[PATCH AUTOSEL 6.0 09/18] char: xillybus: Prevent use-after-free due to race condition

Sasha Levin
2022-12-24T01:33:52Z

From: Eli Billauer <eli.billauer@gmail.com>

[ Upstream commit 282a4b71816b6076029017a7bab3a9dcee12a920 ]

The driver for XillyUSB devices maintains a kref reference count on each
xillyusb_dev structure, which represents a physical device. This reference
count reaches zero when the device has been disconnected and there are no
open file descriptors that are related to the device. When this occurs,
kref_put() calls cleanup_dev(), which clears up the device's data,
including the structure itself.

However, when xillyusb_open() is called, this reference count becomes
tricky: This function needs to obtain the xillyusb_dev structure that
relates to the inode's major and minor (as there can be several such).
xillybus_find_inode() (which is defined in xillybus_class.c) is called
for this purpose. xillybus_find_inode() holds a mutex that is global in
xillybus_class.c to protect the list of devices, and releases this
mutex before returning. As a result, nothing protects the xillyusb_dev's
reference counter from being decremented to zero before xillyusb_open()
increments it on its own behalf. Hence the structure can be freed
due to a rare race condition.

To solve this, a mutex is added. It is locked by xillyusb_open() before
the call to xillybus_find_inode() and is released only after the kref
counter has been incremented on behalf of the newly opened inode. This
protects the kref reference counters of all xillyusb_dev structs from
being decremented by xillyusb_disconnect() during this time segment, as
the call to kref_put() in this function is done with the same lock held.

There is no need to hold the lock on other calls to kref_put(), because
if xillybus_find_inode() finds a struct, xillyusb_disconnect() has not
made the call to remove it, and hence not made its call to kref_put(),
which takes place afterwards. Hence preventing xillyusb_disconnect's
call to kref_put() is enough to ensure that the reference doesn't reach
zero before it's incremented by xillyusb_open().

It would have been more natural to increment the reference count in
xillybus_find_inode() of course, however this function is also called by
Xillybus' driver for PCIe / OF, which registers a completely different
structure. Therefore, xillybus_find_inode() treats these structures as
void pointers, and accordingly can't make any changes.

Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20221030094209.65916-1-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/char/xillybus/xillyusb.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/char/xillybus/xillyusb.c b/drivers/char/xillybus/xillyusb.c
index 39bcbfd908b4..5a5afa14ca8c 100644
--- a/drivers/char/xillybus/xillyusb.c
+++ b/drivers/char/xillybus/xillyusb.c
@@ -184,6 +184,14 @@ struct xillyusb_dev {
struct mutex process_in_mutex; /* synchronize wakeup_all() */
};

+/*
+ * kref_mutex is used in xillyusb_open() to prevent the xillyusb_dev
+ * struct from being freed during the gap between being found by
+ * xillybus_find_inode() and having its reference count incremented.
+ */
+
+static DEFINE_MUTEX(kref_mutex);
+
/* FPGA to host opcodes */
enum {
OPCODE_DATA = 0,
@@ -1237,9 +1245,16 @@ static int xillyusb_open(struct inode *inode, struct file *filp)
int rc;
int index;

+ mutex_lock(&kref_mutex);
+
rc = xillybus_find_inode(inode, (void **)&xdev, &index);
- if (rc)
+ if (rc) {
+ mutex_unlock(&kref_mutex);
return rc;
+ }
+
+ kref_get(&xdev->kref);
+ mutex_unlock(&kref_mutex);

chan = &xdev->channels[index];
filp->private_data = chan;
@@ -1275,8 +1290,6 @@ static int xillyusb_open(struct inode *inode, struct file *filp)
((filp->f_mode & FMODE_WRITE) && chan->open_for_write))
goto unmutex_fail;

- kref_get(&xdev->kref);
-
if (filp->f_mode & FMODE_READ)
chan->open_for_read = 1;

@@ -1413,6 +1426,7 @@ static int xillyusb_open(struct inode *inode, struct file *filp)
return rc;

unmutex_fail:
+ kref_put(&xdev->kref, cleanup_dev);
mutex_unlock(&chan->lock);
return rc;
}
@@ -2227,7 +2241,9 @@ static void xillyusb_disconnect(struct usb_interface *interface)

xdev->dev = NULL;

+ mutex_lock(&kref_mutex);
kref_put(&xdev->kref, cleanup_dev);
+ mutex_unlock(&kref_mutex);
}

static struct usb_driver xillyusb_driver = {
--
2.35.1