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Subject[ 34/99] staging: comedi: fix a race between do_cmd_ioctl() and read/write
Date
3.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ian Abbott <abbotti@mev.co.uk>

commit 4b18f08be01a7b3c7b6df497137b6e3cb28adaa3 upstream.

`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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/staging/comedi/comedi_fops.c | 25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)

--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1401,22 +1401,19 @@ static int do_cmd_ioctl(struct comedi_de
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;
@@ -1426,8 +1423,7 @@ static int do_cmd_ioctl(struct comedi_de
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,
@@ -1479,6 +1475,9 @@ static int do_cmd_ioctl(struct comedi_de

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;
@@ -2041,11 +2040,13 @@ static ssize_t comedi_write(struct file

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;
}
@@ -2144,11 +2145,13 @@ static ssize_t comedi_read(struct file *

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) {
@@ -2186,9 +2189,11 @@ static ssize_t comedi_read(struct file *
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);



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