NULL Pointer Dereference Affecting kernel-rt package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt package and not the kernel-rt package as distributed by RHEL. See How to fix? for RHEL:7 relevant fixed versions and status.

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

net: gso: fix tcp fraglist segmentation after pull from frag_list

Detect tcp gso fraglist skbs with corrupted geometry (see below) and pass these to skb_segment instead of skb_segment_list, as the first can segment them correctly.

Valid SKB_GSO_FRAGLIST skbs

• consist of two or more segments
• the head_skb holds the protocol headers plus first gso_size
• one or more frag_list skbs hold exactly one segment
• all but the last must be gso_size

Optional datapath hooks such as NAT and BPF (bpf_skb_pull_data) can modify these skbs, breaking these invariants.

In extreme cases they pull all data into skb linear. For TCP, this causes a NULL ptr deref in __tcpv4_gso_segment_list_csum at tcp_hdr(seg->next).

Detect invalid geometry due to pull, by checking head_skb size. Don't just drop, as this may blackhole a destination. Convert to be able to pass to regular skb_segment.

Approach and description based on a patch by Willem de Bruijn.

• https://access.redhat.com/security/cve/CVE-2024-49979
• https://git.kernel.org/stable/c/17bd3bd82f9f79f3feba15476c2b2c95a9b11ff8
• https://git.kernel.org/stable/c/2d4a83a44428de45bfe9dccb0192a3711d1097e0
• https://git.kernel.org/stable/c/3fdd8c83e83fa5e82f1b5585245c51e0355c9f46