netdev - [PATCH net] net: esp{4,6}: fix potential MTU calculation overflows

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Date:	Mon,  5 Aug 2013 12:49:35 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org, Benjamin Poirier <bpoirier@...e.de>,
	Steffen Klassert <steffen.klassert@...unet.com>
Subject: [PATCH net] net: esp{4,6}: fix potential MTU calculation overflows

Commit 91657eafb ("xfrm: take net hdr len into account for esp payload
size calculation") introduced a possible interger overflow in
esp{4,6}_get_mtu() handlers in case of x->props.mode equals
XFRM_MODE_TUNNEL. Thus, the following expression will overflow

  unsigned int net_adj;
  ...
  <case ipv{4,6} XFRM_MODE_TUNNEL>
         net_adj = 0;
  ...
  return ((mtu - x->props.header_len - crypto_aead_authsize(esp->aead) -
           net_adj) & ~(align - 1)) + (net_adj - 2);

where (net_adj - 2) would be evaluated as <foo> + (0 - 2) in an unsigned
context. Fix it by simply removing brackets as those operations here
do not need to have special precedence.

Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
Cc: Benjamin Poirier <bpoirier@...e.de>
Cc: Steffen Klassert <steffen.klassert@...unet.com>
---
 Note: only compile tested, maybe Benjamin can comment on why he added
       brackets around this expression. *If* this is valid (which I do
       not think), then this needs at least a big comment explaining so.

 net/ipv4/esp4.c | 2 +-
 net/ipv6/esp6.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index ab3d814..109ee89 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -477,7 +477,7 @@ static u32 esp4_get_mtu(struct xfrm_state *x, int mtu)
 	}
 
 	return ((mtu - x->props.header_len - crypto_aead_authsize(esp->aead) -
-		 net_adj) & ~(align - 1)) + (net_adj - 2);
+		 net_adj) & ~(align - 1)) + net_adj - 2;
 }
 
 static void esp4_err(struct sk_buff *skb, u32 info)
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 40ffd72..aeac0dc 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -425,7 +425,7 @@ static u32 esp6_get_mtu(struct xfrm_state *x, int mtu)
 		net_adj = 0;
 
 	return ((mtu - x->props.header_len - crypto_aead_authsize(esp->aead) -
-		 net_adj) & ~(align - 1)) + (net_adj - 2);
+		 net_adj) & ~(align - 1)) + net_adj - 2;
 }
 
 static void esp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
-- 
1.7.11.7

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