CVE-2026-53362

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

ipv6: account for fraggap on the paged allocation path

In _ip6append_data(), when the paged-allocation branch is taken

(MSGMORE / NETIFF_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

skbsharedinfo.

An unprivileged user can trigger this via a UDPv6 socket using

MSGMORE together with MSGSPLICE_PAGES.

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

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

_ip6appenddata()'s handling of MSGSPLICE_PAGES"), the negative

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

MSGSPLICEPAGES 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 MSGSPLICEPAGES

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