Re: [PATCH] ppp: Prevent out-of-bounds access in ppp_sync_txmunge

Purva Yeshi
2025-02-25T06:20:57Z

On 19/02/25 00:20, Kuniyuki Iwashima wrote:
> From: Purva Yeshi <purvayeshi550@gmail.com>
> Date: Tue, 18 Feb 2025 11:58:17 +0530
>> On 18/02/25 02:46, Kuniyuki Iwashima wrote:
>>> From: Purva Yeshi <purvayeshi550@gmail.com>
>>> Date: Sun, 16 Feb 2025 11:34:46 +0530
>>>> Fix an issue detected by syzbot with KMSAN:
>>>>
>>>> BUG: KMSAN: uninit-value in ppp_sync_txmunge
>>>> drivers/net/ppp/ppp_synctty.c:516 [inline]
>>>> BUG: KMSAN: uninit-value in ppp_sync_send+0x21c/0xb00
>>>> drivers/net/ppp/ppp_synctty.c:568
>>>>
>>>> Ensure sk_buff is valid and has at least 3 bytes before accessing its
>>>> data field in ppp_sync_txmunge(). Without this check, the function may
>>>> attempt to read uninitialized or invalid memory, leading to undefined
>>>> behavior.
>>>>
>>>> To address this, add a validation check at the beginning of the function
>>>> to safely handle cases where skb is NULL or too small. If either condition
>>>> is met, free the skb and return NULL to prevent processing an invalid
>>>> packet.
>>>>
>>>> Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
>>>> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
>>>> Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
>>>> Signed-off-by: Purva Yeshi <purvayeshi550@gmail.com>
>>>> ---
>>>> drivers/net/ppp/ppp_synctty.c | 6 ++++++
>>>> 1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
>>>> index 644e99fc3..e537ea3d9 100644
>>>> --- a/drivers/net/ppp/ppp_synctty.c
>>>> +++ b/drivers/net/ppp/ppp_synctty.c
>>>> @@ -506,6 +506,12 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
>>>> unsigned char *data;
>>>> int islcp;
>>>>
>>>> + /* Ensure skb is not NULL and has at least 3 bytes */
>>>> + if (!skb || skb->len < 3) {
>>>
>>> When is skb NULL ?
>>
>> skb pointer can be NULL in cases where memory allocation for the socket
>> buffer fails, or if an upstream function incorrectly passes a NULL
>> reference due to improper error handling.
>
> Which caller passes NULL skb ?
>
> If it's really possible, you'll see null-ptr-deref at
>
> data = skb->data;
>
> below instead of KMSAN's uninit splat.

Understood. I’ll check where the skb pointer is receiving uninitialized
data.

>
>
>>
>> Additionally, skb->len being less than 3 can occur if the received
>> packet is truncated or malformed, leading to out-of-bounds memory access
>> when attempting to read data[2].
>>
>>>
>>>
>>>> + kfree_skb(skb);
>>>> + return NULL;
>>>> + }
>>>> +
>>>> data = skb->data;
>>>> proto = get_unaligned_be16(data);
>>>>
>>>> --
>>>> 2.34.1