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Commit 735b4333 authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by David S. Miller
Browse files

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: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 969bf05e

kernel/bpf/verifier.c

+3 −20
Original line number Diff line number Diff line
@@ -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);