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Subject[PATCH] crypto: ctr: avoid VLA use
Date
All ciphers implemented in Linux have a block size less than or
equal to 16 bytes and the most demanding hw require 16 bits
alignment for the block buffer.
We avoid 2 VLAs[1] by always allocating 16 bytes with 16 bits
alignment, unless the architecture support efficient unaligned
accesses.
We also check, at runtime, that our assumptions still stand,
possibly dynamically allocating a new buffer, just in case
something changes in the future.

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

Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
---

Notes:
Can we maybe skip the runtime check?

crypto/ctr.c | 50 ++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 42 insertions(+), 8 deletions(-)

diff --git a/crypto/ctr.c b/crypto/ctr.c
index 854d924..f37adf0 100644
--- a/crypto/ctr.c
+++ b/crypto/ctr.c
@@ -35,6 +35,16 @@ struct crypto_rfc3686_req_ctx {
struct skcipher_request subreq CRYPTO_MINALIGN_ATTR;
};

+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+#define DECLARE_CIPHER_BUFFER(name) u8 name[16]
+#else
+#define DECLARE_CIPHER_BUFFER(name) u8 __aligned(16) name[16]
+#endif
+
+#define CHECK_CIPHER_BUFFER(name, size, align) \
+ likely(size <= sizeof(name) && \
+ name == PTR_ALIGN(((u8 *) name), align + 1))
+
static int crypto_ctr_setkey(struct crypto_tfm *parent, const u8 *key,
unsigned int keylen)
{
@@ -52,22 +62,35 @@ static int crypto_ctr_setkey(struct crypto_tfm *parent, const u8 *key,
return err;
}

-static void crypto_ctr_crypt_final(struct blkcipher_walk *walk,
- struct crypto_cipher *tfm)
+static int crypto_ctr_crypt_final(struct blkcipher_walk *walk,
+ struct crypto_cipher *tfm)
{
unsigned int bsize = crypto_cipher_blocksize(tfm);
unsigned long alignmask = crypto_cipher_alignmask(tfm);
u8 *ctrblk = walk->iv;
- u8 tmp[bsize + alignmask];
- u8 *keystream = PTR_ALIGN(tmp + 0, alignmask + 1);
u8 *src = walk->src.virt.addr;
u8 *dst = walk->dst.virt.addr;
unsigned int nbytes = walk->nbytes;
+ DECLARE_CIPHER_BUFFER(tmp);
+ u8 *keystream, *tmp2;
+
+ if (CHECK_CIPHER_BUFFER(tmp, bsize, alignmask))
+ keystream = tmp;
+ else {
+ tmp2 = kmalloc(bsize + alignmask, GFP_ATOMIC);
+ if (!tmp2)
+ return -ENOMEM;
+ keystream = PTR_ALIGN(tmp2 + 0, alignmask + 1);
+ }

crypto_cipher_encrypt_one(tfm, keystream, ctrblk);
crypto_xor_cpy(dst, keystream, src, nbytes);

crypto_inc(ctrblk, bsize);
+
+ if (unlikely(keystream != tmp))
+ kfree(tmp2);
+ return 0;
}

static int crypto_ctr_crypt_segment(struct blkcipher_walk *walk,
@@ -106,8 +129,17 @@ static int crypto_ctr_crypt_inplace(struct blkcipher_walk *walk,
unsigned int nbytes = walk->nbytes;
u8 *ctrblk = walk->iv;
u8 *src = walk->src.virt.addr;
- u8 tmp[bsize + alignmask];
- u8 *keystream = PTR_ALIGN(tmp + 0, alignmask + 1);
+ DECLARE_CIPHER_BUFFER(tmp);
+ u8 *keystream, *tmp2;
+
+ if (CHECK_CIPHER_BUFFER(tmp, bsize, alignmask))
+ keystream = tmp;
+ else {
+ tmp2 = kmalloc(bsize + alignmask, GFP_ATOMIC);
+ if (!tmp2)
+ return -ENOMEM;
+ keystream = PTR_ALIGN(tmp2 + 0, alignmask + 1);
+ }

do {
/* create keystream */
@@ -120,6 +152,8 @@ static int crypto_ctr_crypt_inplace(struct blkcipher_walk *walk,
src += bsize;
} while ((nbytes -= bsize) >= bsize);

+ if (unlikely(keystream != tmp))
+ kfree(tmp2);
return nbytes;
}

@@ -147,8 +181,8 @@ static int crypto_ctr_crypt(struct blkcipher_desc *desc,
}

if (walk.nbytes) {
- crypto_ctr_crypt_final(&walk, child);
- err = blkcipher_walk_done(desc, &walk, 0);
+ err = crypto_ctr_crypt_final(&walk, child);
+ err = blkcipher_walk_done(desc, &walk, err);
}

return err;
--
1.9.1
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