Re: [PATCH] netlink: add buffer boundary checking

[Mark Salyzyn <salyzyn@android.com>]

On 7/24/20 2:14 PM, Jacob Keller wrote:
>
> On 7/23/2020 12:35 PM, Eric Dumazet wrote:
>> On 7/23/20 11:21 AM, Mark Salyzyn wrote:
>>> Many of the nla_get_* inlines fail to check attribute's length before
>>> copying the content resulting in possible out-of-boundary accesses.
>>> Adjust the inlines to perform nla_len checking, for the most part
>>> using the nla_memcpy function to faciliate since these are not
>>> necessarily performance critical and do not need a likely fast path.
>>>
>>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>>> Cc: netdev@vger.kernel.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Cc: kernel-team@android.com
>>> Cc: "David S. Miller" <davem@davemloft.net>
>>> Cc: Jakub Kicinski <kuba@kernel.org>
>>> Cc: Thomas Graf <tgraf@suug.ch>
>>> Fixes: bfa83a9e03cf ("[NETLINK]: Type-safe netlink messages/attributes interface")
>>> ---
>>>   include/net/netlink.h | 66 +++++++++++++++++++++++++++++++++++--------
>>>   1 file changed, 54 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/include/net/netlink.h b/include/net/netlink.h
>>> index c0411f14fb53..11c0f153be7c 100644
>>> --- a/include/net/netlink.h
>>> +++ b/include/net/netlink.h
>>> @@ -1538,7 +1538,11 @@ static inline int nla_put_bitfield32(struct sk_buff *skb, int attrtype,
>>>    */
>>>   static inline u32 nla_get_u32(const struct nlattr *nla)
>>>   {
>>> -	return *(u32 *) nla_data(nla);
>>> +	u32 tmp;
>>> +
>>> +	nla_memcpy(&tmp, nla, sizeof(tmp));
>>> +
>>> +	return tmp;
>> I believe this will hide bugs, that syzbot was able to catch.
>>
>> Instead, you could perhaps introduce a CONFIG_DEBUG_NETLINK option,
>> and add a WARN_ON_ONCE(nla_len(nla) < sizeof(u32)) so that we can detect bugs in callers.
>>
>>
> I also think this is a better approach.

We (another engineer here) are looking into that and will get back to 
everyone.

Sincerely -- Mark Salyzyn