CVE-2026-68462: bpf: Reject negative const offsets for buffer pointers

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

bpf: Reject negative const offsets for buffer pointers

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

Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for
pointers") moved constant pointer offsets from reg->off to reg->var_off.
However, __check_buffer_access() continued to check only the instruction
offset. An access with reg->var_off 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->var_off.value. An eight-byte access starting at -8 wraps
the calculated end to zero, allowing the program to load and attach
without increasing max_tp_access.

After ensuring that reg->var_off 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
PTR_TO_TP_BUFFER and PTR_TO_BUF.

The Linux kernel CVE team has assigned CVE-2026-68462 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 7.1 with commit 022ac075088366b62e130da5e1b200bc93a47191 and fixed in 7.1.5 with commit 314bd592085c0720ef519f6edbc5f41440ff78d4
	Issue introduced in 7.1 with commit 022ac075088366b62e130da5e1b200bc93a47191 and fixed in 7.2-rc4 with commit fd4cfa8c8f9a17cdec0539334d28754bc1d8a5d9

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-68462
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/bpf/verifier.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/314bd592085c0720ef519f6edbc5f41440ff78d4
	https://git.kernel.org/stable/c/fd4cfa8c8f9a17cdec0539334d28754bc1d8a5d9