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

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:

quoted

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 <redacted>
Co-developed-by: Udipto Goswami <redacted>
Signed-off-by: Udipto Goswami <redacted>

A few formatting nits below, but other than that this is:

Reviewed-by: John Keeping <redacted>

quoted

---
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.

quoted

  	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.

quoted

  	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.
  */

quoted

+	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.

quoted

  
  	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