[PATCH 4.17 236/336] mmc: pwrseq: Use kmalloc_array instead of stack VLA

From: Greg Kroah-Hartman
Date: Wed Aug 01 2018 - 14:11:33 EST

4.17-stable review patch. If anyone has any objections, please let me know.


From: "Tobin C. Harding" <me@xxxxxxxx>

[ Upstream commit 486e6661367b40f927aadbed73237693396cbf94 ]

The use of stack Variable Length Arrays needs to be avoided, as they
can be a vector for stack exhaustion, which can be both a runtime bug
(kernel Oops) or a security flaw (overwriting memory beyond the
stack). Also, in general, as code evolves it is easy to lose track of
how big a VLA can get. Thus, we can end up having runtime failures
that are hard to debug. As part of the directive[1] to remove all VLAs
from the kernel, and build with -Wvla.

Currently driver is using a VLA declared using the number of descriptors. This
array is used to store integer values and is later used as an argument to
`gpiod_set_array_value_cansleep()` This can be avoided by using
`kmalloc_array()` to allocate memory for the array of integer values. Memory is
free'd before return from function.

>From the code it appears that it is safe to sleep so we can use GFP_KERNEL
(based _cansleep() suffix of function `gpiod_set_array_value_cansleep()`.

It can be expected that this patch will result in a small increase in overhead
due to the use of `kmalloc_array()`

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Tobin C. Harding <me@xxxxxxxx>
Signed-off-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
drivers/mmc/core/pwrseq_simple.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

--- a/drivers/mmc/core/pwrseq_simple.c
+++ b/drivers/mmc/core/pwrseq_simple.c
@@ -40,14 +40,18 @@ static void mmc_pwrseq_simple_set_gpios_
struct gpio_descs *reset_gpios = pwrseq->reset_gpios;

if (!IS_ERR(reset_gpios)) {
- int i;
- int values[reset_gpios->ndescs];
+ int i, *values;
+ int nvalues = reset_gpios->ndescs;

- for (i = 0; i < reset_gpios->ndescs; i++)
+ values = kmalloc_array(nvalues, sizeof(int), GFP_KERNEL);
+ if (!values)
+ return;
+ for (i = 0; i < nvalues; i++)
values[i] = value;

- gpiod_set_array_value_cansleep(
- reset_gpios->ndescs, reset_gpios->desc, values);
+ gpiod_set_array_value_cansleep(nvalues, reset_gpios->desc, values);
+ kfree(values);