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    Subject[PATCH 4.14 236/237] tcp: increase source port perturb table to 2^16
    Date
    From: Willy Tarreau <w@1wt.eu>

    commit 4c2c8f03a5ab7cb04ec64724d7d176d00bcc91e5 upstream.

    Moshe Kol, Amit Klein, and Yossi Gilad reported being able to accurately
    identify a client by forcing it to emit only 40 times more connections
    than there are entries in the table_perturb[] table. The previous two
    improvements consisting in resalting the secret every 10s and adding
    randomness to each port selection only slightly improved the situation,
    and the current value of 2^8 was too small as it's not very difficult
    to make a client emit 10k connections in less than 10 seconds.

    Thus we're increasing the perturb table from 2^8 to 2^16 so that the
    same precision now requires 2.6M connections, which is more difficult in
    this time frame and harder to hide as a background activity. The impact
    is that the table now uses 256 kB instead of 1 kB, which could mostly
    affect devices making frequent outgoing connections. However such
    components usually target a small set of destinations (load balancers,
    database clients, perf assessment tools), and in practice only a few
    entries will be visited, like before.

    A live test at 1 million connections per second showed no performance
    difference from the previous value.

    Reported-by: Moshe Kol <moshe.kol@mail.huji.ac.il>
    Reported-by: Yossi Gilad <yossi.gilad@mail.huji.ac.il>
    Reported-by: Amit Klein <aksecurity@gmail.com>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/inet_hashtables.c | 9 +++++----
    1 file changed, 5 insertions(+), 4 deletions(-)

    --- a/net/ipv4/inet_hashtables.c
    +++ b/net/ipv4/inet_hashtables.c
    @@ -591,11 +591,12 @@ EXPORT_SYMBOL_GPL(inet_unhash);
    * Note that we use 32bit integers (vs RFC 'short integers')
    * because 2^16 is not a multiple of num_ephemeral and this
    * property might be used by clever attacker.
    - * RFC claims using TABLE_LENGTH=10 buckets gives an improvement,
    - * we use 256 instead to really give more isolation and
    - * privacy, this only consumes 1 KB of kernel memory.
    + * RFC claims using TABLE_LENGTH=10 buckets gives an improvement, though
    + * attacks were since demonstrated, thus we use 65536 instead to really
    + * give more isolation and privacy, at the expense of 256kB of kernel
    + * memory.
    */
    -#define INET_TABLE_PERTURB_SHIFT 8
    +#define INET_TABLE_PERTURB_SHIFT 16
    #define INET_TABLE_PERTURB_SIZE (1 << INET_TABLE_PERTURB_SHIFT)
    static u32 *table_perturb;


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