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    Subject[PATCH 5.10 06/11] tcp: add small random increments to the source port
    Date
    From: Willy Tarreau <w@1wt.eu>

    commit ca7af0402550f9a0b3316d5f1c30904e42ed257d upstream.

    Here we're randomly adding between 0 and 7 random increments to the
    selected source port in order to add some noise in the source port
    selection that will make the next port less predictable.

    With the default port range of 32768-60999 this means a worst case
    reuse scenario of 14116/8=1764 connections between two consecutive
    uses of the same port, with an average of 14116/4.5=3137. This code
    was stressed at more than 800000 connections per second to a fixed
    target with all connections closed by the client using RSTs (worst
    condition) and only 2 connections failed among 13 billion, despite
    the hash being reseeded every 10 seconds, indicating a perfectly
    safe situation.

    Cc: Moshe Kol <moshe.kol@mail.huji.ac.il>
    Cc: Yossi Gilad <yossi.gilad@mail.huji.ac.il>
    Cc: 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>
    Cc: 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
    @@ -833,11 +833,12 @@ next_port:
    return -EADDRNOTAVAIL;

    ok:
    - /* If our first attempt found a candidate, skip next candidate
    - * in 1/16 of cases to add some noise.
    + /* Here we want to add a little bit of randomness to the next source
    + * port that will be chosen. We use a max() with a random here so that
    + * on low contention the randomness is maximal and on high contention
    + * it may be inexistent.
    */
    - if (!i && !(prandom_u32() % 16))
    - i = 2;
    + i = max_t(int, i, (prandom_u32() & 7) * 2);
    WRITE_ONCE(table_perturb[index], READ_ONCE(table_perturb[index]) + i + 2);

    /* Head lock still held and bh's disabled */

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