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Date: Mon, 26 Mar 2018 17:33:14 +1100
From: "Tobin C. Harding" <me@...in.cc>
To: Ulf Hansson <ulf.hansson@...aro.org>
Cc: "Tobin C. Harding" <me@...in.cc>,
linux-mmc@...r.kernel.org,
linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com,
Tycho Andersen <tycho@...ho.ws>
Subject: [PATCH] mmc: pwrseq: Use kmalloc_array instead of stack VLA
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@...in.cc>
---
Patch is untested. I was not able to fully grok the call chain involved
with this change so was unable to quantify the total degradation
of performance.
Line calling to gpiod_set_array_value_cansleep() is 83 characters long,
patch attempts to leave the code as clean as possible. Open to
suggestions for improvement. I tried various forms of while loop and
pointer arithmetic for the setting of `values` but in the end settled
for array indexing, again open to suggestions.
thanks,
Tobin.
drivers/mmc/core/pwrseq_simple.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/drivers/mmc/core/pwrseq_simple.c b/drivers/mmc/core/pwrseq_simple.c
index 13ef162cf066..a8b9fee4d62a 100644
--- 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_value(struct mmc_pwrseq_simple *pwrseq,
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);
}
}
--
2.7.4
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