bpf: improve verifier state equivalence

since UNKNOWN_VALUE type is weaker than CONST_IMM we can un-teach
verifier its recognition of constants in conditional branches
without affecting safety.
Ex:
if (reg == 123) {
  .. here verifier was marking reg->type as CONST_IMM
     instead keep reg as UNKNOWN_VALUE
}

Two verifier states with UNKNOWN_VALUE are equivalent, whereas
CONST_IMM_X != CONST_IMM_Y, since CONST_IMM is used for stack range
verification and other cases.
So help search pruning by marking registers as UNKNOWN_VALUE
where possible instead of CONST_IMM.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -1704,12 +1704,11 @@ static int check_cond_jmp_op(struct verifier_env *env,
 			 */
 			regs[insn->dst_reg].type = PTR_TO_MAP_VALUE;
 			/* branch targer cannot access it, since reg == 0 */
-			other_branch->regs[insn->dst_reg].type = CONST_IMM;
-			other_branch->regs[insn->dst_reg].imm = 0;
+			mark_reg_unknown_value(other_branch->regs,
+					       insn->dst_reg);
 		} else {
 			other_branch->regs[insn->dst_reg].type = PTR_TO_MAP_VALUE;
-			regs[insn->dst_reg].type = CONST_IMM;
-			regs[insn->dst_reg].imm = 0;
+			mark_reg_unknown_value(regs, insn->dst_reg);
 		}
 	} else if (BPF_SRC(insn->code) == BPF_X && opcode == BPF_JGT &&
 		   dst_reg->type == PTR_TO_PACKET &&
@@ -1718,22 +1717,6 @@ static int check_cond_jmp_op(struct verifier_env *env,
 	} else if (is_pointer_value(env, insn->dst_reg)) {
 		verbose("R%d pointer comparison prohibited\n", insn->dst_reg);
 		return -EACCES;
-	} else if (BPF_SRC(insn->code) == BPF_K &&
-		   (opcode == BPF_JEQ || opcode == BPF_JNE)) {
-
-		if (opcode == BPF_JEQ) {
-			/* detect if (R == imm) goto
-			 * and in the target state recognize that R = imm
-			 */
-			other_branch->regs[insn->dst_reg].type = CONST_IMM;
-			other_branch->regs[insn->dst_reg].imm = insn->imm;
-		} else {
-			/* detect if (R != imm) goto
-			 * and in the fall-through state recognize that R = imm
-			 */
-			regs[insn->dst_reg].type = CONST_IMM;
-			regs[insn->dst_reg].imm = insn->imm;
-		}
 	}
 	if (log_level)
 		print_verifier_state(&env->cur_state);