[PATCH] staging: comedi: check s->async for poll(), read() and write()

[Ian Abbott]

Some low-level comedi drivers (incorrectly) point `dev->read_subdev` or
`dev->write_subdev` to a subdevice that does not support asynchronous
commands.  Comedi's poll(), read() and write() file operation handlers
assume these subdevices do support asynchronous commands.  In
particular, they assume `s->async` is valid (where `s` points to the
read or write subdevice), which it won't be if it has been set
incorrectly.  This can lead to a NULL pointer dereference.

Check `s->async` is non-NULL in `comedi_poll()`, `comedi_read()` and
`comedi_write()` to avoid the bug.

Signed-off-by: Ian Abbott <abbotti at mev.co.uk>
---
Note: this patch is for staging-next.  Needs backporting for earlier
kernels.
---
 drivers/staging/comedi/comedi_fops.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index c7377d0..4a28d75 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1844,7 +1844,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	}
 
 	s = comedi_read_subdevice(info);
-	if (s) {
+	if (s && s->async) {
 		poll_wait(file, &s->async->wait_head, wait);
 		if (!s->busy || !comedi_is_subdevice_running(s) ||
 		    comedi_buf_read_n_available(s->async) > 0)
@@ -1852,7 +1852,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	}
 
 	s = comedi_write_subdevice(info);
-	if (s) {
+	if (s && s->async) {
 		unsigned int bps = bytes_per_sample(s->async->subdevice);
 
 		poll_wait(file, &s->async->wait_head, wait);
@@ -1887,7 +1887,7 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 	}
 
 	s = comedi_write_subdevice(info);
-	if (!s)
+	if (!s || !s->async)
 		return -EIO;
 
 	async = s->async;
@@ -1982,7 +1982,7 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 	}
 
 	s = comedi_read_subdevice(info);
-	if (!s)
+	if (!s || !s->async)
 		return -EIO;
 
 	async = s->async;
-- 
1.8.1.2