> +			ppp_receive_nonmp_frame(ppp, skb);
> +		else {
> +			++ppp->dev->stats.rx_length_errors;
> +			kfree_skb(skb);
> +			ppp_receive_error(ppp);
> +		}
> 
>  	return;

This fix looks correct, but could you please enclose the while()
loop in braces, that dangling else and the subsequent "return;"
statement look confusing otherwise.

Thanks.

Re: [PATCH] ppp: fix BUG on non-linear SKB (multilink receive)

[Ben McKeegan]

PPP does not correctly call pskb_may_pull() on all necessary receive paths
before reading the PPP protocol, thus causing PPP to report seemingly
random 'unsupported protocols' and eventually trigger BUG_ON(skb->len <
skb->data_len) in skb_pull_rcsum() when receiving multilink protocol in
non-linear skbs.

ppp_receive_nonmp_frame() does not call pskb_may_pull() before reading the
protocol number.  For the non-mp receive path this is not a problem, as
this check is done in ppp_receive_frame().  For the mp receive path,
ppp_mp_reconstruct() usually copies the data into a new linear skb.
However, in the case where the frame is made up of a single mp fragment,
the mp header is pulled and the existing skb used.  This skb was then
passed to ppp_receive_nonmp_frame() without checking if the encapsulated
protocol header could safely be read.

Signed-off-by: Ben McKeegan <ben@netservers.co.uk>
---
diff -uprN linux-2.6.31.6/drivers/net/ppp_generic.c linux-2.6.31.6-ppp-non-linear-skb/drivers/net/ppp_generic.c
--- linux-2.6.31.6/drivers/net/ppp_generic.c	2009-11-10 00:32:31.000000000 +0000
+++ linux-2.6.31.6-ppp-non-linear-skb/drivers/net/ppp_generic.c	2009-11-16 13:14:22.000000000 +0000
@@ -1943,8 +1943,15 @@ ppp_receive_mp_frame(struct ppp *ppp, st
 	}

 	/* Pull completed packets off the queue and receive them. */
-	while ((skb = ppp_mp_reconstruct(ppp)))
-		ppp_receive_nonmp_frame(ppp, skb);
+	while ((skb = ppp_mp_reconstruct(ppp))) {
+	  	if (pskb_may_pull(skb, 2))
+			ppp_receive_nonmp_frame(ppp, skb);
+		else {
+			++ppp->dev->stats.rx_length_errors;
+			kfree_skb(skb);
+			ppp_receive_error(ppp);
+		}
+	}

 	return;

Re: [PATCH] ppp: fix BUG on non-linear SKB (multilink receive)

[David Miller]

From: Ben McKeegan <ben@netservers.co.uk>
Date: Mon, 16 Nov 2009 13:44:25 +0000 (GMT)

> PPP does not correctly call pskb_may_pull() on all necessary receive paths
> before reading the PPP protocol, thus causing PPP to report seemingly
> random 'unsupported protocols' and eventually trigger BUG_ON(skb->len <
> skb->data_len) in skb_pull_rcsum() when receiving multilink protocol in
> non-linear skbs.
> 
> ppp_receive_nonmp_frame() does not call pskb_may_pull() before reading the
> protocol number.  For the non-mp receive path this is not a problem, as
> this check is done in ppp_receive_frame().  For the mp receive path,
> ppp_mp_reconstruct() usually copies the data into a new linear skb.
> However, in the case where the frame is made up of a single mp fragment,
> the mp header is pulled and the existing skb used.  This skb was then
> passed to ppp_receive_nonmp_frame() without checking if the encapsulated
> protocol header could safely be read.
> 
> Signed-off-by: Ben McKeegan <ben@netservers.co.uk>

Applied but:

> +	while ((skb = ppp_mp_reconstruct(ppp))) {
> +	  	if (pskb_may_pull(skb, 2))

There is a mix of space and tab characters leading up to
the "if (pskb_may_pull" there, I fixed it up but please
take care of such trivial issues before submitting your
patch next time.

Thanks.