[PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

[PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

[Ashutosh Desai]

A crafted AX.25 frame with a valid address header but no control byte
causes skb->len to reach zero after skb_pull() strips the header.
The subsequent reads of skb->data[0] (control) and skb->data[1] (PID)
are then out of bounds.

Linearize the skb after confirming the device is an AX.25 interface.
Guard with skb->len < 1 after the pull - one byte suffices for LAPB
control frames which have no PID byte. Add a separate skb->len < 2
check inside the UI branch before accessing the PID byte.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
v5:
- Move skb_linearize() to after ax25_dev_ax25dev() check; avoids
  unnecessary allocation for frames on non-AX.25 interfaces
- Lower general guard from skb->len < 2 to skb->len < 1; the stricter
  limit incorrectly dropped valid 1-byte LAPB control frames (SABM,
  DISC, UA, DM, RR) which carry no PID byte
- Add explicit skb->len < 2 check inside UI branch before the PID
  byte (skb->data[1]) access
v4:
- Linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
  skb->len < 2 check (per David Laight review)
v3:
- Remove incorrect Suggested-by; add Fixes:, Cc: stable@
v2:
- Replace skb->len check with pskb_may_pull(skb, 2)

Link to v4: https://lore.kernel.org/netdev/20260417065407.206499-1-ashutoshdesai993@gmail.com/
Link to v3: https://lore.kernel.org/netdev/20260415063654.3831353-1-ashutoshdesai993@gmail.com/
Link to v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/
Link to v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/

 net/ax25/ax25_in.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index d75b3e9ed93d..c81d6830af48 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -199,6 +199,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
 		goto free;
 
+	if (skb_linearize(skb))
+		goto free;
+
 	/*
 	 *	Parse the address header.
 	 */
@@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	 */
 	skb_pull(skb, ax25_addr_size(&dp));
 
+	if (skb->len < 1)
+		goto free;
+
 	/* For our port addresses ? */
 	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
 		mine = 1;
@@ -227,6 +233,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 
 	/* UI frame - bypass LAPB processing */
 	if ((*skb->data & ~0x10) == AX25_UI && dp.lastrepeat + 1 == dp.ndigi) {
+		if (skb->len < 2)
+			goto free;
+
 		skb_set_transport_header(skb, 2); /* skip control and pid */
 
 		ax25_send_to_raw(&dest, skb, skb->data[1]);
-- 
2.34.1

Re: [PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

[Ashutosh Desai]

[-- Attachment #1: Type: text/plain, Size: 297 bytes --]

On Tue, Apr 21, 2026 at 05:48:58 +0000, Ashutosh Desai wrote:
> [PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

Please ignore this patch. A net-deletions patch removing the entire ax25
and amateur radio subsystem was posted earlier today. This fix is no
longer needed.

[PATCH v4 net] ax25: fix OOB read after address header strip in ax25_rcv()

[Ashutosh Desai]

A crafted AX.25 frame with a valid address header but no control or PID
bytes causes skb->len to drop to zero after skb_pull() strips the
address header. The subsequent reads of skb->data[0] and skb->data[1]
are then out of bounds.

Linearize the skb at entry to ax25_rcv() so all subsequent accesses to
skb->data are safe. Then check skb->len before reading the control and
PID bytes, discarding frames that are too short.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
v4: linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
    skb->len check
v3: remove incorrect Suggested-by; add symptom, Fixes, Cc stable
v2: use pskb_may_pull(skb, 2) instead of skb->len < 2

Link to v3: https://lore.kernel.org/netdev/20260415063654.3831353-1-ashutoshdesai993@gmail.com/
Link to v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/
Link to v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/

 net/ax25/ax25_in.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index d75b3e9ed93d..d14ccebf9cdd 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -190,6 +190,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	ax25_cb *ax25;
 	ax25_dev *ax25_dev;
 
+	if (skb_linearize(skb))
+		goto free;
+
 	/*
 	 *	Process the AX.25/LAPB frame.
 	 */
@@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	 */
 	skb_pull(skb, ax25_addr_size(&dp));
 
+	if (skb->len < 2)
+		goto free;
+
 	/* For our port addresses ? */
 	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
 		mine = 1;
-- 
2.34.1

[PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

[Ashutosh Desai]

A crafted AX.25 frame with a valid address header but no control byte
causes skb->len to reach zero after skb_pull() strips the header.
The subsequent reads of skb->data[0] (control) and skb->data[1] (PID)
are then out of bounds.

Linearize the skb after confirming the device is an AX.25 interface.
Guard with skb->len < 1 after the pull - one byte suffices for LAPB
control frames which have no PID byte. Add a separate skb->len < 2
check inside the UI branch before accessing the PID byte.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
v5:
- Move skb_linearize() to after ax25_dev_ax25dev() check; avoids
  unnecessary allocation for frames on non-AX.25 interfaces
- Lower general guard from skb->len < 2 to skb->len < 1; the stricter
  limit incorrectly dropped valid 1-byte LAPB control frames (SABM,
  DISC, UA, DM, RR) which carry no PID byte
- Add explicit skb->len < 2 check inside UI branch before the PID
  byte (skb->data[1]) access
v4:
- Linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
  skb->len < 2 check (per David Laight review)
v3:
- Remove incorrect Suggested-by; add Fixes:, Cc: stable@
v2:
- Replace skb->len check with pskb_may_pull(skb, 2)

Link to v4: https://lore.kernel.org/netdev/20260417065407.206499-1-ashutoshdesai993@gmail.com/
Link to v3: https://lore.kernel.org/netdev/20260415063654.3831353-1-ashutoshdesai993@gmail.com/
Link to v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/
Link to v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/

 net/ax25/ax25_in.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index d75b3e9ed93d..c81d6830af48 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -199,6 +199,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
 		goto free;
 
+	if (skb_linearize(skb))
+		goto free;
+
 	/*
 	 *	Parse the address header.
 	 */
@@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	 */
 	skb_pull(skb, ax25_addr_size(&dp));
 
+	if (skb->len < 1)
+		goto free;
+
 	/* For our port addresses ? */
 	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
 		mine = 1;
@@ -227,6 +233,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 
 	/* UI frame - bypass LAPB processing */
 	if ((*skb->data & ~0x10) == AX25_UI && dp.lastrepeat + 1 == dp.ndigi) {
+		if (skb->len < 2)
+			goto free;
+
 		skb_set_transport_header(skb, 2); /* skip control and pid */
 
 		ax25_send_to_raw(&dest, skb, skb->data[1]);
-- 
2.34.1

Re: [PATCH v5 net] ax25: fix OOB read after address header strip in ax25_rcv()

[David Laight]

On Tue, 21 Apr 2026 05:46:26 +0000
Ashutosh Desai <ashutoshdesai993@gmail.com> wrote:

> A crafted AX.25 frame with a valid address header but no control byte
> causes skb->len to reach zero after skb_pull() strips the header.
> The subsequent reads of skb->data[0] (control) and skb->data[1] (PID)
> are then out of bounds.
> 
> Linearize the skb after confirming the device is an AX.25 interface.
> Guard with skb->len < 1 after the pull - one byte suffices for LAPB
> control frames which have no PID byte. Add a separate skb->len < 2
> check inside the UI branch before accessing the PID byte.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
> ---
> v5:
> - Move skb_linearize() to after ax25_dev_ax25dev() check; avoids
>   unnecessary allocation for frames on non-AX.25 interfaces

Nitpick: 'on interfaces where AX.25 isn't enabled'
They still have to be AX.25 frames and get discarded.
So they won't really be expected and any allocated memory is
immediately freed.
More relevant would be linearizing before the ax25_addr_parse() call.
In any case I suspect this code never sees non-linear packets.
The packets will all be short, I don't know ax25, but X.25 (which I've
implemented most of in the past) originally had an mtu of 128 bytes
(and real links running at 2400 baud).

	David


> - Lower general guard from skb->len < 2 to skb->len < 1; the stricter
>   limit incorrectly dropped valid 1-byte LAPB control frames (SABM,
>   DISC, UA, DM, RR) which carry no PID byte
> - Add explicit skb->len < 2 check inside UI branch before the PID
>   byte (skb->data[1]) access
> v4:
> - Linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
>   skb->len < 2 check (per David Laight review)
> v3:
> - Remove incorrect Suggested-by; add Fixes:, Cc: stable@
> v2:
> - Replace skb->len check with pskb_may_pull(skb, 2)
> 
> Link to v4: https://lore.kernel.org/netdev/20260417065407.206499-1-ashutoshdesai993@gmail.com/
> Link to v3: https://lore.kernel.org/netdev/20260415063654.3831353-1-ashutoshdesai993@gmail.com/
> Link to v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/
> Link to v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/
> 
>  net/ax25/ax25_in.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
> index d75b3e9ed93d..c81d6830af48 100644
> --- a/net/ax25/ax25_in.c
> +++ b/net/ax25/ax25_in.c
> @@ -199,6 +199,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
>  	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
>  		goto free;
>  
> +	if (skb_linearize(skb))
> +		goto free;
> +
>  	/*
>  	 *	Parse the address header.
>  	 */
> @@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
>  	 */
>  	skb_pull(skb, ax25_addr_size(&dp));
>  
> +	if (skb->len < 1)
> +		goto free;
> +
>  	/* For our port addresses ? */
>  	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
>  		mine = 1;
> @@ -227,6 +233,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
>  
>  	/* UI frame - bypass LAPB processing */
>  	if ((*skb->data & ~0x10) == AX25_UI && dp.lastrepeat + 1 == dp.ndigi) {
> +		if (skb->len < 2)
> +			goto free;
> +
>  		skb_set_transport_header(skb, 2); /* skip control and pid */
>  
>  		ax25_send_to_raw(&dest, skb, skb->data[1]);