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Date: Fri, 26 Sep 2014 00:17:05 -0700
From: Alexei Starovoitov <ast@...mgrid.com>
To: "David S. Miller" <davem@...emloft.net>
Cc: Ingo Molnar <mingo@...nel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andy Lutomirski <luto@...capital.net>,
Daniel Borkmann <dborkman@...hat.com>,
Hannes Frederic Sowa <hannes@...essinduktion.org>,
Chema Gonzalez <chema@...gle.com>,
Eric Dumazet <edumazet@...gle.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Pablo Neira Ayuso <pablo@...filter.org>,
"H. Peter Anvin" <hpa@...or.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>, linux-api@...r.kernel.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v15 net-next 09/11] bpf: verifier (add branch/goto checks)
check that control flow graph of eBPF program is a directed acyclic graph
check_cfg() does:
- detect loops
- detect unreachable instructions
- check that program terminates with BPF_EXIT insn
- check that all branches are within program boundary
Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
---
kernel/bpf/verifier.c | 189 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 189 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7227543e474b..c689ab8e2713 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -313,6 +313,191 @@ static struct bpf_map *ld_imm64_to_map_ptr(struct bpf_insn *insn)
return (struct bpf_map *) (unsigned long) imm64;
}
+/* non-recursive DFS pseudo code
+ * 1 procedure DFS-iterative(G,v):
+ * 2 label v as discovered
+ * 3 let S be a stack
+ * 4 S.push(v)
+ * 5 while S is not empty
+ * 6 t <- S.pop()
+ * 7 if t is what we're looking for:
+ * 8 return t
+ * 9 for all edges e in G.adjacentEdges(t) do
+ * 10 if edge e is already labelled
+ * 11 continue with the next edge
+ * 12 w <- G.adjacentVertex(t,e)
+ * 13 if vertex w is not discovered and not explored
+ * 14 label e as tree-edge
+ * 15 label w as discovered
+ * 16 S.push(w)
+ * 17 continue at 5
+ * 18 else if vertex w is discovered
+ * 19 label e as back-edge
+ * 20 else
+ * 21 // vertex w is explored
+ * 22 label e as forward- or cross-edge
+ * 23 label t as explored
+ * 24 S.pop()
+ *
+ * convention:
+ * 0x10 - discovered
+ * 0x11 - discovered and fall-through edge labelled
+ * 0x12 - discovered and fall-through and branch edges labelled
+ * 0x20 - explored
+ */
+
+enum {
+ DISCOVERED = 0x10,
+ EXPLORED = 0x20,
+ FALLTHROUGH = 1,
+ BRANCH = 2,
+};
+
+static int *insn_stack; /* stack of insns to process */
+static int cur_stack; /* current stack index */
+static int *insn_state;
+
+/* t, w, e - match pseudo-code above:
+ * t - index of current instruction
+ * w - next instruction
+ * e - edge
+ */
+static int push_insn(int t, int w, int e, struct verifier_env *env)
+{
+ if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
+ return 0;
+
+ if (e == BRANCH && insn_state[t] >= (DISCOVERED | BRANCH))
+ return 0;
+
+ if (w < 0 || w >= env->prog->len) {
+ verbose("jump out of range from insn %d to %d\n", t, w);
+ return -EINVAL;
+ }
+
+ if (insn_state[w] == 0) {
+ /* tree-edge */
+ insn_state[t] = DISCOVERED | e;
+ insn_state[w] = DISCOVERED;
+ if (cur_stack >= env->prog->len)
+ return -E2BIG;
+ insn_stack[cur_stack++] = w;
+ return 1;
+ } else if ((insn_state[w] & 0xF0) == DISCOVERED) {
+ verbose("back-edge from insn %d to %d\n", t, w);
+ return -EINVAL;
+ } else if (insn_state[w] == EXPLORED) {
+ /* forward- or cross-edge */
+ insn_state[t] = DISCOVERED | e;
+ } else {
+ verbose("insn state internal bug\n");
+ return -EFAULT;
+ }
+ return 0;
+}
+
+/* non-recursive depth-first-search to detect loops in BPF program
+ * loop == back-edge in directed graph
+ */
+static int check_cfg(struct verifier_env *env)
+{
+ struct bpf_insn *insns = env->prog->insnsi;
+ int insn_cnt = env->prog->len;
+ int ret = 0;
+ int i, t;
+
+ insn_state = kcalloc(insn_cnt, sizeof(int), GFP_KERNEL);
+ if (!insn_state)
+ return -ENOMEM;
+
+ insn_stack = kcalloc(insn_cnt, sizeof(int), GFP_KERNEL);
+ if (!insn_stack) {
+ kfree(insn_state);
+ return -ENOMEM;
+ }
+
+ insn_state[0] = DISCOVERED; /* mark 1st insn as discovered */
+ insn_stack[0] = 0; /* 0 is the first instruction */
+ cur_stack = 1;
+
+peek_stack:
+ if (cur_stack == 0)
+ goto check_state;
+ t = insn_stack[cur_stack - 1];
+
+ if (BPF_CLASS(insns[t].code) == BPF_JMP) {
+ u8 opcode = BPF_OP(insns[t].code);
+
+ if (opcode == BPF_EXIT) {
+ goto mark_explored;
+ } else if (opcode == BPF_CALL) {
+ ret = push_insn(t, t + 1, FALLTHROUGH, env);
+ if (ret == 1)
+ goto peek_stack;
+ else if (ret < 0)
+ goto err_free;
+ } else if (opcode == BPF_JA) {
+ if (BPF_SRC(insns[t].code) != BPF_K) {
+ ret = -EINVAL;
+ goto err_free;
+ }
+ /* unconditional jump with single edge */
+ ret = push_insn(t, t + insns[t].off + 1,
+ FALLTHROUGH, env);
+ if (ret == 1)
+ goto peek_stack;
+ else if (ret < 0)
+ goto err_free;
+ } else {
+ /* conditional jump with two edges */
+ ret = push_insn(t, t + 1, FALLTHROUGH, env);
+ if (ret == 1)
+ goto peek_stack;
+ else if (ret < 0)
+ goto err_free;
+
+ ret = push_insn(t, t + insns[t].off + 1, BRANCH, env);
+ if (ret == 1)
+ goto peek_stack;
+ else if (ret < 0)
+ goto err_free;
+ }
+ } else {
+ /* all other non-branch instructions with single
+ * fall-through edge
+ */
+ ret = push_insn(t, t + 1, FALLTHROUGH, env);
+ if (ret == 1)
+ goto peek_stack;
+ else if (ret < 0)
+ goto err_free;
+ }
+
+mark_explored:
+ insn_state[t] = EXPLORED;
+ if (cur_stack-- <= 0) {
+ verbose("pop stack internal bug\n");
+ ret = -EFAULT;
+ goto err_free;
+ }
+ goto peek_stack;
+
+check_state:
+ for (i = 0; i < insn_cnt; i++) {
+ if (insn_state[i] != EXPLORED) {
+ verbose("unreachable insn %d\n", i);
+ ret = -EINVAL;
+ goto err_free;
+ }
+ }
+ ret = 0; /* cfg looks good */
+
+err_free:
+ kfree(insn_state);
+ kfree(insn_stack);
+ return ret;
+}
+
/* look for pseudo eBPF instructions that access map FDs and
* replace them with actual map pointers
*/
@@ -462,6 +647,10 @@ int bpf_check(struct bpf_prog *prog, union bpf_attr *attr)
if (ret < 0)
goto skip_full_check;
+ ret = check_cfg(env);
+ if (ret < 0)
+ goto skip_full_check;
+
/* ret = do_check(env); */
skip_full_check:
--
1.7.9.5
--
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