netdev - [PATCH] bpf: verifier: BPF_MOV don't mark dst reg if src == dst

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Message-ID: <CAOn4ftsY-MOMXYR3HVUaoWAyHxPj2sNNqUeP2tff4rCK8NvhDw@mail.gmail.com>
Date:   Wed, 25 Jul 2018 23:08:07 +0100
From:   Arthur Fabre <afabre@...udflare.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org
Subject: [PATCH] bpf: verifier: BPF_MOV don't mark dst reg if src == dst

When check_alu_op() handles a BPF_MOV between two registers,
it calls check_reg_arg() on the dst register, marking it as unbounded.
If the src and dst register are the same, this marks the src as
unbounded, which can lead to unexpected errors for further checks that
rely on bounds info.

check_alu_op() now only marks the dst register as unbounded if it
different from the src register.

Signed-off-by: Arthur Fabre <afabre@...udflare.com>
---
 kernel/bpf/verifier.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 63aaac52a265..ddfe3c544a80 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3238,8 +3238,9 @@ static int check_alu_op(struct bpf_verifier_env
*env, struct bpf_insn *insn)
                        }
                }

-               /* check dest operand */
-               err = check_reg_arg(env, insn->dst_reg, DST_OP);
+               /* check dest operand, only mark if dest != src */
+               err = check_reg_arg(env, insn->dst_reg,
+                               insn->dst_reg == insn->src_reg ?
DST_OP_NO_MARK : DST_OP);
                if (err)
                        return err;

-- 
2.18.0