[PATCH] usb: gadget: configfs: Prevent double delete from purge_configs_funcs

[PATCH] usb: gadget: configfs: Prevent double delete from purge_configs_funcs

[Udipto Goswami]

Currently the function purge_configs_funcs isn't protected by any lock
So there is a potential race possible there id udc called composite_unbind
and say at the same time the configfs_composite_bind also got executed.
If bind fails and follows the error path, driver will end up calling the
purge_configfs_funcs again, resulting in double free from func_list.

Fix this by introducing mutex gi->lock to make sure serial execution
can be maintained. Also, usage of list_for_each_entry_safe is risky
here since the tmp variable is still valid for the next iteration, in
order to ensure list traversal is clean, use list_last_entry.

Fixes: 6cd0fe913879 ("usb: gadget: configfs: Preserve function ordering after bind failure")
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
---
 drivers/usb/gadget/configfs.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
index 3a6b4926193e..f1ac87906897 100644
--- a/drivers/usb/gadget/configfs.c
+++ b/drivers/usb/gadget/configfs.c
@@ -1246,14 +1246,14 @@ static void purge_configs_funcs(struct gadget_info *gi)
 {
 	struct usb_configuration	*c;
 
+	mutex_lock(&gi->lock);
 	list_for_each_entry(c, &gi->cdev.configs, list) {
-		struct usb_function *f, *tmp;
+		struct usb_function *f;
 		struct config_usb_cfg *cfg;
 
 		cfg = container_of(c, struct config_usb_cfg, c);
-
-		list_for_each_entry_safe_reverse(f, tmp, &c->functions, list) {
-
+		while (!list_empty(&c->functions)) {
+			f = list_last_entry(&c->functions, struct usb_function, list);
 			list_move(&f->list, &cfg->func_list);
 			if (f->unbind) {
 				dev_dbg(&gi->cdev.gadget->dev,
@@ -1269,6 +1269,7 @@ static void purge_configs_funcs(struct gadget_info *gi)
 		c->highspeed = 0;
 		c->fullspeed = 0;
 	}
+	mutex_unlock(&gi->lock);
 }
 
 static int configfs_composite_bind(struct usb_gadget *gadget,
-- 
2.17.1

Re: [PATCH] usb: gadget: configfs: Prevent double delete from purge_configs_funcs

[John Keeping]

On Thu, Nov 17, 2022 at 11:05:48AM +0530, Udipto Goswami wrote:
> Currently the function purge_configs_funcs isn't protected by any lock
> So there is a potential race possible there id udc called composite_unbind
> and say at the same time the configfs_composite_bind also got executed.
> If bind fails and follows the error path, driver will end up calling the
> purge_configfs_funcs again, resulting in double free from func_list.
> 
> Fix this by introducing mutex gi->lock to make sure serial execution
> can be maintained. Also, usage of list_for_each_entry_safe is risky
> here since the tmp variable is still valid for the next iteration, in
> order to ensure list traversal is clean, use list_last_entry.

I don't understand how this can happen.

purge_configs_funcs() is called from the .bind and .unbind
usb_gadget_driver hooks, and those should not be able to run at the same
time - if bind has not succeeded, how does unbinding make sense?

> Fixes: 6cd0fe913879 ("usb: gadget: configfs: Preserve function ordering after bind failure")
> Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
> ---
>  drivers/usb/gadget/configfs.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> index 3a6b4926193e..f1ac87906897 100644
> --- a/drivers/usb/gadget/configfs.c
> +++ b/drivers/usb/gadget/configfs.c
> @@ -1246,14 +1246,14 @@ static void purge_configs_funcs(struct gadget_info *gi)
>  {
>  	struct usb_configuration	*c;
>  
> +	mutex_lock(&gi->lock);
>  	list_for_each_entry(c, &gi->cdev.configs, list) {
> -		struct usb_function *f, *tmp;
> +		struct usb_function *f;
>  		struct config_usb_cfg *cfg;
>  
>  		cfg = container_of(c, struct config_usb_cfg, c);
> -
> -		list_for_each_entry_safe_reverse(f, tmp, &c->functions, list) {
> -
> +		while (!list_empty(&c->functions)) {
> +			f = list_last_entry(&c->functions, struct usb_function, list);
>  			list_move(&f->list, &cfg->func_list);
>  			if (f->unbind) {
>  				dev_dbg(&gi->cdev.gadget->dev,
> @@ -1269,6 +1269,7 @@ static void purge_configs_funcs(struct gadget_info *gi)
>  		c->highspeed = 0;
>  		c->fullspeed = 0;
>  	}
> +	mutex_unlock(&gi->lock);
>  }
>  
>  static int configfs_composite_bind(struct usb_gadget *gadget,
> -- 
> 2.17.1
> 

Re: [PATCH] usb: gadget: configfs: Prevent double delete from purge_configs_funcs

[Udipto Goswami]

Hi John,

On 11/18/22 10:04 PM, John Keeping wrote:
> On Thu, Nov 17, 2022 at 11:05:48AM +0530, Udipto Goswami wrote:
>> Currently the function purge_configs_funcs isn't protected by any lock
>> So there is a potential race possible there id udc called composite_unbind
>> and say at the same time the configfs_composite_bind also got executed.
>> If bind fails and follows the error path, driver will end up calling the
>> purge_configfs_funcs again, resulting in double free from func_list.
>>
>> Fix this by introducing mutex gi->lock to make sure serial execution
>> can be maintained. Also, usage of list_for_each_entry_safe is risky
>> here since the tmp variable is still valid for the next iteration, in
>> order to ensure list traversal is clean, use list_last_entry.
> 
> I don't understand how this can happen.
> 
> purge_configs_funcs() is called from the .bind and .unbind
> usb_gadget_driver hooks, and those should not be able to run at the same
> time - if bind has not succeeded, how does unbinding make sense?

Consider this,
we are going out of the composition i.e unbinding it,
lets say we have 7 functions and we were only at removing the 2 one 
where in the meantime the bind process got called, the udc driver 
doesn't make sure of this, depends on the controller when they call the 
usb_add_gadget probably.

For dwc3 controller specifically, the mode will still be device  so from 
core.c will end up calling gadget_init() which in turn will call 
usb_add_gadget which further does to check_pending_gadget_drivers, 
eventually calling bind.

So, i don't see any serializing mechanism here which would stop the 
execution of make the controller wait until the unbind is completed 
right. Please correct me if i'm wrong.
> 
>> Fixes: 6cd0fe913879 ("usb: gadget: configfs: Preserve function ordering after bind failure")
>> Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
>> ---
>>   drivers/usb/gadget/configfs.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
>> index 3a6b4926193e..f1ac87906897 100644
>> --- a/drivers/usb/gadget/configfs.c
>> +++ b/drivers/usb/gadget/configfs.c
>> @@ -1246,14 +1246,14 @@ static void purge_configs_funcs(struct gadget_info *gi)
>>   {
>>   	struct usb_configuration	*c;
>>   
>> +	mutex_lock(&gi->lock);
>>   	list_for_each_entry(c, &gi->cdev.configs, list) {
>> -		struct usb_function *f, *tmp;
>> +		struct usb_function *f;
>>   		struct config_usb_cfg *cfg;
>>   
>>   		cfg = container_of(c, struct config_usb_cfg, c);
>> -
>> -		list_for_each_entry_safe_reverse(f, tmp, &c->functions, list) {
>> -
>> +		while (!list_empty(&c->functions)) {
>> +			f = list_last_entry(&c->functions, struct usb_function, list);
>>   			list_move(&f->list, &cfg->func_list);
>>   			if (f->unbind) {
>>   				dev_dbg(&gi->cdev.gadget->dev,
>> @@ -1269,6 +1269,7 @@ static void purge_configs_funcs(struct gadget_info *gi)
>>   		c->highspeed = 0;
>>   		c->fullspeed = 0;
>>   	}
>> +	mutex_unlock(&gi->lock);
>>   }
>>   
>>   static int configfs_composite_bind(struct usb_gadget *gadget,
>> -- 
>> 2.17.1
>>

Thanks,
-Udipto

Re: [PATCH] usb: gadget: configfs: Prevent double delete from purge_configs_funcs

[John Keeping]

On Sun, Nov 20, 2022 at 12:40:54PM +0530, Udipto Goswami wrote:
> On 11/18/22 10:04 PM, John Keeping wrote:
> > On Thu, Nov 17, 2022 at 11:05:48AM +0530, Udipto Goswami wrote:
> > > Currently the function purge_configs_funcs isn't protected by any lock
> > > So there is a potential race possible there id udc called composite_unbind
> > > and say at the same time the configfs_composite_bind also got executed.
> > > If bind fails and follows the error path, driver will end up calling the
> > > purge_configfs_funcs again, resulting in double free from func_list.
> > > 
> > > Fix this by introducing mutex gi->lock to make sure serial execution
> > > can be maintained. Also, usage of list_for_each_entry_safe is risky
> > > here since the tmp variable is still valid for the next iteration, in
> > > order to ensure list traversal is clean, use list_last_entry.
> > 
> > I don't understand how this can happen.
> > 
> > purge_configs_funcs() is called from the .bind and .unbind
> > usb_gadget_driver hooks, and those should not be able to run at the same
> > time - if bind has not succeeded, how does unbinding make sense?
> 
> Consider this,
> we are going out of the composition i.e unbinding it,
> lets say we have 7 functions and we were only at removing the 2 one where in
> the meantime the bind process got called, the udc driver doesn't make sure
> of this, depends on the controller when they call the usb_add_gadget
> probably.
> 
> For dwc3 controller specifically, the mode will still be device  so from
> core.c will end up calling gadget_init() which in turn will call
> usb_add_gadget which further does to check_pending_gadget_drivers,
> eventually calling bind.

I can't find functions called gadget_init() or
check_pending_gadget_drivers() in the kernel.

> So, i don't see any serializing mechanism here which would stop the
> execution of make the controller wait until the unbind is completed right.
> Please correct me if i'm wrong.

The comments in configfs_composite_bind() and
configfs_composite_unbind() indicate that gi->lock is held by the caller
for both these functions.

I checked that this is the case for configfs gadgets, so it's possible
that one of the legacy paths has a bug where it is not holding this
lock, but adding the extra lock calls in the patch below will deadlock
with configfs.

> > 
> > > Fixes: 6cd0fe913879 ("usb: gadget: configfs: Preserve function ordering after bind failure")
> > > Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
> > > ---
> > >   drivers/usb/gadget/configfs.c | 9 +++++----
> > >   1 file changed, 5 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> > > index 3a6b4926193e..f1ac87906897 100644
> > > --- a/drivers/usb/gadget/configfs.c
> > > +++ b/drivers/usb/gadget/configfs.c
> > > @@ -1246,14 +1246,14 @@ static void purge_configs_funcs(struct gadget_info *gi)
> > >   {
> > >   	struct usb_configuration	*c;
> > > +	mutex_lock(&gi->lock);
> > >   	list_for_each_entry(c, &gi->cdev.configs, list) {
> > > -		struct usb_function *f, *tmp;
> > > +		struct usb_function *f;
> > >   		struct config_usb_cfg *cfg;
> > >   		cfg = container_of(c, struct config_usb_cfg, c);
> > > -
> > > -		list_for_each_entry_safe_reverse(f, tmp, &c->functions, list) {
> > > -
> > > +		while (!list_empty(&c->functions)) {
> > > +			f = list_last_entry(&c->functions, struct usb_function, list);
> > >   			list_move(&f->list, &cfg->func_list);
> > >   			if (f->unbind) {
> > >   				dev_dbg(&gi->cdev.gadget->dev,
> > > @@ -1269,6 +1269,7 @@ static void purge_configs_funcs(struct gadget_info *gi)
> > >   		c->highspeed = 0;
> > >   		c->fullspeed = 0;
> > >   	}
> > > +	mutex_unlock(&gi->lock);
> > >   }
> > >   static int configfs_composite_bind(struct usb_gadget *gadget,