In the Linux kernel, the following vulnerability has been resolved: bpf: Mark the zero register precise for a register-form NULL check check_cond_jmp_op() accepts "if rA <op> rB" as a NULL check for a nullable pointer rA when rB is a s…

Overview

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

bpf: Mark the zero register precise for a register-form NULL check

check_cond_jmp_op() accepts "if rA <op> rB" as a NULL check for a nullable pointer rA when rB is a scalar known to be zero, lifts PTR_MAYBE_NULL from rA in the corresponding branch and does not mark rB precise. Consider the following program:

r0 = bpf_get_prandom_u32(); r6 = 1; /* the r6 == 0 path is explored first / if (r0 == 0) goto 1f; r6 = 0; 1: r0 = bpf_map_lookup_elem(map, &0); / absent, NULL at runtime / if (r0 == r6) goto 2f; / taken as a NULL check for r0 */ *(u8 )(r0 + 0); / verifier: map value; runtime: zero */ 2: return 0;

The r6 == 0 path is explored first and the dereference is accepted. The r6 == 1 path is pruned at the checkpoint recorded for (1), so the comparison is never verified with a non-zero r6. At runtime a failed lookup returns NULL, NULL != 1 takes the non-NULL edge and the program dereferences a pointer that is zero.

Remediation

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

Vendor advisories

• Red Hat VEX · Moderate · updated 2026-09-28 · vex