7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eduard Zingerman [ Upstream commit 67b529f521a6676cdfc78b91b0217d7eaa84216b ] reg_not_null() decides that a register holds a non-NULL value by looking at its type alone. For pointer types that allow arithmetic the type only guarantees a non-NULL base, in case of an unbound offset the runtime offset value might still add up to NULL. Consider the followng program: r6 = bpf_map_lookup_elem(map, &0); /* present */ if (r6 == 0) return 0; r7 = bpf_map_lookup_elem(map, &1); /* absent, NULL at runtime */ r8 = r7; r8 -= r6; /* pointer - pointer: unknown scalar, -r6 */ r8 <<= 1; r8 >>= 1; /* any non-negative offset is accepted by */ /* check_reg_sane_offset_ptr() */ r6 += r8; /* verifier: map value; runtime: zero */ if (r7 != r6) return 0; *(u8 *)(r7 + 0); /* r7 is inferred non-NULL, both are zero */ At runtime both registers are zero, the comparison is true and the load faults with NULL pointer dereference. Require the offset to be within +-BPF_MAX_VAR_OFF in reg_not_null(). Fixes: cac616db39c2 ("bpf: Verifier track null pointer branch_taken with JNE and JEQ") Reported-by: Nicholas Carlini Signed-off-by: Eduard Zingerman Link: https://lore.kernel.org/bpf/20260904083325.2083493-1-eddyz87@gmail.com Signed-off-by: Kumar Kartikeya Dwivedi Signed-off-by: Sasha Levin --- kernel/bpf/verifier.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 96aaa9155e59e..84b05958c3250 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -367,6 +367,13 @@ static bool reg_not_null(struct bpf_verifier_env *env, const struct bpf_reg_stat if (type_may_be_null(type)) return false; + /* + * The types below guarantee a non-NULL base, an unbounded offset can + * still wrap base + offset to zero. + */ + if (reg_smin(reg) <= -BPF_MAX_VAR_OFF || reg_smax(reg) >= BPF_MAX_VAR_OFF) + return false; + type = base_type(type); return type == PTR_TO_SOCKET || type == PTR_TO_TCP_SOCK || -- 2.53.0