bpf: fix branch pruning logic

[ Upstream commit c131187d ]

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: 17a52670 ("bpf: verifier (add verifier core)")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/bpf/verifier.c

@@ -191,6 +191,7 @@ struct bpf_insn_aux_data {
 		enum bpf_reg_type ptr_type;	/* pointer type for load/store insns */
 		struct bpf_map *map_ptr;	/* pointer for call insn into lookup_elem */
 	};
+	bool seen; /* this insn was processed by the verifier */
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
@@ -1793,6 +1794,7 @@ static int do_check(struct verifier_env *env)
 			print_bpf_insn(env, insn);
 		}
 
+		env->insn_aux_data[insn_idx].seen = true;
 		if (class == BPF_ALU || class == BPF_ALU64) {
 			err = check_alu_op(env, insn);
 			if (err)
@@ -1988,6 +1990,7 @@ static int do_check(struct verifier_env *env)
 					return err;
 
 				insn_idx++;
+				env->insn_aux_data[insn_idx].seen = true;
 			} else {
 				verbose("invalid BPF_LD mode\n");
 				return -EINVAL;
@@ -2125,6 +2128,7 @@ static int adjust_insn_aux_data(struct verifier_env *env, u32 prog_len,
 				u32 off, u32 cnt)
 {
 	struct bpf_insn_aux_data *new_data, *old_data = env->insn_aux_data;
+	int i;
 
 	if (cnt == 1)
 		return 0;
@@ -2134,6 +2138,8 @@ static int adjust_insn_aux_data(struct verifier_env *env, u32 prog_len,
 	memcpy(new_data, old_data, sizeof(struct bpf_insn_aux_data) * off);
 	memcpy(new_data + off + cnt - 1, old_data + off,
 	       sizeof(struct bpf_insn_aux_data) * (prog_len - off - cnt + 1));
+	for (i = off; i < off + cnt - 1; i++)
+		new_data[i].seen = true;
 	env->insn_aux_data = new_data;
 	vfree(old_data);
 	return 0;
@@ -2152,6 +2158,25 @@ static struct bpf_prog *bpf_patch_insn_data(struct verifier_env *env, u32 off,
 	return new_prog;
 }
 
+/* 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 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'
  */
@@ -2370,6 +2395,9 @@ 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);