CVE-2026-68462

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

bpf: Reject negative const offsets for buffer pointers

The verifier rejects variable offsets for PTRTOTPBUFFER and PTRTO_BUF accesses, but it currently accepts a constant negative offset produced by pointer arithmetic.

Commit 022ac0750883 ("bpf: use reg->varoff instead of reg->off for pointers") moved constant pointer offsets from reg->off to reg->varoff. However, _checkbufferaccess() continued to check only the instruction offset. An access with reg->varoff equal to -8 and an instruction offset of zero therefore passes verification.

For writable raw tracepoints, the access end is also calculated from the unsigned reg->varoff.value. An eight-byte access starting at -8 wraps the calculated end to zero, allowing the program to load and attach without increasing maxtp_access.

After ensuring that reg->varoff is constant, calculate the effective access start using signed arithmetic and reject it when it is negative. Use the validated start to calculate the access end for both PTRTOTPBUFFER and PTRTOBUF.