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From:   Amit Klein <aksecurity@gmail.com>
Date:   Wed, 16 Jun 2021 16:19:54 +0300
Message-ID: <CANEQ_++RSG=cOa-hWcHefqVa5_=FRo+PdwuJbE2A-NHA_RNXXQ@mail.gmail.com>
Subject: Re: [PATCH 5.4 175/244] inet: use bigger hash table for IP ID generation
To:     David Laight <David.Laight@aculab.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>, Willy Tarreau <w@1wt.eu>,
        "David S. Miller" <davem@davemloft.net>,
        Sasha Levin <sashal@kernel.org>
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On Wed, Jun 16, 2021 at 1:19 PM David Laight <David.Laight@aculab.com> wrote:

>
> Can someone explain why this is a good idea for a 'normal' system?
>

This patch mitigates some techniques that leak internal state due to
table hash collisions.

> Why should my desktop system 'waste' 2MB of memory on a massive
> hash table that I don't need.

In the patch's defense, it only consumes 2MB when the physical RAM is >= 16GB.

> It might be needed by systems than handle massive numbers
> of concurrent connections - but that isn't 'most systems'.
>
> Surely it would be better to detect when the number of entries
> is comparable to the table size and then resize the table.

Security-wise, this approach is not effective. The table size was
increased to reduce the likelihood of hash collisions. These start
happening when you have ~N^(1/2) elements (for table size N), so
you'll need to resize pretty quickly anyway.