Thanks a lot for the detailed explanation, I understood now.
Martijn
On Tue, Jul 28, 2020 at 4:50 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Tue, Jul 28, 2020 at 3:50 PM Martijn Coenen <maco@xxxxxxxxxxx> wrote:
> > On Mon, Jul 27, 2020 at 2:04 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > > - task B opens /dev/binder once, creating binder_proc instance P3
> > > - P3 calls P2 (via magic handle 0) with (void*)1 as argument (two-way
> > > transaction)
> > > - P2 receives the handle and uses it to call P3 (two-way transaction)
> > > - P3 calls P2 (via magic handle 0) (two-way transaction)
> > > - P2 calls P2 (via handle 1) (two-way transaction)
> >
> > Why do you need P3 involved at all? Could P2 just straight away make a
> > call on handle 1?
>
> Yes, it could, I think. IIRC these steps are necessary if you want to
> actually get a KASAN splat, instead of just a transaction-to-self with
> no further consequences. It has been a while since I looked at this,
> but I'll try to figure out again what was going on...
>
>
> A UAF occurs in the following code due to the transaction-to-self,
> because the "if (t->to_thread == thread)" is tricked into believing
> that the transaction has already been accepted.
>
> static int binder_thread_release(struct binder_proc *proc,
> struct binder_thread *thread)
> {
> struct binder_transaction *t;
> struct binder_transaction *send_reply = NULL;
> [...]
> t = thread->transaction_stack;
> if (t) {
> [...]
> if (t->to_thread == thread)
> send_reply = t;
> } else {
> [...]
> }
> [...]
> //NOTE: transaction is freed here because top-of-stack is
> // wrongly treated as already-accepted incoming transaction)
> if (send_reply)
> binder_send_failed_reply(send_reply, BR_DEAD_REPLY);
> //NOTE pending transaction work is processed here (transaction has not
> // yet been accepted)
> binder_release_work(proc, &thread->todo);
> [...]
> }
>
> An unaccepted transaction will only have a non-NULL ->to_thread if the
> transaction has a specific target thread; for a non-reply transaction,
> that is only the case if it is a two-way transaction that was sent
> while the binder call stack already contained the target task (iow,
> the transaction looks like a synchronous callback invocation).
>
> With the steps:
>
> - P3 calls P2 (via magic handle 0) with (void*)1 as argument (two-way
> transaction)
> - P2 receives the handle and uses it to call P3 (two-way transaction)
> - P3 calls P2 (via magic handle 0) (two-way transaction)
> - P2 calls P2 (via handle 1) (two-way transaction)
>
> the call stack will look like this:
>
> P3 -> P2 -> P3 -> P2 -> P2
>
> The initial call from P3 to P2 was IIRC just to give P2 a handle to
> P3; IIRC the relevant part of the call stack was:
>
> P2 -> P3 -> P2 -> P2
>
> Because P2 already occurs down in the call stack, the final
> transaction "P2 -> P2" is considered to be a callback and is
> thread-directed.
>
>
> The reason why P3 has to be created from task B is the "if
> (target_node && target_proc->pid == proc->pid)" check for transactions
> to reference 0.
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