Re: [PATCH v8] usb: f_fs: Fix use-after-free for epfile

[Udipto Goswami <quic_ugoswami@quicinc.com>]

Hi Greg,

On 06-01-2022 08:03 pm, Greg Kroah-Hartman wrote:
> On Wed, Jan 05, 2022 at 07:35:26PM +0530, Udipto Goswami wrote:
>> Consider a case where ffs_func_eps_disable is called from
>> ffs_func_disable as part of composition switch and at the
>> same time ffs_epfile_release get called from userspace.
>> ffs_epfile_release will free up the read buffer and call
>> ffs_data_closed which in turn destroys ffs->epfiles and
>> mark it as NULL. While this was happening the driver has
>> already initialized the local epfile in ffs_func_eps_disable
>> which is now freed and waiting to acquire the spinlock. Once
>> spinlock is acquired the driver proceeds with the stale value
>> of epfile and tries to free the already freed read buffer
>> causing use-after-free.
>>
>> Following is the illustration of the race:
>>
>>        CPU1                                  CPU2
>>
>>     ffs_func_eps_disable
>>     epfiles (local copy)
>> 					ffs_epfile_release
>> 					ffs_data_closed
>> 					if (last file closed)
>> 					ffs_data_reset
>> 					ffs_data_clear
>> 					ffs_epfiles_destroy
>> spin_lock
>> dereference epfiles
>>
>> Fix this races by taking epfiles local copy & assigning it under
>> spinlock and if epfiles(local) is null then update it in ffs->epfiles
>> then finally destroy it.
>> Extending the scope further from the race, protecting the ep related
>> structures, and concurrent accesses.
>>
>> Fixes: a9e6f83c2df (usb: gadget: f_fs: stop sleeping in
>> ffs_func_eps_disable)
> No need to line-wrap this line, the scripts will complain about it :(
checkpatch didn't give any error for this though.
>
>> Reviewed-by: John Keeping <john@metanate.com>
>> Signed-off-by: Pratham Pratap <quic_ppratap@quicinc.com>
>> Co-developed-by: Udipto Goswami <quic_ugoswami@quicinc.com>
>> Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
>> ---
>> v8: Fixed compilation errors from previous version.
>>
>>   drivers/usb/gadget/function/f_fs.c | 60 ++++++++++++++++++++++++++++----------
>>   1 file changed, 45 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
>> index 3c584da..541a4af 100644
>> --- a/drivers/usb/gadget/function/f_fs.c
>> +++ b/drivers/usb/gadget/function/f_fs.c
>> @@ -1711,16 +1711,24 @@ static void ffs_data_put(struct ffs_data *ffs)
>>   
>>   static void ffs_data_closed(struct ffs_data *ffs)
>>   {
>> +	struct ffs_epfile *epfiles;
>> +	unsigned long flags;
>> +
>>   	ENTER();
>>   
>>   	if (atomic_dec_and_test(&ffs->opened)) {
>>   		if (ffs->no_disconnect) {
>>   			ffs->state = FFS_DEACTIVATED;
>> -			if (ffs->epfiles) {
>> -				ffs_epfiles_destroy(ffs->epfiles,
>> -						   ffs->eps_count);
>> -				ffs->epfiles = NULL;
>> -			}
>> +			spin_lock_irqsave(&ffs->eps_lock, flags);
>> +			epfiles = ffs->epfiles;
>> +			ffs->epfiles = NULL;
>> +			spin_unlock_irqrestore(&ffs->eps_lock,
>> +							flags);
>> +
>> +			if (epfiles)
>> +				ffs_epfiles_destroy(epfiles,
>> +						 ffs->eps_count);
>> +
>>   			if (ffs->setup_state == FFS_SETUP_PENDING)
>>   				__ffs_ep0_stall(ffs);
>>   		} else {
>> @@ -1767,14 +1775,27 @@ static struct ffs_data *ffs_data_new(const char *dev_name)
>>   
>>   static void ffs_data_clear(struct ffs_data *ffs)
>>   {
>> +	struct ffs_epfile *epfiles;
>> +	unsigned long flags;
>> +
>>   	ENTER();
>>   
>>   	ffs_closed(ffs);
>>   
>>   	BUG_ON(ffs->gadget);
>>   
>> -	if (ffs->epfiles)
>> -		ffs_epfiles_destroy(ffs->epfiles, ffs->eps_count);
>> +	spin_lock_irqsave(&ffs->eps_lock, flags);
>> +	epfiles = ffs->epfiles;
>> +	ffs->epfiles = NULL;
>> +	spin_unlock_irqrestore(&ffs->eps_lock, flags);
>> +
>> +	/*
>> +	 * potential race possible between ffs_func_eps_disable
>> +	 * & ffs_epfile_release therefore maintaining a local
>> +	 * copy of epfile will save us from use-after-free.
>> +	 */
>> +	if (epfiles)
>> +		ffs_epfiles_destroy(epfiles, ffs->eps_count);
>>   
>>   	if (ffs->ffs_eventfd)
>>   		eventfd_ctx_put(ffs->ffs_eventfd);
>> @@ -1790,7 +1811,6 @@ static void ffs_data_reset(struct ffs_data *ffs)
>>   
>>   	ffs_data_clear(ffs);
>>   
>> -	ffs->epfiles = NULL;
>>   	ffs->raw_descs_data = NULL;
>>   	ffs->raw_descs = NULL;
>>   	ffs->raw_strings = NULL;
>> @@ -1870,6 +1890,7 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
>>   {
>>   	struct ffs_epfile *epfile, *epfiles;
>>   	unsigned i, count;
>> +	unsigned long flags;
>>   
>>   	ENTER();
>>   
>> @@ -1895,7 +1916,9 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
>>   		}
>>   	}
>>   
>> +	spin_lock_irqsave(&ffs->eps_lock, flags);
>>   	ffs->epfiles = epfiles;
>> +	spin_unlock_irqrestore(&ffs->eps_lock, flags);
> Why is this lock needed when you set this value?  What is that
> protecting?

Was making it uniform, protection ffs->epfiles all over. Here intention 
is to protect the operation of epfiles getting assigned to ffs->epfiles 
so that we protect the ffs->epfiles instance at the time of creation as 
well.

>
> thanks,
>
> greg k-h