CVE-2026-74612: veth: fix skb length accounting after XDP frag adjustment
In the Linux kernel, the following vulnerability has been resolved:
veth: fix skb length accounting after XDP frag adjustment
veth exposes non-linear skb fragments through an xdpbuff. If an XDP program adjusts the fragment area, vethxdprcvskb() copies xdpfragssize back to skb->datalen but leaves skb->len containing the old fragment contribution.
After a fragment shrink, this makes skbheadlen() larger than the actual linear area. In the reproduced UDP receive path, skbdatagramiter() copied 1024 bytes past the actual linear tail to userspace, starting at struct skbsharedinfo. The copied bytes included the affected skb's nrfrags, xdpfragssize, and a kernel pointer from skbshinfo(skb)->frags[0]. Real packet data was displaced by the same amount and truncated at the end.
Subtract the old datalen before replacing it and add the new datalen afterwards, keeping skb->len and skb->datalen synchronized.
Additionally, bpfxdppulldata() can advance dataend while leaving frags present. The skb is then still non-linear, so the old skbput(skb, off) triggers SKBLINEARASSERT().
Use skbsettailpointer() and update skb->len explicitly instead, following bpfprogrungenericxdp(). Unlike skbput(), skbsettailpointer() does not require a linear skb.
A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by 1024 bytes from its fragment area. Before the fix, all 10 runs produced corrupted payloads. After the fix, all 10 runs matched the expected payload exactly. A forced-tailroom reproducer also exercises bpfxdppulldata() with frags still present; the old code triggers SKBLINEARASSERT(), while this fix passes 10/10 runs.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 6.6.152.1-1
Event History
Frequently Asked Questions
What conditions are required to trigger the issue?
The affected path requires veth to expose a non-linear skb to an XDP program and for that program to adjust the fragment area, including shrinking fragments. The described disclosure occurs when the resulting skb reaches a UDP receive path that iterates over its data.
What is the practical impact of successful exploitation?
A fragment shrink can cause up to 1024 bytes past the actual linear tail to be copied to userspace in the reproduced UDP receive path. The copied data can include skb metadata such as fragment counts, XDP fragment size, and a kernel pointer; legitimate packet data is correspondingly displaced and truncated.
What should be changed to remediate the accounting error?
Apply the referenced stable kernel fixes. The correction keeps skb->len synchronized with skb->data_len by removing the old data_len contribution before replacing it and adding the new contribution afterward; the related non-linear skb handling replaces __skb_put(skb, off) with skb_set_tail_pointer() plus an explicit skb->len update.