[PATCH] dm-crypt: Use MD5 library instead of crypto_shash

Eric Biggers
2025-10-11T18:12:21Z

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>
---
drivers/md/Kconfig | 1 +
drivers/md/dm-crypt.c | 76 ++++++++++++-------------------------------
2 files changed, 22 insertions(+), 55 deletions(-)

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 104aa53550905..dcd232a2ca244 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -297,10 +297,11 @@ config DM_CRYPT
depends on (TRUSTED_KEYS || TRUSTED_KEYS=n)
select CRC32
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
the ciphers you're going to use in the cryptoapi configuration.

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 5ef43231fe77f..04a553529dc27 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -19,10 +19,11 @@
#include <linux/blk-integrity.h>
#include <linux/crc32.h>
#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>
#include <linux/atomic.h>
#include <linux/scatterlist.h>
@@ -118,11 +119,10 @@ struct iv_benbi_private {
int shift;
};

#define LMK_SEED_SIZE 64 /* hash + 0 */
struct iv_lmk_private {
- struct crypto_shash *hash_tfm;
u8 *seed;
};

#define TCW_WHITENING_SIZE 16
struct iv_tcw_private {
@@ -463,14 +463,10 @@ static int crypt_iv_null_gen(struct crypt_config *cc, u8 *iv,

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;
}

static int crypt_iv_lmk_ctr(struct crypt_config *cc, struct dm_target *ti,
@@ -481,26 +477,24 @@ static int crypt_iv_lmk_ctr(struct crypt_config *cc, struct dm_target *ti,
if (cc->sector_size != (1 << SECTOR_SHIFT)) {
ti->error = "Unsupported sector size for LMK";
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 */
if (cc->key_parts == cc->tfms_count) {
lmk->seed = NULL;
return 0;
}

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;
}

return 0;
@@ -512,11 +506,11 @@ static int crypt_iv_lmk_init(struct crypt_config *cc)
int subkey_size = cc->key_size / cc->key_parts;

/* 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;
}

static int crypt_iv_lmk_wipe(struct crypt_config *cc)
@@ -527,99 +521,71 @@ static int crypt_iv_lmk_wipe(struct crypt_config *cc)
memset(lmk->seed, 0, LMK_SEED_SIZE);

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,
struct dm_crypt_request *dmreq)
{
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,
struct dm_crypt_request *dmreq)
{
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)
{
struct iv_tcw_private *tcw = &cc->iv_gen_private.tcw;

base-commit: 8bd9238e511d02831022ff0270865c54ccc482d6
--
2.51.0