[PATCH 5.4 60/61] netfilter: nft_exthdr: fix endianness of tcp option cast

Sasha Levin
2021-08-24T17:19:43Z

From: Sergey Marinkevich <sergey.marinkevich@eltex-co.ru>

[ Upstream commit 2e34328b396a69b73661ba38d47d92b7cf21c2c4 ]

I got a problem on MIPS with Big-Endian is turned on: every time when
NF trying to change TCP MSS it returns because of new.v16 was greater
than old.v16. But real MSS was 1460 and my rule was like this:

add rule table chain tcp option maxseg size set 1400

And 1400 is lesser that 1460, not greater.

Later I founded that main causer is cast from u32 to __be16.

Debugging:

In example MSS = 1400(HEX: 0x578). Here is representation of each byte
like it is in memory by addresses from left to right(e.g. [0x0 0x1 0x2
0x3]). LE — Little-Endian system, BE — Big-Endian, left column is type.

LE BE
u32: [78 05 00 00] [00 00 05 78]

As you can see, u32 representation will be casted to u16 from different
half of 4-byte address range. But actually nf_tables uses registers and
store data of various size. Actually TCP MSS stored in 2 bytes. But
registers are still u32 in definition:

struct nft_regs {
union {
u32 data[20];
struct nft_verdict verdict;
};
};

So, access like regs->data[priv->sreg] exactly u32. So, according to
table presents above, per-byte representation of stored TCP MSS in
register will be:

LE BE
(u32)regs->data[]: [78 05 00 00] [05 78 00 00]
^^ ^^

We see that register uses just half of u32 and other 2 bytes may be
used for some another data. But in nft_exthdr_tcp_set_eval() it casted
just like u32 -> __be16:

new.v16 = src

But u32 overfill __be16, so it get 2 low bytes. For clarity draw
one more table(<xx xx> means that bytes will be used for cast).

LE BE
u32: [<78 05> 00 00] [00 00 <05 78>]
(u32)regs->data[]: [<78 05> 00 00] [05 78 <00 00>]

As you can see, for Little-Endian nothing changes, but for Big-endian we
take the wrong half. In my case there is some other data instead of
zeros, so new MSS was wrongly greater.

For shooting this bug I used solution for ports ranges. Applying of this
patch does not affect Little-Endian systems.

Signed-off-by: Sergey Marinkevich <sergey.marinkevich@eltex-co.ru>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/netfilter/nft_exthdr.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
index 00f4323cfeb8..faa0844c01fb 100644
--- a/net/netfilter/nft_exthdr.c
+++ b/net/netfilter/nft_exthdr.c
@@ -231,7 +231,6 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
unsigned int i, optl, tcphdr_len, offset;
struct tcphdr *tcph;
u8 *opt;
- u32 src;

tcph = nft_tcp_header_pointer(pkt, sizeof(buff), buff, &tcphdr_len);
if (!tcph)
@@ -240,7 +239,6 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
opt = (u8 *)tcph;
for (i = sizeof(*tcph); i < tcphdr_len - 1; i += optl) {
union {
- u8 octet;
__be16 v16;
__be32 v32;
} old, new;
@@ -262,13 +260,13 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
if (!tcph)
return;

- src = regs->data[priv->sreg];
offset = i + priv->offset;

switch (priv->len) {
case 2:
old.v16 = get_unaligned((u16 *)(opt + offset));
- new.v16 = src;
+ new.v16 = (__force __be16)nft_reg_load16(
+ &regs->data[priv->sreg]);

switch (priv->type) {
case TCPOPT_MSS:
@@ -286,7 +284,7 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
old.v16, new.v16, false);
break;
case 4:
- new.v32 = src;
+ new.v32 = regs->data[priv->sreg];
old.v32 = get_unaligned((u32 *)(opt + offset));

if (old.v32 == new.v32)
--
2.30.2