bpf: Implement verifier support for validation of async callbacks.

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@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210715005417.78572-9-alexei.starovoitov@gmail.com
This commit is contained in:
Alexei Starovoitov 2021-07-14 17:54:14 -07:00 committed by Daniel Borkmann
parent 86fc6ee6e2
commit bfc6bb74e4
3 changed files with 131 additions and 9 deletions

View File

@ -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;
};

View File

@ -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:

View File

@ -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;