DEBIAN-CVE-2026-63984

In the Linux kernel, the following vulnerability has been resolved:  ipv6: rpl: fix hdrlen overflow in ipv6rplsrhdecompress()  ipv6rplsrhdecompress() 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 ipv6rplsrhrcv() 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 ipv6rplsrhcompress() 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.