netdev - Re: [PATCH v2 net] netlink: Use copy_to_user() for optval in netlink

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Date:   Fri, 21 Apr 2023 09:56:37 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Patrick McHardy <kaber@...sh.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Christophe Ricard <christophe-h.ricard@...com>,
        David Ahern <dsahern@...il.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Brad Spencer <bspencer@...ckberry.com>
Subject: Re: [PATCH v2 net] netlink: Use copy_to_user() for optval in
 netlink_getsockopt().

On Thu, 2023-04-20 at 23:33 +0000, Kuniyuki Iwashima wrote:
> Brad Spencer provided a detailed report [0] that when calling getsockopt()
> for AF_NETLINK, some SOL_NETLINK options set only 1 byte even though such
> options require more than int as length.

Nit: not "more than" but "at least" (and sizeof(int), I guess).

> The options return a flag value that fits into 1 byte, but such behaviour
> confuses users who do not initialise the variable before calling
> getsockopt() and do not strictly check the returned value as char.
> 
> Currently, netlink_getsockopt() uses put_user() to copy data to optlen and
> optval, but put_user() casts the data based on the pointer, char *optval.
> As a result, only 1 byte is set to optval.

Maybe as a future thing, we should make the getsockopt method prototype
have void here, so this kind of thing becomes a compilation error? That
affects a fair number I guess, though I can't think of any socket
options that really _should_ be just a char, so if it fails anywhere
that might uncover additional bugs (and potentially avoid future ones)?


johannes