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Subject[PATCH v2] USB: wusbcore: crypto: Remove VLA usage
In preparation to enabling -Wvla, remove VLA and replace it
with dynamic memory allocation instead.

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
or a security flaw. 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.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Notice that in this particular case, an alternative to kzalloc is kcalloc,
in which case the code would look as follows instead:

iv = kcalloc(crypto_skcipher_ivsize(tfm_cbc), sizeof(*iv), GFP_KERNEL);

but if the data type of _iv_ never changes, or the type size is always one
byte, kzalloc is good enough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
Changes in v2:
- Fix a memory leak in previous patch.

drivers/usb/wusbcore/crypto.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/wusbcore/crypto.c b/drivers/usb/wusbcore/crypto.c
index 4c00be2d..aff50eb 100644
--- a/drivers/usb/wusbcore/crypto.c
+++ b/drivers/usb/wusbcore/crypto.c
@@ -202,7 +202,7 @@ static int wusb_ccm_mac(struct crypto_skcipher *tfm_cbc,
struct scatterlist sg[4], sg_dst;
void *dst_buf;
size_t dst_size;
- u8 iv[crypto_skcipher_ivsize(tfm_cbc)];
+ u8 *iv;
size_t zero_padding;

/*
@@ -224,7 +224,9 @@ static int wusb_ccm_mac(struct crypto_skcipher *tfm_cbc,
if (!dst_buf)
goto error_dst_buf;

- memset(iv, 0, sizeof(iv));
+ iv = kzalloc(crypto_skcipher_ivsize(tfm_cbc), GFP_KERNEL);
+ if (!iv)
+ goto error_iv;

/* Setup B0 */
scratch->b0.flags = 0x59; /* Format B0 */
@@ -276,6 +278,8 @@ static int wusb_ccm_mac(struct crypto_skcipher *tfm_cbc,
bytewise_xor(mic, &scratch->ax, iv, 8);
result = 8;
error_cbc_crypt:
+ kfree(iv);
+error_iv:
kfree(dst_buf);
error_dst_buf:
return result;
--
2.7.4
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