[PATCH] binder: Address corner cases in deferred copy and fixup

[PATCH] binder: Address corner cases in deferred copy and fixup

[Alessandro Astone]

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@gmail.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 =
 		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));
+	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;
+
 	fd_buf_size = sizeof(u32) * fda->num_fds;
 	if (fda->num_fds >= SIZE_MAX / sizeof(u32)) {
 		binder_user_error("%d:%d got transaction with invalid number of fds (%lld)\n",
-- 
2.35.1

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

[Greg KH]

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@gmail.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.

>  		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?

> +	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?

thanks,

greg k-h

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

[Alessandro Astone]

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@gmail.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

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

[Todd Kjos]

On Wed, Apr 13, 2022 at 4:20 AM Alessandro Astone <ales.astone@gmail.com> wrote:
>
> 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@gmail.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 for the fix! Please go ahead and split it into two patches.

>
> > thanks,
> >
> > greg k-h