CVE-2026-98040

Summary

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/ddca9a3b8833168dda3e62676648a636fb3f221c
• https://git.kernel.org/stable/c/6aed0134d3cda6382385a734ae0158eb7df6b142

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