linux-kernel - Re: [PATCH net-next 1/2] tun: tun_chr_open(): correctly initialize socket uid

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Date:   Wed, 1 Feb 2023 12:01:12 +0100
From:   Pietro Borrello <borrello@...g.uniroma1.it>
To:     stephen@...workplumber.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>, Jakob Koschel <jkl820.git@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] tun: tun_chr_open(): correctly initialize
 socket uid

On Wed, 1 Feb 2023 at 04:10, Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> On Wed, 01 Feb 2023 00:35:45 +0000
> Pietro Borrello <borrello@...g.uniroma1.it> wrote:
>
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index a7d17c680f4a..6713fffb1488 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -3450,6 +3450,11 @@ static int tun_chr_open(struct inode *inode, struct file * file)
> >
> >       sock_init_data(&tfile->socket, &tfile->sk);
> >
> > +     // sock_init_data initializes sk.sk_uid assuming tfile->socket is embedded
> > +     // in a struct socket_alloc and reading its corresponding inode. Since we
> > +     // pass a socket contained in a struct tun_file we have to fix this manually
> > +     tfile->sk.sk_uid = inode->i_uid;
> > +
>
> Do not use C++ style comments in the kernel.

Thanks for pointing it out. I will fix this in v2.

> Rule #1 of code maintenance. Bug fixes should not stand out.

Thanks for the comment. I agree bug fixes should not stand out.
I sent the patches also to sparkle some discussion on how this should be
better fixed.
As briefly mentioned in the cover letter, I am not sure what is the
cleanest fix according
to Linux standards.
Are you suggesting a briefer comment or removing it completely?

The alternative fixes I see, would be:
1) pass a NULL socket and manually initialize it, which I think would
make the fix
to stand out more, but it would be probably cleaner
2) change the API of sock_init_data, but probably not worth it, given
tuntap devices
are the only 2 users among almost 60 to break the socket_alloc assumption
3) introduce a sock_init_data_with_inode which explicitly uses an
inode to initialize
uid, but would be a bad solution for code duplication
4) wrap sock_init_data call to fix uid in a similar fashion as done
here, maybe cleaner

Best regards,
Pietro