From: Alexei Starovoitov <ast@xxxxxxxxxx>
bpf_for_each_map_elem() and bpf_timer_set_callback() helpers are relying on
PTR_TO_FUNC infra in the verifier to validate addresses to subprograms
and pass them into the helpers as function callbacks.
In case of bpf_for_each_map_elem() the callback is invoked synchronously
and the verifier treats it as a normal subprogram call by adding another
bpf_func_state and new frame in __check_func_call().
bpf_timer_set_callback() doesn't invoke the callback directly.
The subprogram will be called asynchronously from bpf_timer_cb().
Teach the verifier to validate such async callbacks as special kind
of jump by pushing verifier state into stack and let pop_stack() process it.
Special care needs to be taken during state pruning.
The call insn doing bpf_timer_set_callback has to be a prune_point.
Otherwise short timer callbacks might not have prune points in front of
bpf_timer_set_callback() which means is_state_visited() will be called
after this call insn is processed in __check_func_call(). Which means that
another async_cb state will be pushed to be walked later and the verifier
will eventually hit BPF_COMPLEXITY_LIMIT_JMP_SEQ limit.
Since push_async_cb() looks like another push_stack() branch the
infinite loop detection will trigger false positive. To recognize
this case mark such states as in_async_callback_fn.
To distinguish infinite loop in async callback vs the same callback called
with different arguments for different map and timer add async_entry_cnt
to bpf_func_state.
Enforce return zero from async callbacks.
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
include/linux/bpf_verifier.h | 9 ++-
kernel/bpf/helpers.c | 8 +--
kernel/bpf/verifier.c | 123 ++++++++++++++++++++++++++++++++++-
3 files changed, 131 insertions(+), 9 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 5d3169b57e6e..242d0b1a0772 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -208,12 +208,19 @@ struct bpf_func_state {
* zero == main subprog
*/
u32 subprogno;
+ /* Every bpf_timer_start will increment async_entry_cnt.
+ * It's used to distinguish:
+ * void foo(void) { for(;;); }
+ * void foo(void) { bpf_timer_set_callback(,foo); }
+ */
+ u32 async_entry_cnt;
+ bool in_callback_fn;
+ bool in_async_callback_fn;
/* The following fields should be last. See copy_func_state() */
int acquired_refs;
struct bpf_reference_state *refs;
int allocated_stack;
- bool in_callback_fn;
struct bpf_stack_state *stack;
};
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 8f44b8c23543..9a1e554f7cab 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1043,7 +1043,6 @@ static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
void *callback_fn;
void *key;
u32 idx;
- int ret;
callback_fn = rcu_dereference_check(t->callback_fn, rcu_read_lock_bh_held());
if (!callback_fn)
@@ -1066,10 +1065,9 @@ static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
key = value - round_up(map->key_size, 8);
}
- ret = BPF_CAST_CALL(callback_fn)((u64)(long)map,
- (u64)(long)key,
- (u64)(long)value, 0, 0);
- WARN_ON(ret != 0); /* Next patch moves this check into the verifier */
+ BPF_CAST_CALL(callback_fn)((u64)(long)map, (u64)(long)key,
+ (u64)(long)value, 0, 0);
+ /* The verifier checked that return value is zero. */
this_cpu_write(hrtimer_running, NULL);
out:
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1511f92b4cf4..ab6ce598a652 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -735,6 +735,10 @@ static void print_verifier_state(struct bpf_verifier_env *env,
if (state->refs[i].id)
verbose(env, ",%d", state->refs[i].id);
}
+ if (state->in_callback_fn)
+ verbose(env, " cb");
+ if (state->in_async_callback_fn)
+ verbose(env, " async_cb");
verbose(env, "\n");
}
@@ -1527,6 +1531,54 @@ static void init_func_state(struct bpf_verifier_env *env,
init_reg_state(env, state);
}
+/* Similar to push_stack(), but for async callbacks */
+static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
+ int insn_idx, int prev_insn_idx,
+ int subprog)
+{
+ struct bpf_verifier_stack_elem *elem;
+ struct bpf_func_state *frame;
+
+ elem = kzalloc(sizeof(struct bpf_verifier_stack_elem), GFP_KERNEL);
+ if (!elem)
+ goto err;
+
+ elem->insn_idx = insn_idx;
+ elem->prev_insn_idx = prev_insn_idx;
+ elem->next = env->head;
+ elem->log_pos = env->log.len_used;
+ env->head = elem;
+ env->stack_size++;
+ if (env->stack_size > BPF_COMPLEXITY_LIMIT_JMP_SEQ) {
+ verbose(env,
+ "The sequence of %d jumps is too complex for async cb.\n",
+ env->stack_size);
+ goto err;
+ }
+ /* Unlike push_stack() do not copy_verifier_state().
+ * The caller state doesn't matter.
+ * This is async callback. It starts in a fresh stack.
+ * Initialize it similar to do_check_common().
+ */
+ elem->st.branches = 1;
+ frame = kzalloc(sizeof(*frame), GFP_KERNEL);
+ if (!frame)
+ goto err;
+ init_func_state(env, frame,
+ BPF_MAIN_FUNC /* callsite */,
+ 0 /* frameno within this callchain */,
+ subprog /* subprog number within this prog */);
+ elem->st.frame[0] = frame;
+ return &elem->st;
+err:
+ free_verifier_state(env->cur_state, true);
+ env->cur_state = NULL;
+ /* pop all elements and return */
+ while (!pop_stack(env, NULL, NULL, false));
+ return NULL;
+}
+
+
enum reg_arg_type {
SRC_OP, /* register is used as source operand */
DST_OP, /* register is used as destination operand */
@@ -5704,6 +5756,30 @@ static int __check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn
}
}
+ if (insn->code == (BPF_JMP | BPF_CALL) &&
+ insn->imm == BPF_FUNC_timer_set_callback) {
+ struct bpf_verifier_state *async_cb;
+
+ /* there is no real recursion here. timer callbacks are async */
+ async_cb = push_async_cb(env, env->subprog_info[subprog].start,
+ *insn_idx, subprog);
+ if (!async_cb)
+ return -EFAULT;
+ callee = async_cb->frame[0];
+ callee->async_entry_cnt = caller->async_entry_cnt + 1;
+
+ /* Convert bpf_timer_set_callback() args into timer callback args */
+ err = set_callee_state_cb(env, caller, callee, *insn_idx);
+ if (err)
+ return err;
+
+ clear_caller_saved_regs(env, caller->regs);
+ mark_reg_unknown(env, caller->regs, BPF_REG_0);
+ caller->regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG;
+ /* continue with next insn after call */
+ return 0;
+ }
+
callee = kzalloc(sizeof(*callee), GFP_KERNEL);
if (!callee)
return -ENOMEM;
@@ -5856,6 +5932,7 @@ static int set_timer_callback_state(struct bpf_verifier_env *env,
/* unused */
__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
+ callee->in_async_callback_fn = true;
return 0;
}
@@ -9224,7 +9301,8 @@ static int check_return_code(struct bpf_verifier_env *env)
struct tnum range = tnum_range(0, 1);
enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
int err;
- const bool is_subprog = env->cur_state->frame[0]->subprogno;
+ struct bpf_func_state *frame = env->cur_state->frame[0];
+ const bool is_subprog = frame->subprogno;
/* LSM and struct_ops func-ptr's return type could be "void" */
if (!is_subprog &&
@@ -9249,6 +9327,22 @@ static int check_return_code(struct bpf_verifier_env *env)
}
reg = cur_regs(env) + BPF_REG_0;
+
+ if (frame->in_async_callback_fn) {
+ /* enforce return zero from async callbacks like timer */
+ if (reg->type != SCALAR_VALUE) {
+ verbose(env, "In async callback the register R0 is not a known value (%s)\n",
+ reg_type_str[reg->type]);
+ return -EINVAL;
+ }
+
+ if (!tnum_in(tnum_const(0), reg->var_off)) {
+ verbose_invalid_scalar(env, reg, &range, "async callback", "R0");
+ return -EINVAL;
+ }
+ return 0;
+ }
+
if (is_subprog) {
if (reg->type != SCALAR_VALUE) {
verbose(env, "At subprogram exit the register R0 is not a scalar value (%s)\n",
@@ -9496,6 +9590,13 @@ static int visit_insn(int t, int insn_cnt, struct bpf_verifier_env *env)
return DONE_EXPLORING;
case BPF_CALL:
+ if (insns[t].imm == BPF_FUNC_timer_set_callback)
+ /* Mark this call insn to trigger is_state_visited() check
+ * before call itself is processed by __check_func_call().
+ * Otherwise new async state will be pushed for further
+ * exploration.
+ */
+ init_explored_state(env, t);
return visit_func_call_insn(t, insn_cnt, insns, env,
insns[t].src_reg == BPF_PSEUDO_CALL);
@@ -10503,9 +10604,25 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
states_cnt++;
if (sl->state.insn_idx != insn_idx)
goto next;
+
if (sl->state.branches) {
- if (states_maybe_looping(&sl->state, cur) &&
- states_equal(env, &sl->state, cur)) {
+ struct bpf_func_state *frame = sl->state.frame[sl->state.curframe];
+
+ if (frame->in_async_callback_fn &&
+ frame->async_entry_cnt != cur->frame[cur->curframe]->async_entry_cnt) {
+ /* Different async_entry_cnt means that the verifier is
+ * processing another entry into async callback.
+ * Seeing the same state is not an indication of infinite
+ * loop or infinite recursion.
+ * But finding the same state doesn't mean that it's safe
+ * to stop processing the current state. The previous state
+ * hasn't yet reached bpf_exit, since state.branches > 0.
+ * Checking in_async_callback_fn alone is not enough either.
+ * Since the verifier still needs to catch infinite loops
+ * inside async callbacks.
+ */
+ } else if (states_maybe_looping(&sl->state, cur) &&
+ states_equal(env, &sl->state, cur)) {
verbose_linfo(env, insn_idx, "; ");
verbose(env, "infinite loop detected at insn %d\n", insn_idx);
return -EINVAL;
--
2.30.2
