iio: Fix crash when scan_bytes is computed with active_scan_mask == NULL

if device has available_scan_masks set and the buffer is enabled without
any scan_elements enabled, in a NULL pointer is dereferenced in iio_compute_scan_bytes()

[   18.993713] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[   19.002593] pgd = debd4000
[   19.005432] [00000000] *pgd=9ebc0831, *pte=00000000, *ppte=00000000
[   19.012329] Internal error: Oops: 17 [#1] PREEMPT ARM
[   19.017639] Modules linked in:
[   19.020843] CPU: 0    Not tainted  (3.9.11-00036-g75c888a-dirty #207)
[   19.027587] PC is at _find_first_bit_le+0xc/0x2c
[   19.032440] LR is at iio_compute_scan_bytes+0x2c/0xf4
[   19.037719] pc : [<c021dc60>]    lr : [<c03198d0>]    psr: 200d0013
[   19.037719] sp : debd9ed0  ip : 00000000  fp : 000802bc
[   19.049713] r10: 00000000  r9 : 00000000  r8 : deb67250
[   19.055206] r7 : 00000000  r6 : 00000000  r5 : 00000000  r4 : deb67000
[   19.062011] r3 : de96ec00  r2 : 00000000  r1 : 00000004  r0 : 00000000
[   19.068847] Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[   19.076324] Control: 10c5387d  Table: 9ebd4019  DAC: 00000015

problem is the rollback code in iio_update_buffers(), old_mask may be NULL (e.g. on first
call)

I'm not too confident about the fix; works for me...

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index e73033f3839a..5862c88ed5ad 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -528,8 +528,15 @@ int iio_update_buffers(struct iio_dev *indio_dev,
                          * Note can only occur when adding a buffer.
                          */
                         list_del(&insert_buffer->buffer_list);
-                        indio_dev->active_scan_mask = old_mask;
-                        success = -EINVAL;
+                        if (old_mask) {
+                                indio_dev->active_scan_mask = old_mask;
+                                success = -EINVAL;
+                        }
+                        else {
+                                kfree(compound_mask);
+                                ret = -EINVAL;
+                                goto error_ret;
+                        }
                 }
         } else {
                 indio_dev->active_scan_mask = compound_mask;