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Date: Fri, 23 Feb 2018 17:40:51 +0000
From: Edward Cree <ecree@...arflare.com>
To: netdev <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>
Subject: [RFC PATCH bpf-next 05/12] bpf/verifier: detect loops dynamically
rather than statically
Add in a new chain of parent states, which does not cross function-call
boundaries, and check whether our current insn_idx appears anywhere in
the chain. Since all jump targets have state-list marks (now placed
by prepare_cfg_marks(), which replaces check_cfg()), it suffices to
thread this chain through the existing explored_states and check it
only in is_state_visited().
By using this parent-state chain to detect loops, we open up the
possibility that in future we could determine a loop to be bounded and
thus accept it.
A few selftests had to be changed to ensure that all the insns walked
before reaching the back-edge are valid.
Signed-off-by: Edward Cree <ecree@...arflare.com>
---
include/linux/bpf_verifier.h | 2 +
kernel/bpf/verifier.c | 280 +++++++++-------------------
tools/testing/selftests/bpf/test_verifier.c | 12 +-
3 files changed, 97 insertions(+), 197 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 0bc49c768585..24ddbc1ed3b2 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -120,6 +120,8 @@ struct bpf_func_state {
* zero == main subprog
*/
u32 subprogno;
+ /* loop detection; points into an explored_state */
+ struct bpf_func_state *parent;
/* should be second to last. See copy_func_state() */
int allocated_stack;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7a08b5e8e071..e7d898f4fd29 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -429,6 +429,7 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,
dst_state->frame[i] = NULL;
}
dst_state->curframe = src->curframe;
+
for (i = 0; i <= src->curframe; i++) {
dst = dst_state->frame[i];
if (!dst) {
@@ -716,6 +717,7 @@ static void init_func_state(struct bpf_verifier_env *env,
int entry, int frameno, int subprogno)
{
state->insn_idx = entry;
+ state->parent = NULL;
state->frameno = frameno;
state->subprogno = subprogno;
init_reg_state(env, state);
@@ -3747,211 +3749,85 @@ static int check_return_code(struct bpf_verifier_env *env)
return 0;
}
-/* 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 STATE_LIST_MARK ((struct bpf_verifier_state_list *) -1L)
-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 bpf_verifier_env *env)
+static int mark_jump_target(struct bpf_verifier_env *env, int from, int off)
{
- if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
- return 0;
-
- if (e == BRANCH && insn_state[t] >= (DISCOVERED | BRANCH))
- return 0;
+ int to = from + off + 1;
- if (w < 0 || w >= env->prog->len) {
- verbose(env, "jump out of range from insn %d to %d\n", t, w);
+ if (to < 0 || to >= env->prog->len) {
+ verbose(env, "jump out of range from insn %d to %d\n", from, to);
return -EINVAL;
}
-
- if (e == BRANCH)
- /* mark branch target for state pruning */
- env->explored_states[w] = STATE_LIST_MARK;
-
- 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(env, "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(env, "insn state internal bug\n");
- return -EFAULT;
- }
+ /* mark branch target for state pruning */
+ env->explored_states[to] = STATE_LIST_MARK;
return 0;
}
-/* non-recursive depth-first-search to detect loops in BPF program
- * loop == back-edge in directed graph
- */
-static int check_cfg(struct bpf_verifier_env *env)
+/* Mark jump/branch targets for control flow tracking & state pruning */
+static int prepare_cfg_marks(struct bpf_verifier_env *env)
{
struct bpf_insn *insns = env->prog->insnsi;
- int insn_cnt = env->prog->len;
- int ret = 0;
- int i, t;
+ int insn_cnt = env->prog->len, i, err = 0;
- 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;
- }
+ for (i = 0; i < insn_cnt; i++) {
+ u8 opcode = BPF_OP(insns[i].code);
- 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;
- if (t + 1 < insn_cnt)
- env->explored_states[t + 1] = STATE_LIST_MARK;
- if (insns[t].src_reg == BPF_PSEUDO_CALL) {
- env->explored_states[t] = STATE_LIST_MARK;
- ret = push_insn(t, t + insns[t].imm + 1, BRANCH, 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;
+ if (BPF_CLASS(insns[i].code) != BPF_JMP) {
+ if (i + 1 == insn_cnt) {
+ verbose(env, "no exit/jump at end of program (insn %d)\n",
+ i);
+ return -EINVAL;
}
- /* 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;
- /* tell verifier to check for equivalent states
- * after every call and jump
- */
- if (t + 1 < insn_cnt)
- env->explored_states[t + 1] = STATE_LIST_MARK;
- } else {
- /* conditional jump with two edges */
- env->explored_states[t] = STATE_LIST_MARK;
- 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;
+ continue;
}
- } 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(env, "pop stack internal bug\n");
- ret = -EFAULT;
- goto err_free;
+ switch (opcode) {
+ case BPF_EXIT:
+ continue;
+ case BPF_CALL:
+ /* following insn is target of function return */
+ err = mark_jump_target(env, i, 0);
+ if (err)
+ return err;
+ if (insns[i].src_reg == BPF_PSEUDO_CALL)
+ err = mark_jump_target(env, i, insns[i].imm);
+ break;
+ case BPF_JA:
+ /* unconditional jump; mark target */
+ err = mark_jump_target(env, i, insns[i].off);
+ break;
+ default:
+ /* conditional jump; mark following insn and target */
+ err = mark_jump_target(env, i, 0);
+ if (err)
+ return err;
+ err = mark_jump_target(env, i, insns[i].off);
+ }
+ if (err)
+ return err;
}
- goto peek_stack;
-check_state:
+ /* Second pass, to detect statically unreachable code. Any target of
+ * a jump or call will have a state-list mark
+ */
for (i = 0; i < insn_cnt; i++) {
- if (insn_state[i] != EXPLORED) {
- verbose(env, "unreachable insn %d\n", i);
- ret = -EINVAL;
- goto err_free;
- }
+ /* insn following a non-jump is statically reachable */
+ if (BPF_CLASS(insns[i].code) != BPF_JMP)
+ continue;
+ /* insn following a CALL or conditional jump will have been
+ * marked by that insn (see above). So if following insn is
+ * not marked, then we're an EXIT or JA and thus the
+ * following insn is statically unreachable.
+ * If we're last insn, and we're not an EXIT or JA, then first
+ * pass would have complained in mark_jump_target().
+ */
+ if (i + 1 < insn_cnt)
+ if (env->explored_states[i + 1] != STATE_LIST_MARK) {
+ verbose(env, "unreachable insn %d\n", i + 1);
+ return -EINVAL;
+ }
}
- ret = 0; /* cfg looks good */
-
-err_free:
- kfree(insn_state);
- kfree(insn_stack);
- return ret;
+ return 0;
}
/* check %cur's range satisfies %old's */
@@ -4287,11 +4163,24 @@ static int propagate_liveness(struct bpf_verifier_env *env,
return err;
}
-static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
+static struct bpf_func_state *find_loop(struct bpf_verifier_env *env)
+{
+ struct bpf_func_state *cur = cur_frame(env);
+ int insn_idx = cur->insn_idx;
+
+ while ((cur = cur->parent) != NULL)
+ if (cur->insn_idx == insn_idx)
+ return cur;
+ return NULL;
+}
+
+static int is_state_visited(struct bpf_verifier_env *env, int prev_insn_idx,
+ int insn_idx)
{
struct bpf_verifier_state_list *new_sl;
struct bpf_verifier_state_list *sl;
struct bpf_verifier_state *cur = env->cur_state, *new;
+ struct bpf_func_state *old;
int i, j, err;
sl = env->explored_states[insn_idx];
@@ -4301,6 +4190,14 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
*/
return 0;
+ /* Check our parentage chain: have we looped? */
+ old = find_loop(env);
+ if (old != NULL) {
+ verbose(env, "back-edge from insn %d to %d\n", prev_insn_idx,
+ insn_idx);
+ return -EINVAL;
+ }
+
while (sl != STATE_LIST_MARK) {
if (states_equal(env, &sl->state, cur)) {
/* reached equivalent register/stack state,
@@ -4342,7 +4239,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
}
new_sl->next = env->explored_states[insn_idx];
env->explored_states[insn_idx] = new_sl;
- /* connect new state to parentage chain */
+ /* connect new state's regs to parentage chain */
for (i = 0; i < BPF_REG_FP; i++)
cur_regs(env)[i].parent = &new->frame[new->curframe]->regs[i];
/* clear write marks in current state: the writes we did are not writes
@@ -4359,6 +4256,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
struct bpf_func_state *frame = cur->frame[j];
struct bpf_func_state *newframe = new->frame[j];
+ frame->parent = newframe;
for (i = 0; i < frame->allocated_stack / BPF_REG_SIZE; i++) {
frame->stack[i].spilled_ptr.live = REG_LIVE_NONE;
frame->stack[i].spilled_ptr.parent =
@@ -4404,7 +4302,7 @@ static int do_check(struct bpf_verifier_env *env)
u8 class;
int err;
- if (insn_idx >= insn_cnt) {
+ if (insn_idx < 0 || insn_idx >= insn_cnt) {
verbose(env, "invalid insn idx %d insn_cnt %d\n",
insn_idx, insn_cnt);
return -EFAULT;
@@ -4433,7 +4331,7 @@ static int do_check(struct bpf_verifier_env *env)
aux->subprogno = frame->subprogno;
aux->seen = true;
- err = is_state_visited(env, insn_idx);
+ err = is_state_visited(env, prev_insn_idx, insn_idx);
if (err < 0)
return err;
if (err == 1) {
@@ -5581,7 +5479,7 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
env->allow_ptr_leaks = capable(CAP_SYS_ADMIN);
- ret = check_cfg(env);
+ ret = prepare_cfg_marks(env);
if (ret < 0)
goto skip_full_check;
diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index 9c7531887ee3..722a16b2e9c4 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -644,14 +644,13 @@ static struct bpf_test tests[] = {
.insns = {
BPF_ALU64_REG(BPF_MOV, BPF_REG_0, BPF_REG_2),
},
- .errstr = "jump out of range",
+ .errstr = "no exit/jump at end of program",
.result = REJECT,
},
{
"loop (back-edge)",
.insns = {
BPF_JMP_IMM(BPF_JA, 0, 0, -1),
- BPF_EXIT_INSN(),
},
.errstr = "back-edge",
.result = REJECT,
@@ -659,11 +658,11 @@ static struct bpf_test tests[] = {
{
"loop2 (back-edge)",
.insns = {
+ BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_0),
BPF_MOV64_REG(BPF_REG_3, BPF_REG_0),
BPF_JMP_IMM(BPF_JA, 0, 0, -4),
- BPF_EXIT_INSN(),
},
.errstr = "back-edge",
.result = REJECT,
@@ -671,6 +670,7 @@ static struct bpf_test tests[] = {
{
"conditional loop",
.insns = {
+ BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_0),
BPF_MOV64_REG(BPF_REG_3, BPF_REG_0),
@@ -9338,7 +9338,7 @@ static struct bpf_test tests[] = {
BPF_EXIT_INSN(),
},
.prog_type = BPF_PROG_TYPE_TRACEPOINT,
- .errstr = "back-edge from insn 0 to 0",
+ .errstr = "frames is too deep",
.result = REJECT,
},
{
@@ -9666,7 +9666,7 @@ static struct bpf_test tests[] = {
BPF_EXIT_INSN(),
},
.prog_type = BPF_PROG_TYPE_TRACEPOINT,
- .errstr = "back-edge",
+ .errstr = "frames is too deep",
.result = REJECT,
},
{
@@ -9678,7 +9678,7 @@ static struct bpf_test tests[] = {
BPF_EXIT_INSN(),
},
.prog_type = BPF_PROG_TYPE_TRACEPOINT,
- .errstr = "back-edge",
+ .errstr = "frames is too deep",
.result = REJECT,
},
{
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