seg6: validate SRH length before reading fixed fields

Overview

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

seg6: validate SRH length before reading fixed fields

seg6_validate_srh() reads fixed SRH fields such as srh->type and srh->hdrlen before checking that the supplied length covers the fixed struct ipv6_sr_hdr fields.

The BPF SEG6 encap path reaches this with a BPF program-supplied pointer and length: bpf_lwt_push_encap() and the SEG6 local BPF END_B6 and END_B6_ENCAP actions call bpf_push_seg6_encap(), which forwards the length to seg6_validate_srh() with no minimum-size guard. A 2-byte SEG6 encap header can therefore make the validator read srh->type at offset 2 beyond the caller-supplied buffer.

Reject lengths shorter than the fixed SRH at the top of seg6_validate_srh(), before any field is read. This fixes the BPF helper path and keeps the common validator robust.

Affected

• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < 715eb12e453df752f1b4baaf972c3acff0ab9402
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < 7247d05c987c3eec4bb7c2306dbd77ecdf3b7c73
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < 071f1a38d7ddbadee29c09b9e3ee0ff3a61e6a0e
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < 804bb969f194c93497ba632b98343794c6367fdc
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < 0fc7069d39239978130c37ebceaec85c8948d3f1
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < 8dba7a94a269b88e500aafc25ad567ef6a423698
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < c9961336aa5ff83092f23e33ee86666a9dbd1b2a
• Linux >= fe94cc290f535709d3c5ebd1e472dfd0aec7ee79 < a75d99f46bf21b45965ce39c5cfb3b8bb5ffb1aa
• Linux 4.18

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.