[PATCH 1/2] HID: uhid: Fix worker destroying device without any protection

[Jann Horn <jannh@xxxxxxxxxx>]

uhid has to run hid_add_device() from workqueue context while allowing
parallel use of the userspace API (which is protected with ->devlock).
But hid_add_device() can fail. Currently, that is handled by immediately
destroying the associated HID device, without using ->devlock - but if
there are concurrent requests from userspace, that's wrong and leads to
NULL dereferences and/or memory corruption (via use-after-free).

Fix it by leaving the HID device as-is in the worker. We can clean it up
later, either in the UHID_DESTROY command handler or in the ->release()
handler.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 67f8ecc550b5 ("HID: uhid: fix timeout when probe races with IO")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---

Notes:
    This crasher triggers an ASAN UAF warning:
    
    int main(void) {
      int dev = open("/dev/uhid", O_RDWR);
      if (dev == -1) err(1, "open");
    
      while (1) {
        struct uhid_event ev_create = {
          .type = UHID_CREATE2,
          /* choose vendor+product IDs that will be rejected by hid_add_device() */
          .u.create2 = { .rd_size = 1, .vendor = 0x07c0, .product = 0x1500 }
        };
        if (write(dev, &ev_create, sizeof(ev_create)) <= 0)
          err(1, "write CREATE");
    
        while (1) {
          struct uhid_event ev_input = {
            .type = UHID_INPUT2,
            .u.input2 = {
              .data = { 0xff },
              .size = 1
            }
          };
          int res = write(dev, &ev_input, sizeof(ev_input));
          if (res == -1 && errno == EINVAL)
            break;
        }
      }
    }
    
    It results in a splat like this:
    
    BUG: KASAN: use-after-free in __lock_acquire+0x3eb9/0x5550
    Read of size 8 at addr ffff88800c2218f8 by task uhid_uaf/588
    
    CPU: 1 PID: 588 Comm: uhid_uaf Not tainted 5.16.0-08301-gfb3b0673b7d5 #886
    [...]
    Call Trace:
     [...]
     lock_acquire+0x1b9/0x4e0
     _raw_spin_lock_irqsave+0x3e/0x60
     down_trylock+0x13/0x70
     hid_input_report+0x3d/0x500
     uhid_char_write+0x210/0xdb0
     vfs_write+0x1c7/0x920
     ksys_write+0x176/0x1d0
     do_syscall_64+0x43/0x90
     entry_SYSCALL_64_after_hwframe+0x44/0xae
    [...]

 drivers/hid/uhid.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 8fe3efcb8327..fc06d8bb42e0 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -28,11 +28,22 @@
 
 struct uhid_device {
 	struct mutex devlock;
+
+	/* This flag tracks whether the HID device is usable for commands from
+	 * userspace. The flag is already set before hid_add_device(), which
+	 * runs in workqueue context, to allow hid_add_device() to communicate
+	 * with userspace.
+	 * However, if hid_add_device() fails, the flag is cleared without
+	 * holding devlock.
+	 * We guarantee that if @running changes from true to false while you're
+	 * holding @devlock, it's still fine to access @hid.
+	 */
 	bool running;
 
 	__u8 *rd_data;
 	uint rd_size;
 
+	/* When this is NULL, userspace may use UHID_CREATE/UHID_CREATE2. */
 	struct hid_device *hid;
 	struct uhid_event input_buf;
 
@@ -63,9 +74,18 @@ static void uhid_device_add_worker(struct work_struct *work)
 	if (ret) {
 		hid_err(uhid->hid, "Cannot register HID device: error %d\n", ret);
 
-		hid_destroy_device(uhid->hid);
-		uhid->hid = NULL;
+		/* We used to call hid_destroy_device() here, but that's really
+		 * messy to get right because we have to coordinate with
+		 * concurrent writes from userspace that might be in the middle
+		 * of using uhid->hid.
+		 * Just leave uhid->hid as-is for now, and clean it up when
+		 * userspace tries to close or reinitialize the uhid instance.
+		 *
+		 * However, we do have to clear the ->running flag and do a
+		 * wakeup to make sure userspace knows that the device is gone.
+		 */
 		uhid->running = false;
+		wake_up_interruptible(&uhid->report_wait);
 	}
 }
 
@@ -474,7 +494,7 @@ static int uhid_dev_create2(struct uhid_device *uhid,
 	void *rd_data;
 	int ret;
 
-	if (uhid->running)
+	if (uhid->hid)
 		return -EALREADY;
 
 	rd_size = ev->u.create2.rd_size;
@@ -556,7 +576,7 @@ static int uhid_dev_create(struct uhid_device *uhid,
 
 static int uhid_dev_destroy(struct uhid_device *uhid)
 {
-	if (!uhid->running)
+	if (!uhid->hid)
 		return -EINVAL;
 
 	uhid->running = false;
@@ -565,6 +585,7 @@ static int uhid_dev_destroy(struct uhid_device *uhid)
 	cancel_work_sync(&uhid->worker);
 
 	hid_destroy_device(uhid->hid);
+	uhid->hid = NULL;
 	kfree(uhid->rd_data);
 
 	return 0;

base-commit: fb3b0673b7d5b477ed104949450cd511337ba3c6
-- 
2.34.1.703.g22d0c6ccf7-goog