[PATCH bpf 5/8] bpf: don't predict JMP32 pointer vs zero comparisons

[Eduard Zingerman <eddyz87@gmail.com>]

Consider the following program:

        r1 = map_value;      /* low 32 bits are zero at runtime */
        r6 = 0xdead000000000000;
        if w1 != 0 goto l1;
    l0: r1 += r6;
        r2 = *(u64 *)(r1 + 0);
        exit;
    l1: r6 = 0;
        goto l0;

At the moment is_branch_taken() reports the jump as always taken,
because it does not distinguish between BPF_JMP and BPF_JMP32
comparisons when processing 'if w1 != 0 ...'.

Fixes: cac616db39c2 ("bpf: Verifier track null pointer branch_taken with JNE and JEQ")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 kernel/bpf/verifier.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d16dd1fb08b6..bb8e9efcfbad 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16309,6 +16309,13 @@ static int is_branch_taken(struct bpf_verifier_env *env, struct bpf_reg_state *r
 	if (__is_pointer_value(false, reg1) || __is_pointer_value(false, reg2)) {
 		u64 val;
 
+		/*
+		 * The low 32 bits of a valid pointer may well be zero, hence
+		 * nothing below applies to a 32-bit comparison.
+		 */
+		if (is_jmp32)
+			return -1;
+
 		/* arrange that reg2 is a scalar, and reg1 is a pointer */
 		if (!is_reg_const(reg2, is_jmp32)) {
 			opcode = flip_opcode(opcode);
-- 
2.55.0