Hello,
在 2023/10/24 12:57, Alexei Starovoitov 写道:
On Mon, Oct 23, 2023 at 7:42 PM Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx> wrote:
The newly added open-coded css_task iter would try to hold the global
css_set_lock in bpf_iter_css_task_new, so the bpf side has to be careful in
where it allows to use this iter. The mainly concern is dead locking on
css_set_lock. check_css_task_iter_allowlist() in verifier enforced css_task
can only be used in bpf_lsm hooks and sleepable bpf_iter.
This patch relax the allowlist for css_task iter. Any lsm and any iter
(even non-sleepable) and any sleepable are safe since they would not hold
the css_set_lock before entering BPF progs context.
This patch also fixes the misused BPF_TRACE_ITER in
check_css_task_iter_allowlist which compared bpf_prog_type with
bpf_attach_type.
Fixes: 9c66dc94b62ae ("bpf: Introduce css_task open-coded iterator kfuncs")
Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
---
kernel/bpf/verifier.c | 21 ++++++++++++-------
.../selftests/bpf/progs/iters_task_failure.c | 4 ++--
2 files changed, 15 insertions(+), 10 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e9bc5d4a25a1..9f209adc4ccb 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11088,18 +11088,23 @@ static int process_kf_arg_ptr_to_rbtree_node(struct bpf_verifier_env *env,
&meta->arg_rbtree_root.field);
}
+/*
+ * css_task iter allowlist is needed to avoid dead locking on css_set_lock.
+ * LSM hooks and iters (both sleepable and non-sleepable) are safe.
+ * Any sleepable progs are also safe since bpf_check_attach_target() enforce
+ * them can only be attached to some specific hook points.
+ */
static bool check_css_task_iter_allowlist(struct bpf_verifier_env *env)
{
enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
- switch (prog_type) {
- case BPF_PROG_TYPE_LSM:
+ if (prog_type == BPF_PROG_TYPE_LSM)
return true;
- case BPF_TRACE_ITER:
- return env->prog->aux->sleepable;
- default:
- return false;
- }
+
+ if (env->prog->expected_attach_type == BPF_TRACE_ITER)
+ return true;
I think the switch by prog_type has to stay.
Checking attach_type == BPF_TRACE_ITER without considering prog_type
is fragile. It likely works, but we don't do it anywhere else.
Let's stick to what is known to work.
IIUC, do you mean:
static bool check_css_task_iter_allowlist(struct bpf_verifier_env *env)
{
enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
switch (prog_type) {
case BPF_PROG_TYPE_LSM:
return true;
case BPF_PROG_TYPE_TRACING:
if (env->prog->expected_attach_type == BPF_TRACE_ITER)
return true;
return env->prog->aux->sleepable;
default:
return env->prog->aux->sleepable;
}
}
-SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
-__failure __msg("css_task_iter is only allowed in bpf_lsm and bpf iter-s")
+SEC("?fentry/" SYS_PREFIX "sys_getpgid")
+__failure __msg("css_task_iter is only allowed in bpf_lsm, bpf_iter and sleepable progs")
Please add both. fentry that is rejected and fentry.s that is accepted.
Sure.
