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

[Arnaud Lecomte <contact@arnaud-lcm.com>]

Thanks for the review.

You're right, using a merge commit in the `Fixes:` tag was not relevant.
I'll send a v2 shortly with the fix as well as the removal of the skb 
check as it is already done before.

On 08/04/2025 17:33, Simon Horman wrote:
> On Mon, Apr 07, 2025 at 05:26:21PM +0200, 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: 9946eaf552b1 ("Merge tag 'hardening-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux")
> It doesn't seem right to use a Merge commit in a fixes tag.
>
> Looking over the code, the access to data[2] seems to have existed since
> the beginning of git history, in which case I think we can use this:
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>
>> Signed-off-by: Arnaud Lecomte <contact@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..520d895acc60 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 (!skb || !pskb_may_pull(skb, 3)) {
> I doubt that skb can be NULL.
>
>> +		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>
>>
>>