xhci: Fix use-after-free in xhci debugfs

Trying to read from debugfs after the system has resumed from
hibernate causes a use-after-free and thus a protection fault.

Steps to reproduce:
Hibernate system, resume from hibernate, then run
$ cat /sys/kernel/debug/usb/xhci/*/command-ring/enqueue

[ 3902.765086] general protection fault: 0000 [#1] PREEMPT SMP
...
[ 3902.765136] RIP: 0010:xhci_trb_virt_to_dma.part.50+0x5/0x30
...
[ 3902.765178] Call Trace:
[ 3902.765188]  xhci_ring_enqueue_show+0x1e/0x40
[ 3902.765197]  seq_read+0xdb/0x3a0
[ 3902.765204]  ? __handle_mm_fault+0x5fb/0x1210
[ 3902.765211]  full_proxy_read+0x4a/0x70
[ 3902.765219]  __vfs_read+0x23/0x120
[ 3902.765228]  vfs_read+0x8e/0x130
[ 3902.765235]  SyS_read+0x42/0x90
[ 3902.765242]  do_syscall_64+0x6b/0x290
[ 3902.765251]  entry_SYSCALL64_slow_path+0x25/0x25

The issue is caused by the xhci ring structures being reallocated
when the system is resumed, but pointers to the old structures
being retained in the debugfs files "private" field:

The proposed patch fixes this issue by storing a pointer to the xhci_ring
field in the xhci device structure in debugfs rather than directly
storing a pointer to the xhci_ring.

Fixes: 02b6fdc2a153 ("usb: xhci: Add debugfs interface for xHCI driver")
Signed-off-by: Alexander Kappner <agk@godking.net>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-debugfs.c

@@ -162,7 +162,7 @@ static void xhci_debugfs_extcap_regset(struct xhci_hcd *xhci, int cap_id,
 static int xhci_ring_enqueue_show(struct seq_file *s, void *unused)
 {
 	dma_addr_t		dma;
-	struct xhci_ring	*ring = s->private;
+	struct xhci_ring	*ring = *(struct xhci_ring **)s->private;
 
 	dma = xhci_trb_virt_to_dma(ring->enq_seg, ring->enqueue);
 	seq_printf(s, "%pad\n", &dma);
@@ -173,7 +173,7 @@ static int xhci_ring_enqueue_show(struct seq_file *s, void *unused)
 static int xhci_ring_dequeue_show(struct seq_file *s, void *unused)
 {
 	dma_addr_t		dma;
-	struct xhci_ring	*ring = s->private;
+	struct xhci_ring	*ring = *(struct xhci_ring **)s->private;
 
 	dma = xhci_trb_virt_to_dma(ring->deq_seg, ring->dequeue);
 	seq_printf(s, "%pad\n", &dma);
@@ -183,7 +183,7 @@ static int xhci_ring_dequeue_show(struct seq_file *s, void *unused)
 
 static int xhci_ring_cycle_show(struct seq_file *s, void *unused)
 {
-	struct xhci_ring	*ring = s->private;
+	struct xhci_ring	*ring = *(struct xhci_ring **)s->private;
 
 	seq_printf(s, "%d\n", ring->cycle_state);
 
@@ -346,7 +346,7 @@ static void xhci_debugfs_create_files(struct xhci_hcd *xhci,
 }
 
 static struct dentry *xhci_debugfs_create_ring_dir(struct xhci_hcd *xhci,
-						   struct xhci_ring *ring,
+						   struct xhci_ring **ring,
 						   const char *name,
 						   struct dentry *parent)
 {
@@ -387,7 +387,7 @@ void xhci_debugfs_create_endpoint(struct xhci_hcd *xhci,
 
 	snprintf(epriv->name, sizeof(epriv->name), "ep%02d", ep_index);
 	epriv->root = xhci_debugfs_create_ring_dir(xhci,
-						   dev->eps[ep_index].new_ring,
+						   &dev->eps[ep_index].new_ring,
 						   epriv->name,
 						   spriv->root);
 	spriv->eps[ep_index] = epriv;
@@ -423,7 +423,7 @@ void xhci_debugfs_create_slot(struct xhci_hcd *xhci, int slot_id)
 	priv->dev = dev;
 	dev->debugfs_private = priv;
 
-	xhci_debugfs_create_ring_dir(xhci, dev->eps[0].ring,
+	xhci_debugfs_create_ring_dir(xhci, &dev->eps[0].ring,
 				     "ep00", priv->root);
 
 	xhci_debugfs_create_context_files(xhci, priv->root, slot_id);
@@ -488,11 +488,11 @@ void xhci_debugfs_init(struct xhci_hcd *xhci)
 				   ARRAY_SIZE(xhci_extcap_dbc),
 				   "reg-ext-dbc");
 
-	xhci_debugfs_create_ring_dir(xhci, xhci->cmd_ring,
+	xhci_debugfs_create_ring_dir(xhci, &xhci->cmd_ring,
 				     "command-ring",
 				     xhci->debugfs_root);
 
-	xhci_debugfs_create_ring_dir(xhci, xhci->event_ring,
+	xhci_debugfs_create_ring_dir(xhci, &xhci->event_ring,
 				     "event-ring",
 				     xhci->debugfs_root);