From: Sergey Marinkevich <sergey.marinkevich@xxxxxxxxxxx>
[ 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@xxxxxxxxxxx>
Acked-by: Florian Westphal <fw@xxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
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 64e69d6683ca..93fee4106019 100644
--- a/net/netfilter/nft_exthdr.c
+++ b/net/netfilter/nft_exthdr.c
@@ -137,7 +137,6 @@ 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)
@@ -146,7 +145,6 @@ 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;
@@ -167,13 +165,13 @@ 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(
+ ®s->data[priv->sreg]);
switch (priv->type) {
case TCPOPT_MSS:
@@ -191,7 +189,7 @@ 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
