Re: [PATCH v5] usb: f_fs: Fix use-after-free for epfile

[Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>]

Hi John,

Thanks for the review, i'll push v6 with the nits addressed.

On 01-12-2021 10:17 pm, John Keeping wrote:
> On Mon, Nov 29, 2021 at 05:22:57PM +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.
> >
> > Change-Id: I84b46f6c07cbf307a2bf92c366b933dc0e83d91a
> > Signed-off-by: Pratham Pratap <quic_ppratap@xxxxxxxxxxx>
> > Co-developed-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
> > Signed-off-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
> A few formatting nits below, but other than that this is:
>
> Reviewed-by: John Keeping <john@xxxxxxxxxxxx>
>
> > ---
> > v5: Changed the naming of epfile to singular in
> > ffs_func_eps_disable
> >
> >   drivers/usb/gadget/function/f_fs.c | 41 ++++++++++++++++++++++++++++----------
> >   1 file changed, 31 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> > index 3c584da..f7be222 100644
> > --- a/drivers/usb/gadget/function/f_fs.c
> > +++ b/drivers/usb/gadget/function/f_fs.c
> > @@ -1711,16 +1711,23 @@ 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;
> Blank line after variable declarations.
>
> >   	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 +1774,25 @@ 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;
> Again there should be a blank line here.
>
> >   	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.
> > +	 */
> /*
>   * Multi-line comment blocks have the opening "/*" and closing "*/" each
>   * on their own line.  This may benefit from a blank line before the
>   * comment as well.
>   */
>
> > +	if (epfiles)
> > +		ffs_epfiles_destroy(epfiles,
> > +				    ffs->eps_count);
> This was all one line before and you've made it shorter, so it can
> probably stay on one line.
>
> >   
> >   	if (ffs->ffs_eventfd)
> >   		eventfd_ctx_put(ffs->ffs_eventfd);
> > @@ -1919,12 +1937,15 @@ static void ffs_epfiles_destroy(struct ffs_epfile *epfiles, unsigned count)
> >   
> >   static void ffs_func_eps_disable(struct ffs_function *func)
> >   {
> > -	struct ffs_ep *ep         = func->eps;
> > -	struct ffs_epfile *epfile = func->ffs->epfiles;
> > -	unsigned count            = func->ffs->eps_count;
> > +	struct ffs_ep *ep;
> > +	struct ffs_epfile *epfile;
> > +	unsigned short count;
> >   	unsigned long flags;
> >   
> >   	spin_lock_irqsave(&func->ffs->eps_lock, flags);
> > +	count = func->ffs->eps_count;
> > +	epfile = func->ffs->epfiles;
> > +	ep = func->eps;
> >   	while (count--) {
> >   		/* pending requests get nuked */
> >   		if (ep->ep)
> > --
> > 2.7.4