Linux Kernel Unspecified Vulnerability

In the Linux kernel, the following vulnerability has been resolved:

ipv6: account for fraggap on the paged allocation path

In __ip6_append_data(), when the paged-allocation branch is taken

(MSG_MORE / NETIF_F_SG / large fraglen), alloclen and pagedlen are

computed as

alloclen = fragheaderlen + transhdrlen;

pagedlen = datalen - transhdrlen;

datalen already includes fraggap (datalen = length + fraggap). When

fraggap is non-zero, this is not the first skb and transhdrlen is zero.

The fraggap bytes carried over from the previous skb are copied just past

the fragment headers in the new skb's linear area. The linear area is

therefore undersized by fraggap bytes while pagedlen is overstated by the

same amount, and the copy writes past skb->end into the trailing

skb_shared_info.

An unprivileged user can trigger this via a UDPv6 socket using

MSG_MORE together with MSG_SPLICE_PAGES.

The bad accounting was introduced by commit 773ba4fe9104 ("ipv6:

avoid partial copy for zc"). Before commit ce650a166335 ("udp6: Fix

__ip6_append_data()'s handling of MSG_SPLICE_PAGES"), the negative

copy value caused -EINVAL to be returned. That later commit allowed

MSG_SPLICE_PAGES to proceed in this case, making the corruption

triggerable.

The non-paged branch sets alloclen to fraglen, which already accounts

for fraggap because datalen does. Bring the paged branch in line by

adding fraggap to alloclen and subtracting it from pagedlen.

After this adjustment, copy no longer collapses to -fraggap on the

paged path, so remove the stale comment describing that old arithmetic.

Since a negative copy is no longer expected for a valid MSG_SPLICE_PAGES

case, remove the MSG_SPLICE_PAGES exception from the negative copy check.