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.