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@xxxxxxxxxx mailing list a year or so ago if you are curious. It's
not a big deal.
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>
---
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
