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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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 :(

> 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?

thanks,

greg k-h