netdev - Re: Coverity: __sock_gen_cookie(): Error handling issues

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Date:   Thu, 17 Nov 2022 16:25:29 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     linux-kernel@...r.kernel.org,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Paolo Abeni <pabeni@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-next@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: Coverity: __sock_gen_cookie(): Error handling issues

On Thu, Nov 17, 2022 at 03:22:22PM -0800, Eric Dumazet wrote:
> On Thu, Nov 17, 2022 at 3:14 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Thu, Nov 17, 2022 at 02:49:55PM -0800, Eric Dumazet wrote:
> > > On Thu, Nov 17, 2022 at 2:22 PM coverity-bot <keescook@...omium.org> wrote:
> > > >
> > > > Hello!
> > > >
> > > > This is an experimental semi-automated report about issues detected by
> > > > Coverity from a scan of next-20221117 as part of the linux-next scan project:
> > > > https://scan.coverity.com/projects/linux-next-weekly-scan
> > > >
> > > > You're getting this email because you were associated with the identified
> > > > lines of code (noted below) that were touched by commits:
> > > >
> > > >   Wed Nov 16 12:42:01 2022 +0000
> > > >     4ebf802cf1c6 ("net: __sock_gen_cookie() cleanup")
> > > >
> > > > Coverity reported the following:
> > > >
> > > > *** CID 1527347:  Error handling issues  (CHECKED_RETURN)
> > > > net/core/sock_diag.c:33 in __sock_gen_cookie()
> > > > 27     {
> > > > 28      u64 res = atomic64_read(&sk->sk_cookie);
> > > > 29
> > > > 30      if (!res) {
> > > > 31              u64 new = gen_cookie_next(&sock_cookie);
> > > > 32
> > > > vvv     CID 1527347:  Error handling issues  (CHECKED_RETURN)
> > > > vvv     Calling "atomic64_try_cmpxchg" without checking return value (as is done elsewhere 8 out of 9 times).
> > > > 33              atomic64_try_cmpxchg(&sk->sk_cookie, &res, new);
> > >
> > >
> > > Hmmm. for some reason I thought @res was always updated...
> > >
> > > A fix would be to read sk->sk_cookie, but I guess your tool will still
> > > complain we do not care
> > > of  atomic64_try_cmpxchg() return value ?
> > >
> > > diff --git a/net/core/sock_diag.c b/net/core/sock_diag.c
> > > index b11593cae5a09b15a10d6ba35bccc22263cb8fc8..58efb9c1c8dd4f8e5a3009a0176e1b96487daaff
> > > 100644
> > > --- a/net/core/sock_diag.c
> > > +++ b/net/core/sock_diag.c
> > > @@ -31,6 +31,10 @@ u64 __sock_gen_cookie(struct sock *sk)
> > >                 u64 new = gen_cookie_next(&sock_cookie);
> > >
> > >                 atomic64_try_cmpxchg(&sk->sk_cookie, &res, new);
> > > +               /* Another cpu/thread might have won the race,
> > > +                * reload the final value.
> > > +                */
> > > +               res = atomic64_read(&sk->sk_cookie);
> > >         }
> > >         return res;
> > >  }
> >
> > I think it's saying it was expecting an update loop -- i.e. to make sure
> > the value actually got swapped (the "try" part...)?
> 
> The value has been updated, either by us or someone else.
> 
> We do not particularly care who won the race, since the value is
> updated once only.

Ah! Okay, now I understand the added comment. Thanks :)

-- 
Kees Cook