linux-kernel - Re: [patch 37/37] sit: fix off-by-one in ipip6_tunnel_get

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Date:	Sun, 11 Oct 2009 03:29:49 +0200
From:	Wolfgang Walter <wolfgang.walter@...m.de>
To:	David Miller <davem@...emloft.net>
Cc:	Fred.L.Templin@...ing.com, gregkh@...e.de,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	contact@...chahlusiak.de, yoshfuji@...ux-ipv6.org
Subject: Re: [patch 37/37] sit: fix off-by-one in ipip6_tunnel_get_prl

On Saturday 10 October 2009, David Miller wrote:
> From: "Templin, Fred L" <Fred.L.Templin@...ing.com>
> Date: Fri, 9 Oct 2009 17:34:49 -0700
> 
> > Wait a moment - I remember now that this code came
> > from Yoshifuji, and I believe there was a reason for
> > the cmax+1. The application is expected to know this
> > and to post a large enough buffer.

But wouldn't this corrupt kernel memory?

It seems:

kp is never larger then

	sizeof(struct ip_tunnel_prl) * cmax

so kp[cmax] is of by one.

userspace can't allocate a large enough buffer as doesn't know the size of the 
prl.

If the comment is correct and non-root can call this function it could set
	a.addr = htonl(INADDR_ANY);
	a.datalen = sizeof(*kp);

=> cmax == 1
=> kp[1] would be written if prl has 2 or more entries.

So I think the patch is correct.

Probably it would even be better to check for

	c >= ca

as this even more obviously correct.

> > 
> > Can we put this on hold until I have had a chance to
> > check my e-mail archives and my local iproute changes
> > (will respond on monday)?
> 
> Sure, we can keep it out of -stable for now.
> 
> But it is in Linus's tree so if you find we shouldn't do this
> you'll need to send me a revert for net-2.6
> 
> Otherwise if it's good, you'll have to remind me to resubmit
> it to -stable.
> 

This function seems to be a bit strange:

1)

	if (!kp) {
		/* We don't try hard to allocate much memory for
		 * non-root users.
		 * For root users, retry allocating enough memory for
		 * the answer.
		 */
		kp = kcalloc(ca, sizeof(*kp), GFP_ATOMIC);
		if (!kp) {
			ret = -ENOMEM;
			goto out;

why ist ret set to -ENOMEM
it will be set to 0  here:

	out:
		read_unlock(&ipip6_lock);

		len = sizeof(*kp) * c;
		ret = 0;


Because c == 0 len will be 0, so only
	put_user(len, &a->datalen)

will be executed
I'm not sure what this means for put_user(len, &a->datalen) but probably it 
will succed. Therefor the return value is 0 in this case.


I think the

	ret = 0

should be removed as ret is initialized to 0 so it will be zero if not 
modifiied.

And either

	if (! ret) {
		len = sizeof(*kp) * c;
		if ((len && copy_to_user(a + 1, kp, len)) || put_user(len, &a->datalen))
			ret = -EFAULT;
	}

or

	len = sizeof(*kp) * c;
	if (len && (opy_to_user(a + 1, kp, len) || put_user(len, &a->datalen))
		ret = -EFAULT;




2) Then

	c = 0;
	for (prl = t->prl; prl; prl = prl->next) {


c is already 0 here, so this seems to be unecessary


3)  If the caller should know that the list is to large for the buffer he 
supplied by returning -EFAULT then cmax should be increased by one:

	cmax = kprl.datalen / sizeof(kprl) + 1;


But probably user space should initialise its buffer with 0 and assume that the 
list was to large if the last entry's addr  != 0.


Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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