[PATCH v6 07/18] crypto: shash: Remove VLA usage in unaligned hashing

Kees Cook
2018-07-24T16:49:25Z

In the quest to remove all stack VLA usage from the kernel[1], this uses
the newly defined max alignment to perform unaligned hashing to avoid
VLAs, and drops the helper function while adding sanity checks on the
resulting buffer sizes. Additionally, the __aligned_largest macro is
removed since this helper was the only user.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
---
crypto/shash.c | 27 ++++++++++++++++-----------
include/linux/compiler-gcc.h | 1 -
2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/crypto/shash.c b/crypto/shash.c
index 86d76b5c626c..d21f04d70dce 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -73,13 +73,6 @@ int crypto_shash_setkey(struct crypto_shash *tfm, const u8 *key,
}
EXPORT_SYMBOL_GPL(crypto_shash_setkey);

-static inline unsigned int shash_align_buffer_size(unsigned len,
- unsigned long mask)
-{
- typedef u8 __aligned_largest u8_aligned;
- return len + (mask & ~(__alignof__(u8_aligned) - 1));
-}
-
static int shash_update_unaligned(struct shash_desc *desc, const u8 *data,
unsigned int len)
{
@@ -88,11 +81,17 @@ static int shash_update_unaligned(struct shash_desc *desc, const u8 *data,
unsigned long alignmask = crypto_shash_alignmask(tfm);
unsigned int unaligned_len = alignmask + 1 -
((unsigned long)data & alignmask);
- u8 ubuf[shash_align_buffer_size(unaligned_len, alignmask)]
- __aligned_largest;
+ /*
+ * We cannot count on __aligned() working for large values:
+ * https://patchwork.kernel.org/patch/9507697/
+ */
+ u8 ubuf[MAX_ALGAPI_ALIGNMASK * 2];
u8 *buf = PTR_ALIGN(&ubuf[0], alignmask + 1);
int err;

+ if (WARN_ON(buf + unaligned_len > ubuf + sizeof(ubuf)))
+ return -EINVAL;
+
if (unaligned_len > len)
unaligned_len = len;

@@ -124,11 +123,17 @@ static int shash_final_unaligned(struct shash_desc *desc, u8 *out)
unsigned long alignmask = crypto_shash_alignmask(tfm);
struct shash_alg *shash = crypto_shash_alg(tfm);
unsigned int ds = crypto_shash_digestsize(tfm);
- u8 ubuf[shash_align_buffer_size(ds, alignmask)]
- __aligned_largest;
+ /*
+ * We cannot count on __aligned() working for large values:
+ * https://patchwork.kernel.org/patch/9507697/
+ */
+ u8 ubuf[MAX_ALGAPI_ALIGNMASK + HASH_MAX_DIGESTSIZE];
u8 *buf = PTR_ALIGN(&ubuf[0], alignmask + 1);
int err;

+ if (WARN_ON(buf + ds > ubuf + sizeof(ubuf)))
+ return -EINVAL;
+
err = shash->final(desc, buf);
if (err)
goto out;
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index f1a7492a5cc8..1f1cdef36a82 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -125,7 +125,6 @@
*/
#define __pure __attribute__((pure))
#define __aligned(x) __attribute__((aligned(x)))
-#define __aligned_largest __attribute__((aligned))
#define __printf(a, b) __attribute__((format(printf, a, b)))
#define __scanf(a, b) __attribute__((format(scanf, a, b)))
#define __attribute_const__ __attribute__((__const__))
--
2.17.1