CVE-2026-63984

Summary

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

ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()

ipv6_rpl_srh_decompress() computes:

outhdr->hdrlen = (((n + 1) * sizeof(struct in6_addr)) >> 3);

hdrlen is __u8. For n >= 127 the result exceeds 255 and silently truncates. With n=127 (cmpri=15, cmpre=15, pad=0, hdrlen=16):

(128 * 16) >> 3 = 256, truncated to 0 as __u8

The caller in ipv6_rpl_srh_rcv() then places the compressed header at buf + ((ohdr->hdrlen + 1) << 3). With hdrlen=0 this is buf + 8, but the decompressed region occupies buf[0..2055] (8-byte header plus 128 full addresses). The compressed header overlaps the decompressed data, and ipv6_rpl_srh_compress() writes into this overlap, corrupting the routing header of the forwarded packet.

The existing guard at exthdrs.c:546 checks (n + 1) > 255, which prevents n+1 from overflowing unsigned char (the segments_left field), but does not prevent the computed hdrlen from overflowing __u8. n=127 passes because 128 <= 255, yet hdrlen=256 does not fit.

Tighten the bound to (n + 1) > 127. This caps n at 126, giving hdrlen = (127 * 16) >> 3 = 254, which fits in __u8. The compressed header then lands at buf + ((254 + 1) << 3) = buf + 2040, exactly past the decompressed region (buf[0..2039]). No overlap. 127 segments is well beyond any realistic RPL deployment.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/75b3680047bf09af8e7e471a7a6ddf2ce5847f56
• https://git.kernel.org/stable/c/fd238c51b0fa5390cceca9f1ac5a9ffda8063eed
• https://git.kernel.org/stable/c/3618b34942b76471d044369bfd30d58c39068bf1
• https://git.kernel.org/stable/c/97e06791368c01f0ad2a4b3269c2abe19485ca32
• https://git.kernel.org/stable/c/de02fc049352af5a9595f015511222d0a85c326b
• https://git.kernel.org/stable/c/6fe1cb312038516cb4d9fa089d700af7059f1a64
• https://git.kernel.org/stable/c/c0487a9c1e116cf349e2d1f302d9019670460858
• https://git.kernel.org/stable/c/9d5e7a46a9f6d8f503b41bfefef70659845f1679

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