linux-kernel - Re: [PATCH] netlink: add buffer boundary checking

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Date:   Fri, 24 Jul 2020 14:14:25 -0700
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Mark Salyzyn <salyzyn@...roid.com>,
        linux-kernel@...r.kernel.org
Cc:     kernel-team@...roid.com, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Thomas Graf <tgraf@...g.ch>
Subject: Re: [PATCH] netlink: add buffer boundary checking



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@...roid.com>
>> Cc: netdev@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: kernel-team@...roid.com
>> Cc: "David S. Miller" <davem@...emloft.net>
>> Cc: Jakub Kicinski <kuba@...nel.org>
>> Cc: Thomas Graf <tgraf@...g.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.