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

[Udipto Goswami <quic_ugoswami@quicinc.com>]

Hi Greg,

On 07-01-2022 03:34 pm, Greg Kroah-Hartman wrote:
> On Fri, Jan 07, 2022 at 01:52:05PM +0530, Udipto Goswami wrote:
>> 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.
> It's not a checkpatch error, it will show up in the scripts we run on
> commits in our trees.  linux-next will report a problem with this, and
> so I have the same scripts as well.  They were posted to the
> users@kernel.org mailing list a year or so ago if you are curious.  It's
> not a big deal.
>
>>>> 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.
> But it is not needed here, so no need to add it.

we haven't encountered this race although, if ffs_func_set_alt() which 
is asynchronously scheduled ffs_reset_work() -> 
ffs_data_reset()->ffs_data_clear() this can take place while user space 
just opened ep0.

in this case the epfiles_destroy() if took place just after this 
operation of ffs->epfiles = epfiles, the epfiles_create, the 
ffs->epfiles will be freed. It will be better if we protect this case as 
well.

>
> thanks,
>
> greg k-h