linux-kernel - Re: [PATCH] binder: Address corner cases in deferred copy and fixup

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Message-ID: <b14ed993-6c3a-ca0a-1b5c-b365628bd1fc@gmail.com>
Date:   Wed, 13 Apr 2022 13:20:39 +0200
From:   Alessandro Astone <ales.astone@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     tkjos@...roid.com, brauner@...nel.org, arve@...roid.com,
        linux-kernel@...r.kernel.org, maco@...roid.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH] binder: Address corner cases in deferred copy and fixup

On 13/04/2022 12:00, Greg KH wrote:

> On Wed, Apr 13, 2022 at 10:54:27AM +0200, Alessandro Astone wrote:
>> When handling BINDER_TYPE_FDA object we are pushing a parent fixup
>> with a certain skip_size but no scatter-gather copy object, since
>> the copy is handled standalone.
>> If BINDER_TYPE_FDA is the last children the scatter-gather copy
>> loop will never stop to skip it, thus we are left with an item in
>> the parent fixup list. This will trigger the BUG_ON().
>>
>> Furthermore, it is possible to receive BINDER_TYPE_FDA object
>> with num_fds=0 which will confuse the scatter-gather code.
>>
>> In the android userspace I could only find these usecases in the
>> libstagefright OMX implementation, so it might be that they're
>> doing something very weird, but nonetheless the kernel should not
>> panic about it.
>>
>> Fixes: 09184ae9b575 ("binder: defer copies of pre-patched txn data")
>> Signed-off-by: Alessandro Astone <ales.astone@...il.com>
>> ---
>>   drivers/android/binder.c | 11 +++++++++--
>>   1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
>> index 8351c5638880..18ad6825ba30 100644
>> --- a/drivers/android/binder.c
>> +++ b/drivers/android/binder.c
>> @@ -2295,7 +2295,7 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
>>   {
>>   	int ret = 0;
>>   	struct binder_sg_copy *sgc, *tmpsgc;
>> -	struct binder_ptr_fixup *pf =
>> +	struct binder_ptr_fixup *tmppf, *pf =
> Just make this a new line:
> 	struct binder_ptr_fixup *tmppf;
> above the existing line.
>
Ack.

>>   		list_first_entry_or_null(pf_head, struct binder_ptr_fixup,
>>   					 node);
>>   
>> @@ -2349,7 +2349,11 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
>>   		list_del(&sgc->node);
>>   		kfree(sgc);
>>   	}
>> -	BUG_ON(!list_empty(pf_head));
> So you are hitting this BUG_ON() today?
>
Correct, both on 5.17, stable 5.17.2 and current master

>> +	list_for_each_entry_safe(pf, tmppf, pf_head, node) {
>> +		BUG_ON(pf->skip_size == 0);
>> +		list_del(&pf->node);
>> +		kfree(pf);
>> +	}
>>   	BUG_ON(!list_empty(sgc_head));
>>   
>>   	return ret > 0 ? -EINVAL : ret;
>> @@ -2486,6 +2490,9 @@ static int binder_translate_fd_array(struct list_head *pf_head,
>>   	struct binder_proc *proc = thread->proc;
>>   	int ret;
>>   
>> +	if (fda->num_fds == 0)
>> +		return 0;
> Why return 0?
>
> This feels like a separate issue from above, should this be 2 different
> commits?
>
return 0 because I want it to be handled as it was handled before
	09184ae9b575 ("binder: defer copies of pre-patched txn data")

Function `binder_do_deferred_txn_copies` distinguishes between a copy-fixup
and a skip with `if (pf->skip_size)` so if the skip_size is 0, which happens
if fda->num_fds is 0, it would accidentally enter the wrong branch.
By returning 0 early i make sure a skip of size 0 is not added. It's not an
error because it was never an error before commit 09184ae9b575 and some
userspace in android is hitting this path.

I would agree it's a separate issue.
I originally merged it in this same patch because
1) Both are fixups to 09184ae9b575
2) Both are triggered by the same real-world android transaction that looks
    something like this:
      obj[0] BINDER_TYPE_PTR, parent
      obj[1] BINDER_TYPE_PTR, child
      obj[2] BINDER_TYPE_PTR, child
      obj[3] BINDER_TYPE_FDA with num_fds=0
3) In the other hunk of this patch i replace the BUG_ON with:
      BUG_ON(pf->skip_size == 0)
    to only BUG if any item remaining in the pf_head list is not a skip,
    but as observed we may receive skips of size 0.
4) With this hunk only, you would no longer reproduce the BUG_ON because the
    only transaction we receive in android with BINDER_TYPE_FDA as last child
    coincidentally always has num_fds=0. Certainly some weird behaviour...

So if I split them, patch A would depend on patch B (see point 3), but the
BUG of patch B would only be reproducible without patch A (see point 4).

But let me know if you still prefer them split.

> thanks,
>
> greg k-h