Re: [PATCH v2] iov_iter: Fix out-of-bound access in iov_iter_advance()

[Al Viro]

On Fri, Apr 01, 2016 at 08:39:19PM +0200, Takashi Iwai wrote:
> 
> /* Get packet from user space buffer */
> static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> 			    void *msg_control, struct iov_iter *from,
> 			    int noblock)
> {
> ....
> 	struct virtio_net_hdr gso = { 0 };
> ....

Here len must be equal to iov_iter_count(from).

> 	if (tun->flags & IFF_VNET_HDR) {
> 		if (len < tun->vnet_hdr_sz)
> 			return -EINVAL;

... and be at least tun->vnet_hdr_sz

> 		len -= tun->vnet_hdr_sz;
> 
> 		n = copy_from_iter(&gso, sizeof(gso), from);
> 		if (n != sizeof(gso))
> 			return -EFAULT;

We'd consumed sizeof(gso)

> 		if ((gso.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
> 		    tun16_to_cpu(tun, gso.csum_start) + tun16_to_cpu(tun, gso.csum_offset) + 2 > tun16_to_cpu(tun, gso.hdr_len))
> 			gso.hdr_len = cpu_to_tun16(tun, tun16_to_cpu(tun, gso.csum_start) + tun16_to_cpu(tun, gso.csum_offset) + 2);
> 
> 		if (tun16_to_cpu(tun, gso.hdr_len) > len)
> 			return -EINVAL;
> ==>		iov_iter_advance(from, tun->vnet_hdr_sz - sizeof(gso));

... and skipped tun->vnet_hdr_sz - sizeof(gso).  How the hell can that
overrun the end of iterator?  Is ->vnet_hdr_sz less that struct virtio_net_hdr
somehow?

> So, tun_get_user() calls copy_from_iter(), and the iterator is
> advanced, and call iov_iter_advance() from that point for the rest
> size.  And this size can be zero or greater.  We can put simply a zero
> check there, and actually Jiri suggested it at first.

> Hm, so do you mean that it's invalid to call this function with
> size=0?  Or shouldn't we return the actually advanced size?  Currently
> the function assumes the size suffices implicitly.

Zero is certainly valid.  But note that if _that_ is what you are concerned
about, the warning is not serious.  Look:

#define iterate_iovec(i, n, __v, __p, skip, STEP) {     \

n is 0

        size_t left;                                    \
        size_t wanted = n;                              \
        __p = i->iov;                                   \

        __v.iov_len = min(n, __p->iov_len - skip);      \

min(0, some unsigned crap) => 0.

        if (likely(__v.iov_len)) {                      \        
not taken
                __v.iov_base = __p->iov_base + skip;    \
                left = (STEP);                          \
                __v.iov_len -= left;                    \
                skip += __v.iov_len;                    \
                n -= __v.iov_len;                       \
        } else {                                        \
                left = 0;                               \
        }                                               \
        while (unlikely(!left && n)) {                  \
never executed
                __p++;                                  \
                __v.iov_len = min(n, __p->iov_len);     \
                if (unlikely(!__v.iov_len))             \
                        continue;                       \
                __v.iov_base = __p->iov_base;           \
                left = (STEP);                          \
                __v.iov_len -= left;                    \
                skip = __v.iov_len;                     \
                n -= __v.iov_len;                       \
        }                                               \
        n = wanted - n;                                 \
0 is stored in n again, no-op
}
with similar working for kvec and bvec cases.

IF the warning is actually about zero-length case, it's a red herring.
Yes, in theory the array of iovec/kvec/bvec might reach the end of a page,
with the next one not being mapped at all.  In that case we would oops
there, and I'm fine with adding if (!n) return; there.  However, I'm _not_
OK with the first part - there we would be papering over a real bug in
the caller.