staging: comedi: comedi_test: fix race when cancelling command
authorIan Abbott <abbotti@mev.co.uk>
Fri, 4 Jan 2013 11:33:21 +0000 (11:33 +0000)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Thu, 17 Jan 2013 16:50:56 +0000 (08:50 -0800)
commit9a194265172c86fa49299c9fcce7fe9590d66220
tree6410444961988a2a4a1cd94788d0168e674abdbb
parenta72725d609f46f4099fb2f4bbfb128d2ce84074e
staging: comedi: comedi_test: fix race when cancelling command

commit c0729eeefdcd76db338f635162bf0739fd2c5f6f upstream.

Éric Piel reported a kernel oops in the "comedi_test" module.  It was a
NULL pointer dereference within `waveform_ai_interrupt()` (actually a
timer function) that sometimes occurred when a running asynchronous
command is cancelled (either by the `COMEDI_CANCEL` ioctl or by closing
the device file).

This seems to be a race between the caller of `waveform_ai_cancel()`
which on return from that function goes and tears down the running
command, and the timer function which uses the command.  In particular,
`async->cmd.chanlist` gets freed (and the pointer set to NULL) by
`do_become_nonbusy()` in "comedi_fops.c" but a previously scheduled
`waveform_ai_interrupt()` timer function will dereference that pointer
regardless, leading to the oops.

Fix it by replacing the `del_timer()` call in `waveform_ai_cancel()`
with `del_timer_sync()`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reported-by: Éric Piel <piel@delmic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/staging/comedi/drivers/comedi_test.c