1. 18 1月, 2013 1 次提交
    • J
      net: increase fragment memory usage limits · c2a93660
      Jesper Dangaard Brouer 提交于
      Increase the amount of memory usage limits for incomplete
      IP fragments.
      
      Arguing for new thresh high/low values:
      
       High threshold = 4 MBytes
       Low  threshold = 3 MBytes
      
      The fragmentation memory accounting code, tries to account for the
      real memory usage, by measuring both the size of frag queue struct
      (inet_frag_queue (ipv4:ipq/ipv6:frag_queue)) and the SKB's truesize.
      
      We want to be able to handle/hold-on-to enough fragments, to ensure
      good performance, without causing incomplete fragments to hurt
      scalability, by causing the number of inet_frag_queue to grow too much
      (resulting longer searches for frag queues).
      
      For IPv4, how much memory does the largest frag consume.
      
      Maximum size fragment is 64K, which is approx 44 fragments with
      MTU(1500) sized packets. Sizeof(struct ipq) is 200.  A 1500 byte
      packet results in a truesize of 2944 (not 2048 as I first assumed)
      
        (44*2944)+200 = 129736 bytes
      
      The current default high thresh of 262144 bytes, is obviously
      problematic, as only two 64K fragments can fit in the queue at the
      same time.
      
      How many 64K fragment can we fit into 4 MBytes:
      
        4*2^20/((44*2944)+200) = 32.34 fragment in queues
      
      An attacker could send a separate/distinct fake fragment packets per
      queue, causing us to allocate one inet_frag_queue per packet, and thus
      attacking the hash table and its lists.
      
      How many frag queue do we need to store, and given a current hash size
      of 64, what is the average list length.
      
      Using one MTU sized fragment per inet_frag_queue, each consuming
      (2944+200) 3144 bytes.
      
        4*2^20/(2944+200) = 1334 frag queues -> 21 avg list length
      
      An attack could send small fragments, the smallest packet I could send
      resulted in a truesize of 896 bytes (I'm a little surprised by this).
      
        4*2^20/(896+200)  = 3827 frag queues -> 59 avg list length
      
      When increasing these number, we also need to followup with
      improvements, that is going to help scalability.  Simply increasing
      the hash size, is not enough as the current implementation does not
      have a per hash bucket locking.
      Signed-off-by: NJesper Dangaard Brouer <brouer@redhat.com>
      Signed-off-by: NDavid S. Miller <davem@davemloft.net>
      c2a93660
  2. 17 1月, 2013 16 次提交
  3. 16 1月, 2013 23 次提交