netdev - [PATCH bpf-next 06/19] bpf: verifier: hard wire branches to dead code

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20181219182930.8825-7-jakub.kicinski@netronome.com>
Date:   Wed, 19 Dec 2018 10:29:17 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     alexei.starovoitov@...il.com, daniel@...earbox.net
Cc:     oss-drivers@...ronome.com, netdev@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: [PATCH bpf-next 06/19] bpf: verifier: hard wire branches to dead code

Loading programs with dead code becomes more and more
common, as people begin to patch constants at load time.
Turn conditional jumps to unconditional ones, to avoid
potential branch misprediction penalty.

This optimization is enabled for privileged users only.

For branches which just fall through we could just mark
them as not seen and have dead code removal take care of
them, but that seems less clean.

v0.2:
 - don't call capable(CAP_SYS_ADMIN) twice (Jiong).

Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
---
 kernel/bpf/verifier.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e608c7556e10..7db2a48ea7f7 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6252,6 +6252,40 @@ static void sanitize_dead_code(struct bpf_verifier_env *env)
 	}
 }
 
+static bool insn_is_cond_jump(u8 code)
+{
+	u8 op;
+
+	if (BPF_CLASS(code) != BPF_JMP)
+		return false;
+
+	op = BPF_OP(code);
+	return op != BPF_JA && op != BPF_EXIT && op != BPF_CALL;
+}
+
+static void opt_hard_wire_dead_code_branches(struct bpf_verifier_env *env)
+{
+	struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
+	struct bpf_insn ja = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
+	struct bpf_insn *insn = env->prog->insnsi;
+	const int insn_cnt = env->prog->len;
+	int i;
+
+	for (i = 0; i < insn_cnt; i++, insn++) {
+		if (!insn_is_cond_jump(insn->code))
+			continue;
+
+		if (!aux_data[i + 1].seen)
+			ja.off = insn->off;
+		else if (!aux_data[i + 1 + insn->off].seen)
+			ja.off = 0;
+		else
+			continue;
+
+		memcpy(insn, &ja, sizeof(ja));
+	}
+}
+
 /* convert load instructions that access fields of a context type into a
  * sequence of instructions that access fields of the underlying structure:
  *     struct __sk_buff    -> struct sk_buff
@@ -6892,6 +6926,7 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
 	struct bpf_verifier_env *env;
 	struct bpf_verifier_log *log;
 	int ret = -EINVAL;
+	bool is_priv;
 
 	/* no program is valid */
 	if (ARRAY_SIZE(bpf_verifier_ops) == 0)
@@ -6955,7 +6990,8 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
 	if (!env->explored_states)
 		goto skip_full_check;
 
-	env->allow_ptr_leaks = capable(CAP_SYS_ADMIN);
+	is_priv = capable(CAP_SYS_ADMIN);
+	env->allow_ptr_leaks = is_priv;
 
 	ret = check_subprogs(env);
 	if (ret < 0)
@@ -6986,6 +7022,11 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
 		ret = check_max_stack_depth(env);
 
 	/* instruction rewrites happen after this point */
+	if (is_priv) {
+		if (ret == 0)
+			opt_hard_wire_dead_code_branches(env);
+	}
+
 	if (ret == 0)
 		sanitize_dead_code(env);
 
-- 
2.19.2