[PATCH 4/4][Z] bpf: fix branch pruning logic

[Seth Forshee]

From: Alexei Starovoitov <ast at fb.com>

when the verifier detects that register contains a runtime constant
and it's compared with another constant it will prune exploration
of the branch that is guaranteed not to be taken at runtime.
This is all correct, but malicious program may be constructed
in such a way that it always has a constant comparison and
the other branch is never taken under any conditions.
In this case such path through the program will not be explored
by the verifier. It won't be taken at run-time either, but since
all instructions are JITed the malicious program may cause JITs
to complain about using reserved fields, etc.
To fix the issue we have to track the instructions explored by
the verifier and sanitize instructions that are dead at run time
with NOPs. We cannot reject such dead code, since llvm generates
it for valid C code, since it doesn't do as much data flow
analysis as the verifier does.

Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
Signed-off-by: Alexei Starovoitov <ast at kernel.org>
Acked-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
(backported from commit c131187db2d3fa2f8bf32fdf4e9a4ef805168467)
CVE-2017-17862
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
---
 include/linux/bpf_verifier.h |  1 +
 kernel/bpf/verifier.c        | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index a13b031dc6b8..aeca014c52fe 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -67,6 +67,7 @@ struct bpf_verifier_state_list {
 
 struct bpf_insn_aux_data {
 	enum bpf_reg_type ptr_type;	/* pointer type for load/store insns */
+	bool seen; /* this insn was processed by the verifier */
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index fed5b346cd18..d34a572b3388 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2748,6 +2748,7 @@ static int do_check(struct bpf_verifier_env *env)
 		if (err)
 			return err;
 
+		env->insn_aux_data[insn_idx].seen = true;
 		if (class == BPF_ALU || class == BPF_ALU64) {
 			err = check_alu_op(env, insn);
 			if (err)
@@ -2945,6 +2946,7 @@ static int do_check(struct bpf_verifier_env *env)
 					return err;
 
 				insn_idx++;
+				env->insn_aux_data[insn_idx].seen = true;
 			} else {
 				verbose("invalid BPF_LD mode\n");
 				return -EINVAL;
@@ -3100,6 +3102,25 @@ static void convert_pseudo_ld_imm64(struct bpf_verifier_env *env)
 			insn->src_reg = 0;
 }
 
+/* The verifier does more data flow analysis than llvm and will not explore
+ * branches that are dead at run time. Malicious programs can have dead code
+ * too. Therefore replace all dead at-run-time code with nops.
+ */
+static void sanitize_dead_code(struct bpf_verifier_env *env)
+{
+	struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
+	struct bpf_insn nop = BPF_MOV64_REG(BPF_REG_0, BPF_REG_0);
+	struct bpf_insn *insn = env->prog->insnsi;
+	const int insn_cnt = env->prog->len;
+	int i;
+
+	for (i = 0; i < insn_cnt; i++) {
+		if (aux_data[i].seen)
+			continue;
+		memcpy(insn + i, &nop, sizeof(nop));
+	}
+}
+
 /* convert load instructions that access fields of 'struct __sk_buff'
  * into sequence of instructions that access fields of 'struct sk_buff'
  */
@@ -3259,6 +3280,9 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
 	while (pop_stack(env, NULL) >= 0);
 	free_states(env);
 
+	if (ret == 0)
+		sanitize_dead_code(env);
+
 	if (ret == 0)
 		/* program is valid, convert *(u32*)(ctx + off) accesses */
 		ret = convert_ctx_accesses(env);
-- 
2.14.1