Re: PROBLEM: DoS Attack on Fragment Cache

[David Ahern]

[ cc author of 648700f76b03b7e8149d13cc2bdb3355035258a9 ]

On 4/16/21 3:58 PM, Keyu Man wrote:
> Hi,
> 
>  
> 
>     My name is Keyu Man. We are a group of researchers from University
> of California, Riverside. Zhiyun Qian is my advisor. We found the code
> in processing IPv4/IPv6 fragments will potentially lead to DoS Attacks.
> Specifically, after the latest kernel receives an IPv4 fragment, it will
> try to fit it into a queue by calling function
> 
>  
> 
>     struct inet_frag_queue *inet_frag_find(struct fqdir *fqdir, void
> *key) in net/ipv4/inet_fragment.c.
> 
>  
> 
>     However, this function will first check if the existing fragment
> memory exceeds the fqdir->high_thresh. If it exceeds, then drop the
> fragment regardless whether it belongs to a new queue or an existing queue.
> 
>     Chances are that an attacker can fill the cache with fragments that
> will never be assembled (i.e., only sends the first fragment with new
> IPIDs every time) to exceed the threshold so that all future incoming
> fragmented IPv4 traffic would be blocked and dropped. Since there is no
> GC mechanism, the victim host has to wait for 30s when the fragments are
> expired to continue receive incoming fragments normally.
> 
>     In practice, given the 4MB fragment cache, the attacker only needs
> to send 1766 fragments to exhaust the cache and DoS the victim for 30s,
> whose cost is pretty low. Besides, IPv6 would also be affected since the
> issue resides in inet part.
> 
> This issue is introduced in commit
> 648700f76b03b7e8149d13cc2bdb3355035258a9 (inet: frags: use rhashtables
> for reassembly units) which removes fqdir->low_thresh, and GC worker as
> well. We would gently request to bring GC worker back to the kernel to
> prevent the DoS attacks.
> 
> Looking forward to hear from you
> 
>  
> 
>     Thanks,
> 
> Keyu Man
>