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@xxxxxxxxxxxx>
---
kernel/bpf/verifier.c | 183 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 183 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7227543e474b..effab7d1c7e8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -313,6 +313,185 @@ 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,
+};
+
+#define PUSH_INT(I) \
+ do { \
+ if (cur_stack >= insn_cnt) { \
+ ret = -E2BIG; \
+ goto free_st; \
+ } \
+ stack[cur_stack++] = I; \
+ } while (0)
+
+#define PEEK_INT() \
+ ({ \
+ int _ret; \
+ if (cur_stack == 0) \
+ _ret = -1; \
+ else \
+ _ret = stack[cur_stack - 1]; \
+ _ret; \
+ })
+
+#define POP_INT() \
+ ({ \
+ int _ret; \
+ if (cur_stack == 0) \
+ _ret = -1; \
+ else \
+ _ret = stack[--cur_stack]; \
+ _ret; \
+ })
+
+#define PUSH_INSN(T, W, E) \
+ do { \
+ int w = W; \
+ if (E == FALLTHROUGH && st[T] >= (DISCOVERED | FALLTHROUGH)) \
+ break; \
+ if (E == BRANCH && st[T] >= (DISCOVERED | BRANCH)) \
+ break; \
+ if (w < 0 || w >= insn_cnt) { \
+ verbose("jump out of range from insn %d to %d\n", T, w); \
+ ret = -EINVAL; \
+ goto free_st; \
+ } \
+ if (st[w] == 0) { \
+ /* tree-edge */ \
+ st[T] = DISCOVERED | E; \
+ st[w] = DISCOVERED; \
+ PUSH_INT(w); \
+ goto peek_stack; \
+ } else if ((st[w] & 0xF0) == DISCOVERED) { \
+ verbose("back-edge from insn %d to %d\n", T, w); \
+ ret = -EINVAL; \
+ goto free_st; \
+ } else if (st[w] == EXPLORED) { \
+ /* forward- or cross-edge */ \
+ st[T] = DISCOVERED | E; \
+ } else { \
+ verbose("insn state internal bug\n"); \
+ ret = -EFAULT; \
+ goto free_st; \
+ } \
+ } while (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 cur_stack = 0;
+ int *stack;
+ int ret = 0;
+ int *st;
+ int i, t;
+
+ st = kzalloc(sizeof(int) * insn_cnt, GFP_KERNEL);
+ if (!st)
+ return -ENOMEM;
+
+ stack = kzalloc(sizeof(int) * insn_cnt, GFP_KERNEL);
+ if (!stack) {
+ kfree(st);
+ return -ENOMEM;
+ }
+
+ st[0] = DISCOVERED; /* mark 1st insn as discovered */
+ PUSH_INT(0);
+
+peek_stack:
+ while ((t = PEEK_INT()) != -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) {
+ PUSH_INSN(t, t + 1, FALLTHROUGH);
+ } else if (opcode == BPF_JA) {
+ if (BPF_SRC(insns[t].code) != BPF_K) {
+ ret = -EINVAL;
+ goto free_st;
+ }
+ /* unconditional jump with single edge */
+ PUSH_INSN(t, t + insns[t].off + 1, FALLTHROUGH);
+ } else {
+ /* conditional jump with two edges */
+ PUSH_INSN(t, t + 1, FALLTHROUGH);
+ PUSH_INSN(t, t + insns[t].off + 1, BRANCH);
+ }
+ } else {
+ /* all other non-branch instructions with single
+ * fall-through edge
+ */
+ PUSH_INSN(t, t + 1, FALLTHROUGH);
+ }
+
+mark_explored:
+ st[t] = EXPLORED;
+ if (POP_INT() == -1) {
+ verbose("pop_int internal bug\n");
+ ret = -EFAULT;
+ goto free_st;
+ }
+ }
+
+
+ for (i = 0; i < insn_cnt; i++) {
+ if (st[i] != EXPLORED) {
+ verbose("unreachable insn %d\n", i);
+ ret = -EINVAL;
+ goto free_st;
+ }
+ }
+
+free_st:
+ kfree(st);
+ kfree(stack);
+ return ret;
+}
+
/* look for pseudo eBPF instructions that access map FDs and
* replace them with actual map pointers
*/
@@ -462,6 +641,10 @@ 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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