Hi Greg,
Apologies for the delay, please find the following comments as per my
understanding inline.
On 13-12-2021 07:38 pm, Greg Kroah-Hartman wrote:
On Tue, Dec 07, 2021 at 11:37:54AM +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.
Fixes: a9e6f83c2df (usb: gadget: f_fs: stop sleeping in
ffs_func_eps_disable)
Reviewed-by: John Keeping <john@xxxxxxxxxxxx>
Signed-off-by: Pratham Pratap <quic_ppratap@xxxxxxxxxxx>
Co-developed-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
Signed-off-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
---
v6: Addressing minor code formatting.
drivers/usb/gadget/function/f_fs.c | 44 +++++++++++++++++++++++++++++---------
1 file changed, 34 insertions(+), 10 deletions(-)
What commit does this fix?
Does this need to go to stable kernel releases?
thanks,
greg k-h
This is fixing Commit: a9e6f83c2df (usb: gadget: f_fs: stop sleeping in
ffs_func_eps_disable)
Yes this fix is needed for >=5.4 Kernel releases.
diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 3c584da..6f23a66 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);
You are accessing epfiles outside of the lock. How is that ok?
There are 2 parties that are trying to free the ffs->epfiles. We use the
lock to serialize which party gets to free the ffs->epfiles. Where we
are making it NULL we protected that, The freeing part doesn't need to
be inside spinlock because now it is epfiles(local_ variable) with which
we are trying to operate now. We removed the ffs->epfiles as the common
structure to operate and made the local variable epfiles to do the job.
+
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);
How will it save you from a use-after-free as you never increased a
reference count on the pointer here?
From the above we already serialized the one who gets to free it. So
whoever gets to access will mark it NULL so the other function trying to
access ffs->epfiles backs off. I think reference count is not applicable
here.
And don't write code with new races. Or are you trying to say this
fixes a race?
Yes this is for an existing race which we encountered.
Just saving off a pointer feels wrong here. What happens if the lock is
properly grabbed, then freed, right after you unlock? You aren't really
protecting this properly in a normal way that reference counts and locks
work from what I can tell, only trying to reduce the race window but not
fixing it.
This I Doubt because the reference count might not be decremented at
anycase.
thanks,
greg k-h
