dm-crypt: Use MD5 library instead of crypto_shash

The lmk IV mode, which dm-crypt supports for Loop-AES compatibility,
involves an MD5 computation.  Update its implementation to use the MD5
library API instead of crypto_shash.  This has many benefits, such as:

- Simpler code.  Notably, much of the error-handling code is no longer
  needed, since the library functions can't fail.

- Reduced stack usage.  crypt_iv_lmk_one() now allocates only 112 bytes
  on the stack instead of 520 bytes.

- The library functions are strongly typed, preventing bugs like
  https://lore.kernel.org/r/f1625ddc-e82e-4b77-80c2-dc8e45b54848@gmail.com

- Slightly improved performance, as the library provides direct access
  to the MD5 code without unnecessary overhead such as indirect calls.

To preserve the existing behavior of lmk support being disabled when the
kernel is booted with "fips=1", make crypt_iv_lmk_ctr() check
fips_enabled itself.  Previously it relied on crypto_alloc_shash("md5")
failing.  (I don't know for sure that lmk *actually* needs to be
disallowed in FIPS mode; this just preserves the existing behavior.)

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>

2 files changed, 22 insertions, 55 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 104aa5355090..dcd232a2ca24 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -299,6 +299,7 @@ config DM_CRYPT
 	select CRYPTO
 	select CRYPTO_CBC
 	select CRYPTO_ESSIV
+	select CRYPTO_LIB_MD5 # needed by lmk IV mode
 	help
 	  This device-mapper target allows you to create a device that
 	  transparently encrypts the data on it. You'll need to activate
diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 5ef43231fe77..04a553529dc2 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -21,6 +21,7 @@
 #include <linux/mempool.h>
 #include <linux/slab.h>
 #include <linux/crypto.h>
+#include <linux/fips.h>
 #include <linux/workqueue.h>
 #include <linux/kthread.h>
 #include <linux/backing-dev.h>
@@ -120,7 +121,6 @@ struct iv_benbi_private {
 
 #define LMK_SEED_SIZE 64 /* hash + 0 */
 struct iv_lmk_private {
-	struct crypto_shash *hash_tfm;
 	u8 *seed;
 };
 
@@ -465,10 +465,6 @@ static void crypt_iv_lmk_dtr(struct crypt_config *cc)
 {
 	struct iv_lmk_private *lmk = &cc->iv_gen_private.lmk;
 
-	if (lmk->hash_tfm && !IS_ERR(lmk->hash_tfm))
-		crypto_free_shash(lmk->hash_tfm);
-	lmk->hash_tfm = NULL;
-
 	kfree_sensitive(lmk->seed);
 	lmk->seed = NULL;
 }
@@ -483,11 +479,10 @@ static int crypt_iv_lmk_ctr(struct crypt_config *cc, struct dm_target *ti,
 		return -EINVAL;
 	}
 
-	lmk->hash_tfm = crypto_alloc_shash("md5", 0,
-					   CRYPTO_ALG_ALLOCATES_MEMORY);
-	if (IS_ERR(lmk->hash_tfm)) {
-		ti->error = "Error initializing LMK hash";
-		return PTR_ERR(lmk->hash_tfm);
+	if (fips_enabled) {
+		ti->error = "LMK support is disabled due to FIPS";
+		/* ... because it uses MD5. */
+		return -EINVAL;
 	}
 
 	/* No seed in LMK version 2 */
@@ -498,7 +493,6 @@ static int crypt_iv_lmk_ctr(struct crypt_config *cc, struct dm_target *ti,
 
 	lmk->seed = kzalloc(LMK_SEED_SIZE, GFP_KERNEL);
 	if (!lmk->seed) {
-		crypt_iv_lmk_dtr(cc);
 		ti->error = "Error kmallocing seed storage in LMK";
 		return -ENOMEM;
 	}
@@ -514,7 +508,7 @@ static int crypt_iv_lmk_init(struct crypt_config *cc)
 	/* LMK seed is on the position of LMK_KEYS + 1 key */
 	if (lmk->seed)
 		memcpy(lmk->seed, cc->key + (cc->tfms_count * subkey_size),
-		       crypto_shash_digestsize(lmk->hash_tfm));
+		       MD5_DIGEST_SIZE);
 
 	return 0;
 }
@@ -529,55 +523,31 @@ static int crypt_iv_lmk_wipe(struct crypt_config *cc)
 	return 0;
 }
 
-static int crypt_iv_lmk_one(struct crypt_config *cc, u8 *iv,
-			    struct dm_crypt_request *dmreq,
-			    u8 *data)
+static void crypt_iv_lmk_one(struct crypt_config *cc, u8 *iv,
+			     struct dm_crypt_request *dmreq, u8 *data)
 {
 	struct iv_lmk_private *lmk = &cc->iv_gen_private.lmk;
-	SHASH_DESC_ON_STACK(desc, lmk->hash_tfm);
-	union {
-		struct md5_state md5state;
-		u8 state[CRYPTO_MD5_STATESIZE];
-	} u;
+	struct md5_ctx ctx;
 	__le32 buf[4];
-	int i, r;
 
-	desc->tfm = lmk->hash_tfm;
-
-	r = crypto_shash_init(desc);
-	if (r)
-		return r;
+	md5_init(&ctx);
 
-	if (lmk->seed) {
-		r = crypto_shash_update(desc, lmk->seed, LMK_SEED_SIZE);
-		if (r)
-			return r;
-	}
+	if (lmk->seed)
+		md5_update(&ctx, lmk->seed, LMK_SEED_SIZE);
 
 	/* Sector is always 512B, block size 16, add data of blocks 1-31 */
-	r = crypto_shash_update(desc, data + 16, 16 * 31);
-	if (r)
-		return r;
+	md5_update(&ctx, data + 16, 16 * 31);
 
 	/* Sector is cropped to 56 bits here */
 	buf[0] = cpu_to_le32(dmreq->iv_sector & 0xFFFFFFFF);
 	buf[1] = cpu_to_le32((((u64)dmreq->iv_sector >> 32) & 0x00FFFFFF) | 0x80000000);
 	buf[2] = cpu_to_le32(4024);
 	buf[3] = 0;
-	r = crypto_shash_update(desc, (u8 *)buf, sizeof(buf));
-	if (r)
-		return r;
+	md5_update(&ctx, (u8 *)buf, sizeof(buf));
 
 	/* No MD5 padding here */
-	r = crypto_shash_export(desc, &u.md5state);
-	if (r)
-		return r;
-
-	for (i = 0; i < MD5_HASH_WORDS; i++)
-		__cpu_to_le32s(&u.md5state.hash[i]);
-	memcpy(iv, &u.md5state.hash, cc->iv_size);
-
-	return 0;
+	cpu_to_le32_array(ctx.state.h, ARRAY_SIZE(ctx.state.h));
+	memcpy(iv, ctx.state.h, cc->iv_size);
 }
 
 static int crypt_iv_lmk_gen(struct crypt_config *cc, u8 *iv,
@@ -585,17 +555,15 @@ static int crypt_iv_lmk_gen(struct crypt_config *cc, u8 *iv,
 {
 	struct scatterlist *sg;
 	u8 *src;
-	int r = 0;
 
 	if (bio_data_dir(dmreq->ctx->bio_in) == WRITE) {
 		sg = crypt_get_sg_data(cc, dmreq->sg_in);
 		src = kmap_local_page(sg_page(sg));
-		r = crypt_iv_lmk_one(cc, iv, dmreq, src + sg->offset);
+		crypt_iv_lmk_one(cc, iv, dmreq, src + sg->offset);
 		kunmap_local(src);
 	} else
 		memset(iv, 0, cc->iv_size);
-
-	return r;
+	return 0;
 }
 
 static int crypt_iv_lmk_post(struct crypt_config *cc, u8 *iv,
@@ -603,21 +571,19 @@ static int crypt_iv_lmk_post(struct crypt_config *cc, u8 *iv,
 {
 	struct scatterlist *sg;
 	u8 *dst;
-	int r;
 
 	if (bio_data_dir(dmreq->ctx->bio_in) == WRITE)
 		return 0;
 
 	sg = crypt_get_sg_data(cc, dmreq->sg_out);
 	dst = kmap_local_page(sg_page(sg));
-	r = crypt_iv_lmk_one(cc, iv, dmreq, dst + sg->offset);
+	crypt_iv_lmk_one(cc, iv, dmreq, dst + sg->offset);
 
 	/* Tweak the first block of plaintext sector */
-	if (!r)
-		crypto_xor(dst + sg->offset, iv, cc->iv_size);
+	crypto_xor(dst + sg->offset, iv, cc->iv_size);
 
 	kunmap_local(dst);
-	return r;
+	return 0;
 }
 
 static void crypt_iv_tcw_dtr(struct crypt_config *cc)