linux-kernel - Re: [PATCH] net: don't unconditionally copy_from

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Date:   Thu, 26 Aug 2021 12:46:26 -0700
From:   Peter Collingbourne <pcc@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Colin Ian King <colin.king@...onical.com>,
        Cong Wang <cong.wang@...edance.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "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] net: don't unconditionally copy_from_user a struct ifreq
 for socket ioctls

On Thu, Aug 26, 2021 at 1:12 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Peter Collingbourne
> > Sent: 26 August 2021 02:27
> >
> > A common implementation of isatty(3) involves calling a ioctl passing
> > a dummy struct argument and checking whether the syscall failed --
> > bionic and glibc use TCGETS (passing a struct termios), and musl uses
> > TIOCGWINSZ (passing a struct winsize). If the FD is a socket, we will
> > copy sizeof(struct ifreq) bytes of data from the argument and return
> > -EFAULT if that fails. The result is that the isatty implementations
> > may return a non-POSIX-compliant value in errno in the case where part
> > of the dummy struct argument is inaccessible, as both struct termios
> > and struct winsize are smaller than struct ifreq (at least on arm64).
> >
> > Although there is usually enough stack space following the argument
> > on the stack that this did not present a practical problem up to now,
> > with MTE stack instrumentation it's more likely for the copy to fail,
> > as the memory following the struct may have a different tag.
> >
> > Fix the problem by adding an early check for whether the ioctl is a
> > valid socket ioctl, and return -ENOTTY if it isn't.
> ..
> > +bool is_dev_ioctl_cmd(unsigned int cmd)
> > +{
> > +     switch (cmd) {
> > +     case SIOCGIFNAME:
> > +     case SIOCGIFHWADDR:
> > +     case SIOCGIFFLAGS:
> > +     case SIOCGIFMETRIC:
> > +     case SIOCGIFMTU:
> > +     case SIOCGIFSLAVE:
> > +     case SIOCGIFMAP:
> > +     case SIOCGIFINDEX:
> > +     case SIOCGIFTXQLEN:
> > +     case SIOCETHTOOL:
> > +     case SIOCGMIIPHY:
> > +     case SIOCGMIIREG:
> > +     case SIOCSIFNAME:
> > +     case SIOCSIFMAP:
> > +     case SIOCSIFTXQLEN:
> > +     case SIOCSIFFLAGS:
> > +     case SIOCSIFMETRIC:
> > +     case SIOCSIFMTU:
> > +     case SIOCSIFHWADDR:
> > +     case SIOCSIFSLAVE:
> > +     case SIOCADDMULTI:
> > +     case SIOCDELMULTI:
> > +     case SIOCSIFHWBROADCAST:
> > +     case SIOCSMIIREG:
> > +     case SIOCBONDENSLAVE:
> > +     case SIOCBONDRELEASE:
> > +     case SIOCBONDSETHWADDR:
> > +     case SIOCBONDCHANGEACTIVE:
> > +     case SIOCBRADDIF:
> > +     case SIOCBRDELIF:
> > +     case SIOCSHWTSTAMP:
> > +     case SIOCBONDSLAVEINFOQUERY:
> > +     case SIOCBONDINFOQUERY:
> > +     case SIOCGIFMEM:
> > +     case SIOCSIFMEM:
> > +     case SIOCSIFLINK:
> > +     case SIOCWANDEV:
> > +     case SIOCGHWTSTAMP:
> > +             return true;
>
> That is horrid.
> Can't you at least use _IOC_TYPE() to check for socket ioctls.
> Clearly it can succeed for 'random' driver ioctls, but will fail
> for the tty ones.

Yes, that works, since all of the ioctls listed above are in the range
where the _IOC_TYPE() check would succeed. It now also makes sense to
move the check inline into the header. I've done all of that in v2.

> The other sane thing is to check _IOC_SIZE().
> Since all the SIOCxxxx have a correct _IOC_SIZE() that can be
> used to check the user copy length.
> (Unlike socket options the correct length is always supplied.

FWIW, it doesn't look like any of them have the _IOC_SIZE() bits set,
so that won't work. _IOC_TYPE() seems better anyway.

Peter