Re: [PATCH net-next v1] l2tp: fix reading optional fields

[Jacob Wen <jian.w.wen@oracle.com>]

On 1/23/19 10:51 AM, Eric Dumazet wrote:
>
> On 01/22/2019 06:30 PM, Jacob Wen wrote:
>> Use pskb_may_pull() to make sure the optional fields are in skb linear
>> parts.
>>
>> Signed-off-by: Jacob Wen <jian.w.wen@oracle.com>
>> ---
>> v1: fix warning: ISO C90 forbids mixed declarations and code
>> ---
>>   net/l2tp/l2tp_core.c | 12 +++++++++++-
>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
>> index 26f1d435696a..c0dfa2bcd218 100644
>> --- a/net/l2tp/l2tp_core.c
>> +++ b/net/l2tp/l2tp_core.c
>> @@ -627,6 +627,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
>>   
>>   	/* Parse and check optional cookie */
>>   	if (session->peer_cookie_len > 0) {
>> +		if (!pskb_may_pull(skb, ptr - optr + session->peer_cookie_len))
>> +			goto discard;
> This is completely buggy.
>
> After pskb_may_pull(), the ptr and optr pointers might point to freed data.
>
> So the memcmp() might crash hard.
Fixed in v2. Thanks.
>
>>   		if (memcmp(ptr, &session->peer_cookie[0], session->peer_cookie_len)) {
>>   			l2tp_info(tunnel, L2TP_MSG_DATA,
>>   				  "%s: cookie mismatch (%u/%u). Discarding.\n",
>> @@ -649,6 +651,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
>>   	L2TP_SKB_CB(skb)->has_seq = 0;
>>   	if (tunnel->version == L2TP_HDR_VER_2) {
>>   		if (hdrflags & L2TP_HDRFLAG_S) {
>> +			if (!pskb_may_pull(skb, ptr - optr + 4))
>> +				goto discard;
>>   			ns = ntohs(*(__be16 *) ptr);
>>   			ptr += 2;
>>   			nr = ntohs(*(__be16 *) ptr);
>> @@ -663,8 +667,12 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
>>   				 session->name, ns, nr, session->nr);
>>   		}
>>   	} else if (session->l2specific_type == L2TP_L2SPECTYPE_DEFAULT) {
>> -		u32 l2h = ntohl(*(__be32 *) ptr);
>> +		u32 l2h;
>> +
>> +		if (!pskb_may_pull(skb, ptr - optr + 4))
>> +			goto discard;
>>   
>> +		l2h = ntohl(*(__be32 *) ptr);
>>   		if (l2h & 0x40000000) {
>>   			ns = l2h & 0x00ffffff;
>>   
>> @@ -729,6 +737,8 @@ void l2tp_recv_common(struct l2tp_session *session, struct sk_buff *skb,
>>   	if (tunnel->version == L2TP_HDR_VER_2) {
>>   		/* If offset bit set, skip it. */
>>   		if (hdrflags & L2TP_HDRFLAG_O) {
>> +			if (!pskb_may_pull(skb, ptr - optr + 2))
>> +				goto discard;
>>   			offset = ntohs(*(__be16 *)ptr);
>>   			ptr += 2 + offset;
>>   		}
>>

-- 
Jacob