[PATCH v2] rsi: Remove stack VLA usage

["Tobin C. Harding" <me@tobin.cc>]

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 rsi code uses a VLA based on a function argument to
`rsi_sdio_load_data_master_write()`.  The function call chain is

Both these functions

	rsi_sdio_reinit_device()
	rsi_probe()

start the call chain:

	rsi_hal_device_init()
	rsi_load_fw()
	auto_fw_upgrade()
	ping_pong_write()
	rsi_sdio_load_data_master_write()

[Without familiarity with the code] it appears that none of the 4 locks

	mutex
	rx_mutex
	tx_mutex
	tx_bus_mutex

are held when `rsi_sdio_load_data_master_write()` is called.  It is therefore
safe to use kmalloc with GFP_KERNEL.

We can avoid using the VLA by using `kmalloc()` and free'ing the memory on all
exit paths.

Change buffer from 'u8 array' to 'u8 *'.  Call `kmalloc()` to allocate memory for
the buffer.  Using goto statement to call `kfree()` on all return paths.

It can be expected that this patch will result in a small increase in overhead
due to the use of `kmalloc()` however this code is only called on initialization
(and re-initialization) so this overhead should not degrade performance.

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

Signed-off-by: Tobin C. Harding <me@tobin.cc>
---

This applies onto tip of wireless-drivers next, commit

	(28bf8312a983 mwifiex: get_channel from firmware 

v2:
 - Use kmalloc instead of #define (suggested by Larry)
 - (Apply on top of wireless-drivers-next tree)
 - (Fix up user name on patchwork.kernel.org)

 drivers/net/wireless/rsi/rsi_91x_sdio.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/net/wireless/rsi/rsi_91x_sdio.c b/drivers/net/wireless/rsi/rsi_91x_sdio.c
index 98c7d1dae18e..13705fca59dd 100644
--- a/drivers/net/wireless/rsi/rsi_91x_sdio.c
+++ b/drivers/net/wireless/rsi/rsi_91x_sdio.c
@@ -576,7 +576,7 @@ static int rsi_sdio_load_data_master_write(struct rsi_hw *adapter,
 {
 	u32 num_blocks, offset, i;
 	u16 msb_address, lsb_address;
-	u8 temp_buf[block_size];
+	u8 *temp_buf;
 	int status;
 
 	num_blocks = instructions_sz / block_size;
@@ -585,11 +585,15 @@ static int rsi_sdio_load_data_master_write(struct rsi_hw *adapter,
 	rsi_dbg(INFO_ZONE, "ins_size: %d, num_blocks: %d\n",
 		instructions_sz, num_blocks);
 
+	temp_buf = kmalloc(block_size, GFP_KERNEL);
+	if (!temp_buf)
+		return -ENOMEM;
+
 	/* Loading DM ms word in the sdio slave */
 	status = rsi_sdio_master_access_msword(adapter, msb_address);
 	if (status < 0) {
 		rsi_dbg(ERR_ZONE, "%s: Unable to set ms word reg\n", __func__);
-		return status;
+		goto out_free;
 	}
 
 	for (offset = 0, i = 0; i < num_blocks; i++, offset += block_size) {
@@ -601,7 +605,7 @@ static int rsi_sdio_load_data_master_write(struct rsi_hw *adapter,
 					 temp_buf, block_size);
 		if (status < 0) {
 			rsi_dbg(ERR_ZONE, "%s: failed to write\n", __func__);
-			return status;
+			goto out_free;
 		}
 		rsi_dbg(INFO_ZONE, "%s: loading block: %d\n", __func__, i);
 		base_address += block_size;
@@ -616,7 +620,7 @@ static int rsi_sdio_load_data_master_write(struct rsi_hw *adapter,
 				rsi_dbg(ERR_ZONE,
 					"%s: Unable to set ms word reg\n",
 					__func__);
-				return status;
+				goto out_free;
 			}
 		}
 	}
@@ -632,12 +636,16 @@ static int rsi_sdio_load_data_master_write(struct rsi_hw *adapter,
 					 temp_buf,
 					 instructions_sz % block_size);
 		if (status < 0)
-			return status;
+			goto out_free;
 		rsi_dbg(INFO_ZONE,
 			"Written Last Block in Address 0x%x Successfully\n",
 			offset | RSI_SD_REQUEST_MASTER);
 	}
-	return 0;
+
+	status = 0;
+out_free:
+	kfree(temp_buf);
+	return status;
 }
 
 #define FLASH_SIZE_ADDR                 0x04000016
-- 
2.7.4