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

Purva Yeshi
2025-02-18T06:28:26Z

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.

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