[PATCH 1/1] dm-integrity: replace forgeable discard filler with a keyed sector marker
From: Shukai Ni
Date: Mon Jul 27 2026 - 09:49:39 EST
The discard-block check in dm_integrity_rw_tag() treats a stored tag
of all 0xf6 bytes (DISCARD_FILLER) as proof a block was discarded and
skips HMAC verification. allow_discards is only accepted in
dm-integrity's standalone mode. An attacker with raw write access to
the backing device, but without the integrity key, can stamp any block
with an all-0xf6 tag and have it served as authentic.
Replace the constant filler with a keyed checksum of (salt || sector)
and no block payload, computed by integrity_discard_checksum().
Fixes: 84597a44a9d8 ("dm integrity: add optional discard support")
Co-developed-by: Jo Van Bulck <jo.vanbulck@xxxxxxxxxxxxxx>
Signed-off-by: Jo Van Bulck <jo.vanbulck@xxxxxxxxxxxxxx>
Signed-off-by: Shukai Ni <shukai.ni@xxxxxxxxxxx>
---
drivers/md/dm-integrity.c | 77 ++++++++++++++++++++++++++++-----------
1 file changed, 56 insertions(+), 21 deletions(-)
diff --git a/drivers/md/dm-integrity.c b/drivers/md/dm-integrity.c
index 06e805902..7a58086a7 100644
--- a/drivers/md/dm-integrity.c
+++ b/drivers/md/dm-integrity.c
@@ -44,7 +44,6 @@
#define RECALC_WRITE_SUPER 16
#define BITMAP_BLOCK_SIZE 4096 /* don't change it */
#define BITMAP_FLUSH_INTERVAL (10 * HZ)
-#define DISCARD_FILLER 0xf6
#define SALT_SIZE 16
#define RECHECK_POOL_SIZE 256
@@ -1414,7 +1413,6 @@ static int dm_integrity_rw_tag(struct dm_integrity_c *ic, unsigned char *tag, se
{
unsigned int hash_offset = 0;
unsigned char mismatch_hash = 0;
- unsigned char mismatch_filler = !ic->discard;
do {
unsigned char *data, *dp;
@@ -1457,16 +1455,14 @@ static int dm_integrity_rw_tag(struct dm_integrity_c *ic, unsigned char *tag, se
* individual bytes.
*/
mismatch_hash |= dp[i] ^ tag[i];
- mismatch_filler |= dp[i] ^ DISCARD_FILLER;
hash_offset++;
if (unlikely(hash_offset == ic->tag_size)) {
- if (unlikely(mismatch_hash) && unlikely(mismatch_filler)) {
+ if (unlikely(mismatch_hash)) {
dm_bufio_release(b);
return ts;
}
hash_offset = 0;
mismatch_hash = 0;
- mismatch_filler = !ic->discard;
}
}
}
@@ -1645,7 +1641,8 @@ static void integrity_end_io(struct bio *bio)
}
static void integrity_sector_checksum_shash(struct dm_integrity_c *ic, sector_t sector,
- const char *data, unsigned offset, char *result)
+ const char *data, unsigned offset,
+ unsigned int len, char *result)
{
__le64 sector_le = cpu_to_le64(sector);
SHASH_DESC_ON_STACK(req, ic->internal_shash);
@@ -1674,10 +1671,12 @@ static void integrity_sector_checksum_shash(struct dm_integrity_c *ic, sector_t
goto failed;
}
- r = crypto_shash_update(req, data + offset, ic->sectors_per_block << SECTOR_SHIFT);
- if (unlikely(r < 0)) {
- dm_integrity_io_error(ic, "crypto_shash_update", r);
- goto failed;
+ if (likely(len)) {
+ r = crypto_shash_update(req, data + offset, len);
+ if (unlikely(r < 0)) {
+ dm_integrity_io_error(ic, "crypto_shash_update", r);
+ goto failed;
+ }
}
r = crypto_shash_final(req, result);
@@ -1698,7 +1697,8 @@ static void integrity_sector_checksum_shash(struct dm_integrity_c *ic, sector_t
}
static void integrity_sector_checksum_ahash(struct dm_integrity_c *ic, struct ahash_request **ahash_req,
- sector_t sector, struct page *page, unsigned offset, char *result)
+ sector_t sector, struct page *page, unsigned offset,
+ unsigned int len, char *result)
{
__le64 sector_le = cpu_to_le64(sector);
struct ahash_request *req;
@@ -1707,6 +1707,7 @@ static void integrity_sector_checksum_ahash(struct dm_integrity_c *ic, struct ah
int r;
unsigned int digest_size;
unsigned int nbytes = 0;
+ unsigned int nents = 1 + (len ? 1 : 0);
might_sleep();
@@ -1720,12 +1721,12 @@ static void integrity_sector_checksum_ahash(struct dm_integrity_c *ic, struct ah
ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP, crypto_req_done, &wait);
if (ic->sb->flags & cpu_to_le32(SB_FLAG_FIXED_HMAC)) {
- sg_init_table(sg, 3);
+ sg_init_table(sg, nents + 1);
sg_set_buf(s, (const __u8 *)&ic->sb->salt, SALT_SIZE);
nbytes += SALT_SIZE;
s++;
} else {
- sg_init_table(sg, 2);
+ sg_init_table(sg, nents);
}
if (likely(!is_vmalloc_addr(§or_le))) {
@@ -1738,8 +1739,10 @@ static void integrity_sector_checksum_ahash(struct dm_integrity_c *ic, struct ah
nbytes += sizeof(sector_le);
s++;
- sg_set_page(s, page, ic->sectors_per_block << SECTOR_SHIFT, offset);
- nbytes += ic->sectors_per_block << SECTOR_SHIFT;
+ if (likely(len)) {
+ sg_set_page(s, page, len, offset);
+ nbytes += len;
+ }
ahash_request_set_crypt(req, sg, result, nbytes);
@@ -1763,10 +1766,28 @@ static void integrity_sector_checksum_ahash(struct dm_integrity_c *ic, struct ah
static void integrity_sector_checksum(struct dm_integrity_c *ic, struct ahash_request **ahash_req,
sector_t sector, const char *data, unsigned offset, char *result)
{
+ unsigned int len = ic->sectors_per_block << SECTOR_SHIFT;
+
if (likely(ic->internal_shash != NULL))
- integrity_sector_checksum_shash(ic, sector, data, offset, result);
+ integrity_sector_checksum_shash(ic, sector, data, offset, len, result);
else
- integrity_sector_checksum_ahash(ic, ahash_req, sector, (struct page *)data, offset, result);
+ integrity_sector_checksum_ahash(ic, ahash_req, sector, (struct page *)data,
+ offset, len, result);
+}
+
+/*
+ * Authenticated marker for a discarded block: HMAC_key(salt || sector), with
+ * no data payload. Because a real data tag's input always covers a full
+ * block, its length differs from this marker's, so the two can never
+ * collide structurally, regardless of block content.
+ */
+static void integrity_discard_checksum(struct dm_integrity_c *ic, struct ahash_request **ahash_req,
+ sector_t sector, char *result)
+{
+ if (likely(ic->internal_shash != NULL))
+ integrity_sector_checksum_shash(ic, sector, NULL, 0, 0, result);
+ else
+ integrity_sector_checksum_ahash(ic, ahash_req, sector, NULL, 0, 0, result);
}
static void *integrity_kmap(struct dm_integrity_c *ic, struct page *p)
@@ -1820,6 +1841,7 @@ static noinline void integrity_recheck(struct dm_integrity_io *dio, char *checks
char *mem;
char *buffer = page_to_virt(page);
unsigned int buffer_offset;
+ char on_disk_tag[MAX_T(size_t, HASH_MAX_DIGESTSIZE, MAX_TAG_SIZE)];
int r;
struct dm_io_request io_req;
struct dm_io_region io_loc;
@@ -1847,8 +1869,16 @@ static noinline void integrity_recheck(struct dm_integrity_io *dio, char *checks
}
integrity_sector_checksum(ic, &dio->ahash_req, logical_sector, integrity_identity(ic, buffer), buffer_offset, checksum);
- r = dm_integrity_rw_tag(ic, checksum, &dio->metadata_block,
- &dio->metadata_offset, ic->tag_size, TAG_CMP);
+ r = dm_integrity_rw_tag(ic, on_disk_tag, &dio->metadata_block,
+ &dio->metadata_offset, ic->tag_size, TAG_READ);
+ if (likely(!r)) {
+ r = crypto_memneq(on_disk_tag, checksum, ic->tag_size);
+ if (unlikely(r) && ic->discard) {
+ integrity_discard_checksum(ic, &dio->ahash_req,
+ logical_sector, checksum);
+ r = crypto_memneq(on_disk_tag, checksum, ic->tag_size);
+ }
+ }
if (r) {
if (r > 0) {
DMERR_LIMIT("%pg: Checksum failed at sector 0x%llx",
@@ -1914,13 +1944,18 @@ static void integrity_metadata(struct work_struct *w)
unsigned int bi_size = dio->bio_details.bi_iter.bi_size;
unsigned int max_size = likely(checksums != checksums_onstack) ? PAGE_SIZE : HASH_MAX_DIGESTSIZE;
unsigned int max_blocks = max_size / ic->tag_size;
-
- memset(checksums, DISCARD_FILLER, max_size);
+ sector_t sector = dio->range.logical_sector;
while (bi_size) {
+ unsigned int i;
unsigned int this_step_blocks = bi_size >> (SECTOR_SHIFT + ic->sb->log2_sectors_per_block);
this_step_blocks = min(this_step_blocks, max_blocks);
+ for (i = 0; i < this_step_blocks; i++) {
+ integrity_discard_checksum(ic, &dio->ahash_req, sector,
+ checksums + i * ic->tag_size);
+ sector += ic->sectors_per_block;
+ }
r = dm_integrity_rw_tag(ic, checksums, &dio->metadata_block, &dio->metadata_offset,
this_step_blocks * ic->tag_size, TAG_WRITE);
if (unlikely(r)) {
--
2.53.0