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bpf: improve verifier state equivalence

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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>
This commit is contained in:
Alexei Starovoitov 2016-05-05 19:49:11 -07:00 committed by David S. Miller
parent 969bf05eb3
commit 735b433397
1 changed files with 3 additions and 20 deletions
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23
kernel/bpf/verifier.c
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@ -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);