[PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Subham Pal]

This series resolves two separate issues in the Netfilter H.323 helper:

1. Overly restrictive tuple comparisons across expectation helpers
   (expect_h245, expect_rtp_rtcp, expect_t120, expect_callforwarding)
   that bypass NAT payload mangling under DNAT, hairpin routing, and
   reverse/reply traffic directions.
2. An ASN.1 PER decoding bug in decode_seq() where absent extension fields
   are processed before verifying their presence in the extension bitmap,
   leading to packet bitstream desynchronization or premature aborts.

Subham Pal (2):
  netfilter: nf_conntrack_h323: do not bypass NAT helpers when tuple
    source matches reply destination
  netfilter: nf_conntrack_h323: check extension bitmap before decoding
    components

 net/netfilter/nf_conntrack_h323_asn1.c |  6 +++---
 net/netfilter/nf_conntrack_h323_main.c | 24 ++++++------------------
 2 files changed, 9 insertions(+), 21 deletions(-)

[PATCH net 1/2] netfilter: nf_conntrack_h323: do not bypass NAT helpers when tuple source matches reply destination

[Subham Pal]

[-- Attachment #1: Type: text/plain, Size: 1389 bytes --]

The expectation helpers (expect_h245, expect_rtp_rtcp, expect_t120, and
expect_callforwarding) test whether NAT is required by comparing the
forward source IP against the reverse destination IP:

    memcmp(&ct->tuplehash[dir].tuple.src.u3,
           &ct->tuplehash[!dir].tuple.dst.u3,
           sizeof(ct->tuplehash[dir].tuple.src.u3))

This check is broken for multiple reasons:

1. It incorrectly evaluates to 0 in the reply direction
   (dir == IP_CT_DIR_REPLY) even under SNAT, because tuplehash[REPLY].src
   and tuplehash[ORIGINAL].dst both represent the remote peer's address.
2. In Destination NAT (DNAT) and hairpin setups, the sender's source IP
   is never modified, causing this check to evaluate to false even though
   NAT payload rewriting is necessary.

As a result, NAT payload mangling is bypassed, leaving internal addresses
untranslated inside the signaling packets and leading to call setup and
media stream failures.

Remove the redundant memcmp() checks across all four expectation helpers.
Verifying that nathook is present and that ct->status matches IPS_NAT_MASK
is sufficient to determine whether NAT handling should execute.

Fixes: f587de0e2feb ("[NETFILTER]: nf_conntrack/nf_nat: add H.323 helper port")
Cc: stable@vger.kernel.org
Signed-off-by: Subham Pal <subhampal789@gmail.com>
---
[Note: Raw patch attached to avoid webmail tab/whitespace corruption]

[-- Attachment #2: 0001-netfilter-nf_conntrack_h323-do-not-bypass-NAT-helper.patch --]
[-- Type: text/x-patch, Size: 3991 bytes --]

From eb78748cc3cb6032566a649ceeadf99c788f4573 Mon Sep 17 00:00:00 2001
From: Subham Pal <subhampal789@gmail.com>
Date: Tue, 22 Sep 2026 16:19:05 +0530
Subject: [PATCH net 1/2] netfilter: nf_conntrack_h323: do not bypass NAT
 helpers when tuple source matches reply destination

The expectation helpers (expect_h245, expect_rtp_rtcp, expect_t120, and
expect_callforwarding) test whether NAT is required by comparing the
forward source IP against the reverse destination IP:

    memcmp(&ct->tuplehash[dir].tuple.src.u3,
           &ct->tuplehash[!dir].tuple.dst.u3,
           sizeof(ct->tuplehash[dir].tuple.src.u3))

This check is broken for multiple reasons:

1. It incorrectly evaluates to 0 in the reply direction
   (dir == IP_CT_DIR_REPLY) even under SNAT, because tuplehash[REPLY].src
   and tuplehash[ORIGINAL].dst both represent the remote peer's address.
2. In Destination NAT (DNAT) and hairpin setups, the sender's source IP
   is never modified, causing this check to evaluate to false even though
   NAT payload rewriting is necessary.

As a result, NAT payload mangling is bypassed, leaving internal addresses
untranslated inside the signaling packets and leading to call setup and
media stream failures.

Remove the redundant memcmp() checks across all four expectation helpers.
Verifying that nathook is present and that ct->status matches IPS_NAT_MASK
is sufficient to determine whether NAT handling should execute.

Fixes: f587de0e2feb ("[NETFILTER]: nf_conntrack/nf_nat: add H.323 helper port")
Cc: stable@vger.kernel.org
Signed-off-by: Subham Pal <subhampal789@gmail.com>
---
 net/netfilter/nf_conntrack_h323_main.c | 24 ++++++------------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/net/netfilter/nf_conntrack_h323_main.c b/net/netfilter/nf_conntrack_h323_main.c
index 4cb1665bba02..0c2aa608ae60 100644
--- a/net/netfilter/nf_conntrack_h323_main.c
+++ b/net/netfilter/nf_conntrack_h323_main.c
@@ -250,12 +250,9 @@ static int expect_rtp_rtcp(struct sk_buff *skb, struct nf_conn *ct,
 			  IPPROTO_UDP, NULL, &rtcp_port);
 
 	nathook = rcu_dereference(nfct_h323_nat_hook);
-	if (memcmp(&ct->tuplehash[dir].tuple.src.u3,
-		   &ct->tuplehash[!dir].tuple.dst.u3,
-		   sizeof(ct->tuplehash[dir].tuple.src.u3)) &&
-		   nathook &&
-		   nf_ct_l3num(ct) == NFPROTO_IPV4 &&
-		   ct->status & IPS_NAT_MASK) {
+	if (nathook &&
+		nf_ct_l3num(ct) == NFPROTO_IPV4 &&
+		ct->status & IPS_NAT_MASK) {
 		/* NAT needed */
 		ret = nathook->nat_rtp_rtcp(skb, ct, ctinfo, protoff, data, dataoff,
 					    taddr, port, rtp_port, rtp_exp, rtcp_exp);
@@ -310,10 +307,7 @@ static int expect_t120(struct sk_buff *skb,
 	exp->flags = NF_CT_EXPECT_PERMANENT;	/* Accept multiple channels */
 
 	nathook = rcu_dereference(nfct_h323_nat_hook);
-	if (memcmp(&ct->tuplehash[dir].tuple.src.u3,
-		   &ct->tuplehash[!dir].tuple.dst.u3,
-		   sizeof(ct->tuplehash[dir].tuple.src.u3)) &&
-	    nathook &&
+	if (nathook &&
 	    nf_ct_l3num(ct) == NFPROTO_IPV4 &&
 	    ct->status & IPS_NAT_MASK) {
 		/* NAT needed */
@@ -643,10 +637,7 @@ static int expect_h245(struct sk_buff *skb, struct nf_conn *ct,
 	rcu_assign_pointer(exp->assign_helper, nf_conntrack_helper_h245_ptr);
 
 	nathook = rcu_dereference(nfct_h323_nat_hook);
-	if (memcmp(&ct->tuplehash[dir].tuple.src.u3,
-		   &ct->tuplehash[!dir].tuple.dst.u3,
-		   sizeof(ct->tuplehash[dir].tuple.src.u3)) &&
-	    nathook &&
+	if (nathook &&
 	    nf_ct_l3num(ct) == NFPROTO_IPV4 &&
 	    ct->status & IPS_NAT_MASK) {
 		/* NAT needed */
@@ -770,10 +761,7 @@ static int expect_callforwarding(struct sk_buff *skb,
 	rcu_assign_pointer(exp->assign_helper, nf_conntrack_helper_q931_ptr[0]);
 
 	nathook = rcu_dereference(nfct_h323_nat_hook);
-	if (memcmp(&ct->tuplehash[dir].tuple.src.u3,
-		   &ct->tuplehash[!dir].tuple.dst.u3,
-		   sizeof(ct->tuplehash[dir].tuple.src.u3)) &&
-	    nathook &&
+	if (nathook &&
 	    nf_ct_l3num(ct) == NFPROTO_IPV4 &&
 	    ct->status & IPS_NAT_MASK) {
 		/* Need NAT */
-- 
2.43.0

[PATCH net 2/2] netfilter: nf_conntrack_h323: check extension bitmap before decoding components

[Subham Pal]

[-- Attachment #1: Type: text/plain, Size: 1260 bytes --]

In decode_seq(), the extension components loop iterates through bmp2_len
bits. However, it evaluates whether the component index exceeds the known
upper bound (`i >= f->ub`) and whether it contains a STOP attribute before
checking if the field is actually present in `bmp2`:

    if (i >= f->ub) {
        ...
        len = get_len(bs);
        ...
    }
    if (son->attr & STOP)
        return H323_ERROR_STOP;

    if (!((0x80000000 >> opt) & bmp2))
        continue;

If an unknown extension is marked absent in the bitmap (`bit == 0`), the
parser erroneously attempts to read a non-existent open type length from
the bitstream and advances `bs->cur` by garbage bytes. Furthermore, if a
known field has the STOP attribute but is absent from the transmission,
decoding aborts prematurely.

Move the presence bitmap check to the beginning of the loop body so absent
extensions are skipped immediately before attempting to decode or inspect
attributes. Also append a 'U' suffix to 0x80000000 to prevent signed shift
overflow.

Fixes: f587de0e2feb ("[NETFILTER]: nf_conntrack/nf_nat: add H.323 helper port")
Cc: stable@vger.kernel.org
Signed-off-by: Subham Pal <subhampal789@gmail.com>
---
[Note: Raw patch attached to avoid webmail tab/whitespace corruption]

[-- Attachment #2: 0002-netfilter-nf_conntrack_h323-check-extension-bitmap-b.patch --]
[-- Type: text/x-patch, Size: 2428 bytes --]

From 46835631e6949ed936a44cd8e4648b1adf7304ec Mon Sep 17 00:00:00 2001
From: Subham Pal <subhampal789@gmail.com>
Date: Tue, 22 Sep 2026 16:35:08 +0530
Subject: [PATCH net 2/2] netfilter: nf_conntrack_h323: check extension bitmap
 before decoding components

In decode_seq(), the extension components loop iterates through bmp2_len
bits. However, it evaluates whether the component index exceeds the known
upper bound (`i >= f->ub`) and whether it contains a STOP attribute before
checking if the field is actually present in `bmp2`:

    if (i >= f->ub) {
        ...
        len = get_len(bs);
        ...
    }
    if (son->attr & STOP)
        return H323_ERROR_STOP;

    if (!((0x80000000 >> opt) & bmp2))
        continue;

If an unknown extension is marked absent in the bitmap (`bit == 0`), the
parser erroneously attempts to read a non-existent open type length from
the bitstream and advances `bs->cur` by garbage bytes. Furthermore, if a
known field has the STOP attribute but is absent from the transmission,
decoding aborts prematurely.

Move the presence bitmap check to the beginning of the loop body so absent
extensions are skipped immediately before attempting to decode or inspect
attributes. Also append a 'U' suffix to 0x80000000 to prevent signed shift
overflow.

Fixes: f587de0e2feb ("[NETFILTER]: nf_conntrack/nf_nat: add H.323 helper port")
Cc: stable@vger.kernel.org
Signed-off-by: Subham Pal <subhampal789@gmail.com>
---
 net/netfilter/nf_conntrack_h323_asn1.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/netfilter/nf_conntrack_h323_asn1.c b/net/netfilter/nf_conntrack_h323_asn1.c
index 6830c9da3507..57317a28212b 100644
--- a/net/netfilter/nf_conntrack_h323_asn1.c
+++ b/net/netfilter/nf_conntrack_h323_asn1.c
@@ -596,6 +596,9 @@ static int decode_seq(struct bitstr *bs, const struct field_t *f,
 
 	/* Decode the extension components */
 	for (opt = 0; opt < bmp2_len; opt++, i++, son++) {
+		if (!((0x80000000U >> opt) & bmp2))	/* Not present */
+			continue;
+
 		/* Check Range */
 		if (i >= f->ub) {	/* Newer Version? */
 			if (nf_h323_error_boundary(bs, 2, 0))
@@ -613,9 +616,6 @@ static int decode_seq(struct bitstr *bs, const struct field_t *f,
 			return H323_ERROR_STOP;
 		}
 
-		if (!((0x80000000 >> opt) & bmp2))	/* Not present */
-			continue;
-
 		if (nf_h323_error_boundary(bs, 2, 0))
 			return H323_ERROR_BOUND;
 		len = get_len(bs);
-- 
2.43.0

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Pablo Neira Ayuso]

Hi,

On Wed, Sep 23, 2026 at 09:50:22AM +0530, Subham Pal wrote:
> This series resolves two separate issues in the Netfilter H.323 helper:
> 
> 1. Overly restrictive tuple comparisons across expectation helpers
>    (expect_h245, expect_rtp_rtcp, expect_t120, expect_callforwarding)
>    that bypass NAT payload mangling under DNAT, hairpin routing, and
>    reverse/reply traffic directions.
> 2. An ASN.1 PER decoding bug in decode_seq() where absent extension fields
>    are processed before verifying their presence in the extension bitmap,
>    leading to packet bitstream desynchronization or premature aborts.

I understand these are correctness fixes.

Do you have a reproducer/PoC? Do you tests for this?

> Subham Pal (2):
>   netfilter: nf_conntrack_h323: do not bypass NAT helpers when tuple
>     source matches reply destination
>   netfilter: nf_conntrack_h323: check extension bitmap before decoding
>     components
> 
>  net/netfilter/nf_conntrack_h323_asn1.c |  6 +++---
>  net/netfilter/nf_conntrack_h323_main.c | 24 ++++++------------------
>  2 files changed, 9 insertions(+), 21 deletions(-)

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Subham Pal]

On Wed, Sep 23, 2026 at 2:16 PM Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> I understand these are correctness fixes.
> Do you have a reproducer/PoC? Do you tests for this?

Hi Pablo,

Both issues were found while testing a netfilter based linux
firewall with communicating devices from multiple vendors
(Panasonic, PeopleLink, YeaLink and Plycom).

1. For patch 1 (NAT expectation bypass)
- Environment & Setup
  Linux firewall performing NAT between internal and external
  network segment with Panasonic, PeopleLink, YeaLink
  and Polycom endpoints.
- Observation
  Call setup failing due to missing conntrack expectation from
  initial Q.931.

2. For patch 2 (Failed ASN.1 parsing)
- Environment & Setup
  The same setup but this problem happened particularly with
  Panasonic endpoint.
- Observation
  Panasonic endpoint advertises sequence extension with missing
  optional fields during call setup. Due to misaligned check for presence
  of field, I was getting H323_ERROR_BOUND error in dmesg.

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Subham Pal]

Hi Pablo,

Just following up to check if there are any remaining questions or queries,
regarding the multi-vendor testbed I described, or if the series is
ready to be applied.

Thanks,
Subham Pal

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Pablo Neira Ayuso]

On Thu, Oct 01, 2026 at 10:09:45AM +0530, Subham Pal wrote:
> Hi Pablo,
> 
> Just following up to check if there are any remaining questions or queries,
> regarding the multi-vendor testbed I described, or if the series is
> ready to be applied.

I am not sure this is really fixing up anything.

If any, this should go to nf-next because this does not crash the
kernel.

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Pablo Neira Ayuso]

Hi,

On Wed, Sep 23, 2026 at 04:16:48PM +0530, Subham Pal wrote:
> On Wed, Sep 23, 2026 at 2:16 PM Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > I understand these are correctness fixes.
> > Do you have a reproducer/PoC? Do you tests for this?
> 
> Hi Pablo,
> 
> Both issues were found while testing a netfilter based linux
> firewall with communicating devices from multiple vendors
> (Panasonic, PeopleLink, YeaLink and Plycom).

We will need a reproducer here without requiring such hardware.

> 1. For patch 1 (NAT expectation bypass)
> - Environment & Setup
>   Linux firewall performing NAT between internal and external
>   network segment with Panasonic, PeopleLink, YeaLink
>   and Polycom endpoints.
> - Observation
>   Call setup failing due to missing conntrack expectation from
>   initial Q.931.
> 
> 2. For patch 2 (Failed ASN.1 parsing)
> - Environment & Setup
>   The same setup but this problem happened particularly with
>   Panasonic endpoint.
> - Observation
>   Panasonic endpoint advertises sequence extension with missing
>   optional fields during call setup. Due to misaligned check for presence
>   of field, I was getting H323_ERROR_BOUND error in dmesg.

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Pablo Neira Ayuso]

On Thu, Oct 01, 2026 at 10:44:35AM +0200, Pablo Neira Ayuso wrote:
> Hi,
> 
> On Wed, Sep 23, 2026 at 04:16:48PM +0530, Subham Pal wrote:
> > On Wed, Sep 23, 2026 at 2:16 PM Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > > I understand these are correctness fixes.
> > > Do you have a reproducer/PoC? Do you tests for this?
> > 
> > Hi Pablo,
> > 
> > Both issues were found while testing a netfilter based linux
> > firewall with communicating devices from multiple vendors
> > (Panasonic, PeopleLink, YeaLink and Plycom).
> 
> We will need a reproducer here without requiring such hardware.

This requirement is too strict, apologies, let's remove it.

Re: [PATCH net 0/2] netfilter: nf_conntrack_h323: fixes for NAT bypass and ASN.1 decode

[Subham Pal]

On Thu, Oct 1, 2026 at 9:28 PM Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > We will need a reproducer here without requiring such hardware.
>
> This requirement is too strict, apologies, let's remove it.

Thank you for understanding and waiving that requirement.

Per your earlier note regarding the target tree, I will rebase this series
onto nf-next and post [PATCH nf-next v2 0/2] shortly.

Regards,
Subham Pal