[PATCH v3] xfrm: take iphdr size into account for esp payload size calculation

From: Benjamin Poirier
Date: Wed May 16 2012 - 15:37:05 EST


Corrects the function that determines the esp payload size.
The calculations done in esp4_get_mtu lead to overlength frames in transport
mode for certain mtu values and suboptimal frames for others.

According to what is done, mainly in esp_output(), net_header_len aka
sizeof(struct iphdr) must be taken into account before doing the alignment
calculation.

Signed-off-by: Benjamin Poirier <bpoirier@xxxxxxx>

---
Changes since v2:
* rename l3_adj to net_adj
* fix indentation

Changes since v1:
* introduce l3_adj to preserve the same returned value as before for tunnel
mode

For example, with md5 AH and 3des ESP (transport mode):
mtu = 1499 leads to FRAGFAILS
mtu = 1500 the addition of padding in the esp header could be avoided

Tested with
* transport mode E
* transport mode EA
* transport mode E + ah
* tunnel mode E

Not tested with BEET, but it should be the same as transport mode
draft-nikander-esp-beet-mode-03.txt Section 5.2:
"The wire packet format is identical to the ESP transport mode"
---
net/ipv4/esp4.c | 24 +++++++++---------------
1 files changed, 9 insertions(+), 15 deletions(-)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index 89a47b3..cb982a6 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -459,28 +459,22 @@ static u32 esp4_get_mtu(struct xfrm_state *x, int mtu)
struct esp_data *esp = x->data;
u32 blksize = ALIGN(crypto_aead_blocksize(esp->aead), 4);
u32 align = max_t(u32, blksize, esp->padlen);
- u32 rem;
-
- mtu -= x->props.header_len + crypto_aead_authsize(esp->aead);
- rem = mtu & (align - 1);
- mtu &= ~(align - 1);
+ unsigned int net_adj;

switch (x->props.mode) {
- case XFRM_MODE_TUNNEL:
- break;
- default:
case XFRM_MODE_TRANSPORT:
- /* The worst case */
- mtu -= blksize - 4;
- mtu += min_t(u32, blksize - 4, rem);
- break;
case XFRM_MODE_BEET:
- /* The worst case. */
- mtu += min_t(u32, IPV4_BEET_PHMAXLEN, rem);
+ net_adj = sizeof(struct iphdr);
break;
+ case XFRM_MODE_TUNNEL:
+ net_adj = 0;
+ break;
+ default:
+ BUG();
}

- return mtu - 2;
+ return ((mtu - x->props.header_len - crypto_aead_authsize(esp->aead) -
+ net_adj) & ~(align - 1)) + (net_adj - 2);
}

static void esp4_err(struct sk_buff *skb, u32 info)
--
1.7.7

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