staging: comedi: fix a race between do_cmd_ioctl() and read/write

`do_cmd_ioctl()` is called with the comedi device's mutex locked to
process the `COMEDI_CMD` ioctl to set up comedi's asynchronous command
handling on a comedi subdevice.  `comedi_read()` and `comedi_write()`
are the `read` and `write` handlers for the comedi device, but do not
lock the mutex (for performance reasons, as some things can hold the
mutex for quite a long time).

There is a race condition if `comedi_read()` or `comedi_write()` is
running at the same time and for the same file object and comedi
subdevice as `do_cmd_ioctl()`.  `do_cmd_ioctl()` sets the subdevice's
`busy` pointer to the file object way before it sets the `SRF_RUNNING` flag
in the subdevice's `runflags` member.  `comedi_read() and
`comedi_write()` check the subdevice's `busy` pointer is pointing to the
current file object, then if the `SRF_RUNNING` flag is not set, will call
`do_become_nonbusy()` to shut down the asyncronous command.  Bad things
can happen if the asynchronous command is being shutdown and set up at
the same time.

To prevent the race, don't set the `busy` pointer until
after the `SRF_RUNNING` flag has been set.  Also, make sure the mutex is
held in `comedi_read()` and `comedi_write()` while calling
`do_become_nonbusy()` in order to avoid moving the race condition to a
point within that function.

Change some error handling `goto cleanup` statements in `do_cmd_ioctl()`
to simple `return -ERRFOO` statements as a result of changing when the
`busy` pointer is set.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 6cdef9d..f4a197b 100644 (file)
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1413,22 +1413,19 @@ static int do_cmd_ioctl(struct comedi_device *dev,
                DPRINTK("subdevice busy\n");
                return -EBUSY;
        }
-       s->busy = file;
 
        /* make sure channel/gain list isn't too long */
        if (cmd.chanlist_len > s->len_chanlist) {
                DPRINTK("channel/gain list too long %u > %d\n",
                        cmd.chanlist_len, s->len_chanlist);
-               ret = -EINVAL;
-               goto cleanup;
+               return -EINVAL;
        }
 
        /* make sure channel/gain list isn't too short */
        if (cmd.chanlist_len < 1) {
                DPRINTK("channel/gain list too short %u < 1\n",
                        cmd.chanlist_len);
-               ret = -EINVAL;
-               goto cleanup;
+               return -EINVAL;
        }
 
        async->cmd = cmd;
@@ -1438,8 +1435,7 @@ static int do_cmd_ioctl(struct comedi_device *dev,
            kmalloc(async->cmd.chanlist_len * sizeof(int), GFP_KERNEL);
        if (!async->cmd.chanlist) {
                DPRINTK("allocation failed\n");
-               ret = -ENOMEM;
-               goto cleanup;
+               return -ENOMEM;
        }
 
        if (copy_from_user(async->cmd.chanlist, user_chanlist,
@@ -1491,6 +1487,9 @@ static int do_cmd_ioctl(struct comedi_device *dev,
 
        comedi_set_subdevice_runflags(s, ~0, SRF_USER | SRF_RUNNING);
 
+       /* set s->busy _after_ setting SRF_RUNNING flag to avoid race with
+        * comedi_read() or comedi_write() */
+       s->busy = file;
        ret = s->do_cmd(dev, s);
        if (ret == 0)
                return 0;
@@ -2058,11 +2057,13 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 
                if (!comedi_is_subdevice_running(s)) {
                        if (count == 0) {
+                               mutex_lock(&dev->mutex);
                                if (comedi_is_subdevice_in_error(s))
                                        retval = -EPIPE;
                                else
                                        retval = 0;
                                do_become_nonbusy(dev, s);
+                               mutex_unlock(&dev->mutex);
                        }
                        break;
                }
@@ -2161,11 +2162,13 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 
                if (n == 0) {
                        if (!comedi_is_subdevice_running(s)) {
+                               mutex_lock(&dev->mutex);
                                do_become_nonbusy(dev, s);
                                if (comedi_is_subdevice_in_error(s))
                                        retval = -EPIPE;
                                else
                                        retval = 0;
+                               mutex_unlock(&dev->mutex);
                                break;
                        }
                        if (file->f_flags & O_NONBLOCK) {
@@ -2203,9 +2206,11 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
                buf += n;
                break;          /* makes device work like a pipe */
        }
-       if (comedi_is_subdevice_idle(s) &&
-           async->buf_read_count - async->buf_write_count == 0) {
-               do_become_nonbusy(dev, s);
+       if (comedi_is_subdevice_idle(s)) {
+               mutex_lock(&dev->mutex);
+               if (async->buf_read_count - async->buf_write_count == 0)
+                       do_become_nonbusy(dev, s);
+               mutex_unlock(&dev->mutex);
        }
        set_current_state(TASK_RUNNING);
        remove_wait_queue(&async->wait_head, &wait);