netdev - Re: Posible memory corruption from "mac80211: do not accept/forward invalid EAPOL frames"

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Date:   Wed, 30 Jun 2021 20:01:31 +0200
From:   Felix Fietkau <nbd@....name>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Davis <davikovs@...il.com>, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, Johannes Berg <johannes@...solutions.net>
Subject: Re: Posible memory corruption from "mac80211: do not accept/forward
 invalid EAPOL frames"

On 2021-06-29 19:49, Greg Kroah-Hartman wrote:
> On Tue, Jun 29, 2021 at 07:26:03PM +0200, Felix Fietkau wrote:
>> 
>> Hi,
>> 
>> On 2021-06-29 06:48, Davis wrote:
>> > Greetings!
>> > 
>> > Could it be possible that
>> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v5.12.13&id=a8c4d76a8dd4fb9666fc8919a703d85fb8f44ed8
>> > or at least its backport to 4.4 has the potential for memory
>> > corruption due to incorrect pointer calculation?
>> > Shouldn't the line:
>> >   struct ethhdr *ehdr = (void *)skb_mac_header(skb);
>> > be:
>> >   struct ethhdr *ehdr = (struct ethhdr *) skb->data;
>> > 
>> > Later ehdr->h_dest is referenced, read and (when not equal to expected
>> > value) written:
>> >   if (unlikely(skb->protocol == sdata->control_port_protocol &&
>> >       !ether_addr_equal(ehdr->h_dest, sdata->vif.addr)))
>> >     ether_addr_copy(ehdr->h_dest, sdata->vif.addr);
>> > 
>> > In my case after cherry-picking
>> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v4.4.273&id=e3d4030498c304d7c36bccc6acdedacf55402387
>> > to 4.4 kernel of an ARM device occasional memory corruption was observed.
>> > 
>> > To investigate this issue logging was added - the pointer calculation
>> > was expressed as:
>> >   struct ethhdr *ehdr = (void *)skb_mac_header(skb);
>> >   struct ethhdr *ehdr2 = (struct ethhdr *) skb->data;
>> > and memory writing was replaced by logging:
>> >   if (unlikely(skb->protocol == sdata->control_port_protocol &&
>> >       (!ether_addr_equal(ehdr->h_dest, sdata->vif.addr) ||
>> > !ether_addr_equal(ehdr2->h_dest, sdata->vif.addr))))
>> >     printk(KERN_ERR "Matching1: %u, matching2: %u, addr1: %px, addr2:
>> > %px", !ether_addr_equal(ehdr->h_dest, sdata->vif.addr),
>> > !ether_addr_equal(ehdr2->h_dest, sdata->vif.addr), ehdr->h_dest,
>> > ehdr2->h_dest);
>> > 
>> > During normal use of wifi (in residential environment) logging was
>> > triggered several times, in all cases matching1 was 1 and matching2
>> > was 0.
>> > This makes me think that normal control frames were received and
>> > correctly validated by !ether_addr_equal(ehdr2->h_dest,
>> > sdata->vif.addr), however !ether_addr_equal(ehdr->h_dest,
>> > sdata->vif.addr) was checking incorrect buffer and identified the
>> > frames as malformed/correctable.
>> > This also explains memory corruption - offset difference between both
>> > buffers (addr1 and addr2) was close to 64 KB in all cases, virtually
>> > always a random memory location (around 64 KB away from the correct
>> > buffer) will belong to something else, will have a value that differs
>> > from the expected MAC address and will get overwritten by the
>> > cherry-picked code.
>> It seems that the 4.4 backport is broken. The problem is the fact that
>> skb_mac_header is called before eth_type_trans(). This means that the
>> mac header offset still has the default value of (u16)-1, resulting in
>> the 64 KB memory offset that you observed.
>> 
>> I think that for 4.4, the code should be changed to use skb->data
>> instead of skb_mac_header. 4.9 looks broken in the same way.
>> 5.4 seems fine, so newer kernels should be fine as well.
> 
> Thanks for looking into this, can you submit a patch to fix this up in
> the older kernel trees?
Sorry, I don't have time to prepare and test the patches at the moment.

- Felix