[PATCH v2] net: ppp: Add bound checking for skb d on ppp_sync_txmung

[PATCH v2] net: ppp: Add bound checking for skb d on ppp_sync_txmung

[Arnaud Lecomte]

Ensure we have enough data in linear buffer from skb before accessing
initial bytes. This prevents potential out-of-bounds accesses
when processing short packets.

When ppp_sync_txmung receives an incoming package with an empty
payload:
(remote) gef➤  p *(struct pppoe_hdr *) (skb->head + skb->network_header)
$18 = {
	type = 0x1,
	ver = 0x1,
	code = 0x0,
	sid = 0x2,
        length = 0x0,
	tag = 0xffff8880371cdb96
}

from the skb struct (trimmed)
      tail = 0x16,
      end = 0x140,
      head = 0xffff88803346f400 "4",
      data = 0xffff88803346f416 ":\377",
      truesize = 0x380,
      len = 0x0,
      data_len = 0x0,
      mac_len = 0xe,
      hdr_len = 0x0,

it is not safe to access data[2].

Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
---
Changes in v2:
- Fixed the Fixes: tag to refer to initial commit instead of merge commit (1da177e4c3f4)
- Remove the skb check as it can not be null
- Link to v1: https://lore.kernel.org/r/20250407-bound-checking-ppp_txmung-v1-1-cfcd2efe39e3@arnaud-lcm.com
---
 drivers/net/ppp/ppp_synctty.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
index 644e99fc3623..9c4932198931 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -506,6 +506,11 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
 	unsigned char *data;
 	int islcp;
 
+	/* Ensure we can safely access protocol field and LCP code */
+	if (!pskb_may_pull(skb, 3)) {
+		kfree_skb(skb);
+		return NULL;
+	}
 	data  = skb->data;
 	proto = get_unaligned_be16(data);
 

---
base-commit: 9946eaf552b194bb352c2945b54ff98c8193b3f1
change-id: 20250405-bound-checking-ppp_txmung-4807c854ed85

Best regards,
-- 
Arnaud Lecomte <contact@arnaud-lcm.com>

Re: [PATCH v2] net: ppp: Add bound checking for skb d on ppp_sync_txmung

[Paolo Abeni]

On 4/8/25 5:55 PM, Arnaud Lecomte wrote:
> Ensure we have enough data in linear buffer from skb before accessing
> initial bytes. This prevents potential out-of-bounds accesses
> when processing short packets.
> 
> When ppp_sync_txmung receives an incoming package with an empty
> payload:
> (remote) gef➤  p *(struct pppoe_hdr *) (skb->head + skb->network_header)
> $18 = {
> 	type = 0x1,
> 	ver = 0x1,
> 	code = 0x0,
> 	sid = 0x2,
>         length = 0x0,
> 	tag = 0xffff8880371cdb96
> }
> 
> from the skb struct (trimmed)
>       tail = 0x16,
>       end = 0x140,
>       head = 0xffff88803346f400 "4",
>       data = 0xffff88803346f416 ":\377",
>       truesize = 0x380,
>       len = 0x0,
>       data_len = 0x0,
>       mac_len = 0xe,
>       hdr_len = 0x0,
> 
> it is not safe to access data[2].
> 
> Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
> Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>

A couple of notes for future submission: you should have included the
target tree in the subj prefix (in this case: "[PATCH net v2]..."), and
there is a small typo in the subjected (skb d -> skb data). The patch
looks good I'm applying it.

Thanks,

Paolo

Re: [PATCH v2] net: ppp: Add bound checking for skb d on ppp_sync_txmung

[Arnaud Lecomte]

Thanks Paolo for the feedback, I'll make sure to follow your 
recommendations next time.

Have a lovely day,

Arnaud

On 10/04/2025 11:30, Paolo Abeni wrote:
> On 4/8/25 5:55 PM, Arnaud Lecomte wrote:
>> Ensure we have enough data in linear buffer from skb before accessing
>> initial bytes. This prevents potential out-of-bounds accesses
>> when processing short packets.
>>
>> When ppp_sync_txmung receives an incoming package with an empty
>> payload:
>> (remote) gef➤  p *(struct pppoe_hdr *) (skb->head + skb->network_header)
>> $18 = {
>> 	type = 0x1,
>> 	ver = 0x1,
>> 	code = 0x0,
>> 	sid = 0x2,
>>          length = 0x0,
>> 	tag = 0xffff8880371cdb96
>> }
>>
>> from the skb struct (trimmed)
>>        tail = 0x16,
>>        end = 0x140,
>>        head = 0xffff88803346f400 "4",
>>        data = 0xffff88803346f416 ":\377",
>>        truesize = 0x380,
>>        len = 0x0,
>>        data_len = 0x0,
>>        mac_len = 0xe,
>>        hdr_len = 0x0,
>>
>> it is not safe to access data[2].
>>
>> Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
>> Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
> A couple of notes for future submission: you should have included the
> target tree in the subj prefix (in this case: "[PATCH net v2]..."), and
> there is a small typo in the subjected (skb d -> skb data). The patch
> looks good I'm applying it.
>
> Thanks,
>
> Paolo
>

Re: [PATCH v2] net: ppp: Add bound checking for skb d on ppp_sync_txmung

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Tue, 08 Apr 2025 17:55:08 +0200 you wrote:
> Ensure we have enough data in linear buffer from skb before accessing
> initial bytes. This prevents potential out-of-bounds accesses
> when processing short packets.
> 
> When ppp_sync_txmung receives an incoming package with an empty
> payload:
> (remote) gef➤  p *(struct pppoe_hdr *) (skb->head + skb->network_header)
> $18 = {
> 	type = 0x1,
> 	ver = 0x1,
> 	code = 0x0,
> 	sid = 0x2,
>         length = 0x0,
> 	tag = 0xffff8880371cdb96
> }
> 
> [...]

Here is the summary with links:
  - [v2] net: ppp: Add bound checking for skb d on ppp_sync_txmung
    https://git.kernel.org/netdev/net/c/aabc6596ffb3

You are awesome, thank you!
-- 
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