Subject: Re: [PATCH] tun: make sure interface usage can not overflow

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 29 Sep 2014 15:02:28 +0300

On Mon, Sep 29, 2014 at 02:48:49PM +0300, Michael S. Tsirkin wrote:
> On Sun, Sep 28, 2014 at 04:27:53PM -0700, Kees Cook wrote:
> > This makes the size argument a const, since it is always populated by
> > the caller. Additionally double-checks to make sure the copy_from_user
> > can never overflow, keeping CONFIG_DEBUG_STRICT_USER_COPY_CHECKS happy:
> > 
> >    In function 'copy_from_user',
> >        inlined from '__tun_chr_ioctl' at drivers/net/tun.c:1871:7:
> >        ... copy_from_user() buffer size is not provably correct
> > 
> > Signed-off-by: Kees Cook <[email-masked]>
> 
> What exactly is the issue here?
> __tun_chr_ioctl is called with sizeof(struct compat_ifreq)
> or  sizeof (struct ifreq) as the last argument.
> 
> So this looks like a false positive, but
> CONFIG_DEBUG_STRICT_USER_COPY_CHECKS machinery is supposed
> to avoid false positives.
> 
> On which architecture is this?

Also - which kernel?
Does your kernel include: commit 3df7b41aa5e7797f391d0a41f8b0dce1fe366a09
    x86: Unify copy_from_user() size checking
?

> > ---
> >  drivers/net/tun.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index acaaf6784179..a1f317cba206 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1855,7 +1855,7 @@ unlock:
> >  }
> >  
> >  static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
> > -			    unsigned long arg, int ifreq_len)
> > +			    unsigned long arg, const size_t ifreq_len)
> >  {
> >  	struct tun_file *tfile = file->private_data;
> >  	struct tun_struct *tun;
> > @@ -1869,6 +1869,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
> >  	int ret;
> >  
> >  	if (cmd == TUNSETIFF || cmd == TUNSETQUEUE || _IOC_TYPE(cmd) == 0x89) {
> > +		BUG_ON(ifreq_len > sizeof(ifr));
> >  		if (copy_from_user(&ifr, argp, ifreq_len))
> >  			return -EFAULT;
> >  	} else {
> > -- 
> > 1.9.1
> > 
> > 
> > -- 
> > Kees Cook
> > Chrome OS Security