[PATCH 08/13] ANDROID: binder: don't check prio permissions on restore.

Martijn Coenen
2017-08-25T09:35:17Z

Because we have disabled RT priority inheritance for
the regular binder domain, the following can happen:

1) thread A (prio 98) calls into thread B
2) because RT prio inheritance is disabled, thread B
runs at the lowest nice (prio 100) instead
3) thread B calls back into A; A will run at prio 100
for the duration of the transaction
4) When thread A is done with the call from B, we will
try to restore the prio back to 98. But, we fail
because the process doesn't hold CAP_SYS_NICE,
neither is RLIMIT_RT_PRIO set.

While the proper fix going forward will be to
correctly apply CAP_SYS_NICE or RLIMIT_RT_PRIO,
for now it seems reasonable to not check permissions
on the restore path.

Signed-off-by: Martijn Coenen <maco@android.com>
---
drivers/android/binder.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 8eee4716e777..d6648b5f5988 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -1108,8 +1108,9 @@ static int to_kernel_prio(int policy, int user_priority)
return MAX_USER_RT_PRIO - 1 - user_priority;
}

-static void binder_set_priority(struct task_struct *task,
- struct binder_priority desired)
+static void binder_do_set_priority(struct task_struct *task,
+ struct binder_priority desired,
+ bool verify)
{
int priority; /* user-space prio value */
bool has_cap_nice;
@@ -1122,7 +1123,7 @@ static void binder_set_priority(struct task_struct *task,

priority = to_userspace_prio(policy, desired.prio);

- if (is_rt_policy(policy) && !has_cap_nice) {
+ if (verify && is_rt_policy(policy) && !has_cap_nice) {
long max_rtprio = task_rlimit(task, RLIMIT_RTPRIO);

if (max_rtprio == 0) {
@@ -1133,7 +1134,7 @@ static void binder_set_priority(struct task_struct *task,
}
}

- if (is_fair_policy(policy) && !has_cap_nice) {
+ if (verify && is_fair_policy(policy) && !has_cap_nice) {
long min_nice = rlimit_to_nice(task_rlimit(task, RLIMIT_NICE));

if (min_nice > MAX_NICE) {
@@ -1166,6 +1167,18 @@ static void binder_set_priority(struct task_struct *task,
set_user_nice(task, priority);
}

+static void binder_set_priority(struct task_struct *task,
+ struct binder_priority desired)
+{
+ binder_do_set_priority(task, desired, /* verify = */ true);
+}
+
+static void binder_restore_priority(struct task_struct *task,
+ struct binder_priority desired)
+{
+ binder_do_set_priority(task, desired, /* verify = */ false);
+}
+
static void binder_transaction_priority(struct task_struct *task,
struct binder_transaction *t,
struct binder_priority node_prio,
@@ -3202,7 +3215,7 @@ static void binder_transaction(struct binder_proc *proc,
binder_enqueue_work_ilocked(&t->work, &target_thread->todo);
binder_inner_proc_unlock(target_proc);
wake_up_interruptible_sync(&target_thread->wait);
- binder_set_priority(current, in_reply_to->saved_priority);
+ binder_restore_priority(current, in_reply_to->saved_priority);
binder_free_transaction(in_reply_to);
} else if (!(t->flags & TF_ONE_WAY)) {
BUG_ON(t->buffer->async_transaction != 0);
@@ -3291,7 +3304,7 @@ static void binder_transaction(struct binder_proc *proc,

BUG_ON(thread->return_error.cmd != BR_OK);
if (in_reply_to) {
- binder_set_priority(current, in_reply_to->saved_priority);
+ binder_restore_priority(current, in_reply_to->saved_priority);
thread->return_error.cmd = BR_TRANSACTION_COMPLETE;
binder_enqueue_work(thread->proc,
&thread->return_error.work,
@@ -3891,7 +3904,7 @@ static int binder_thread_read(struct binder_proc *proc,
wait_event_interruptible(binder_user_error_wait,
binder_stop_on_user_error < 2);
}
- binder_set_priority(current, proc->default_priority);
+ binder_restore_priority(current, proc->default_priority);
}

if (non_block) {
--
2.14.1.480.gb18f417b89-goog