netdev - Re: [PATCH v2] net: don't unconditionally copy_from

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Date:   Wed, 1 Sep 2021 07:03:56 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Peter Collingbourne <pcc@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Colin Ian King <colin.king@...onical.com>,
        Cong Wang <cong.wang@...edance.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Arnd Bergmann" <arnd@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] net: don't unconditionally copy_from_user a struct
 ifreq for socket ioctls

On Wed, 1 Sep 2021 08:22:42 +0000 David Laight wrote:
> From: Jakub Kicinski
> > Sent: 31 August 2021 17:30
> > 
> > On Thu, 26 Aug 2021 12:46:01 -0700 Peter Collingbourne wrote:  
> > > @@ -3306,6 +3308,8 @@ static int compat_ifr_data_ioctl(struct net *net, unsigned int cmd,
> > >  	struct ifreq ifreq;
> > >  	u32 data32;
> > >
> > > +	if (!is_socket_ioctl_cmd(cmd))
> > > +		return -ENOTTY;
> > >  	if (copy_from_user(ifreq.ifr_name, u_ifreq32->ifr_name, IFNAMSIZ))
> > >  		return -EFAULT;
> > >  	if (get_user(data32, &u_ifreq32->ifr_data))  
> > 
> > Hi Peter, when resolving the net -> net-next merge conflict I couldn't
> > figure out why this chunk is needed. It seems all callers of
> > compat_ifr_data_ioctl() already made sure it's a socket IOCTL.
> > Please double check my resolution (tip of net-next) and if this is
> > indeed unnecessary perhaps send a cleanup? Thanks!  
> 
> To stop the copy_from_user() faulting when the user buffer
> isn't long enough.
> In particular for iasatty() on arm with tagged pointers.

Let me rephrase. is_socket_ioctl_cmd() is always true here. There were
only two callers, both check cmd is of specific, "sockety" type.