[PATCH v2 1/1] dm-integrity: replace forgeable discard filler with a keyed sector marker
From: Shukai Ni
Date: Tue Jul 28 2026 - 10:59:30 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.
Add a new "allow_discards_keyed" target argument that marks discarded
blocks with a keyed checksum of (salt || sector) instead, 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>
---
.../admin-guide/device-mapper/dm-ima.rst | 7 +-
.../device-mapper/dm-integrity.rst | 13 ++
drivers/md/dm-integrity.c | 122 +++++++++++++++---
3 files changed, 121 insertions(+), 21 deletions(-)
diff --git a/Documentation/admin-guide/device-mapper/dm-ima.rst b/Documentation/admin-guide/device-mapper/dm-ima.rst
index a4aa50a82..2a3b50ffb 100644
--- a/Documentation/admin-guide/device-mapper/dm-ima.rst
+++ b/Documentation/admin-guide/device-mapper/dm-ima.rst
@@ -424,7 +424,8 @@ section above) has the following data format for 'integrity' target.
target_attributes := <target_name> "," <target_version> "," <dev_name> "," <start>
<tag_size> "," <mode> "," [<meta_device> ","] [<block_size> ","] <recalculate> ","
- <allow_discards> "," <fix_padding> "," <fix_hmac> "," <legacy_recalculate> ","
+ <allow_discards> "," <allow_discards_keyed> "," <fix_padding> "," <fix_hmac> ","
+ <legacy_recalculate> ","
<journal_sectors> "," <interleave_sectors> "," <buffer_sectors> ";"
target_name := "target_name=integrity"
@@ -438,6 +439,7 @@ section above) has the following data format for 'integrity' target.
block_size := "block_size=" <N>
recalculate := "recalculate=" <yes_no>
allow_discards := "allow_discards=" <yes_no>
+ allow_discards_keyed := "allow_discards_keyed=" <yes_no>
fix_padding := "fix_padding=" <yes_no>
fix_hmac := "fix_hmac=" <yes_no>
legacy_recalculate := "legacy_recalculate=" <yes_no>
@@ -455,7 +457,8 @@ section above) has the following data format for 'integrity' target.
dm_version=4.45.0;
name=integrity1,uuid=,major=253,minor=1,minor_count=1,num_targets=1;
target_index=0,target_begin=0,target_len=7856,target_name=integrity,target_version=1.10.0,
- dev_name=253:0,start=0,tag_size=32,mode=J,recalculate=n,allow_discards=n,fix_padding=n,
+ dev_name=253:0,start=0,tag_size=32,mode=J,recalculate=n,allow_discards=n,
+ allow_discards_keyed=n,fix_padding=n,
fix_hmac=n,legacy_recalculate=n,journal_sectors=88,interleave_sectors=32768,buffer_sectors=128;
diff --git a/Documentation/admin-guide/device-mapper/dm-integrity.rst b/Documentation/admin-guide/device-mapper/dm-integrity.rst
index c2e18ecc0..9c2130142 100644
--- a/Documentation/admin-guide/device-mapper/dm-integrity.rst
+++ b/Documentation/admin-guide/device-mapper/dm-integrity.rst
@@ -190,6 +190,19 @@ allow_discards
Allow block discard requests (a.k.a. TRIM) for the integrity device.
Discards are only allowed to devices using internal hash.
+ A discarded block is marked with a constant filler tag that anyone
+ with raw write access to the backing device can forge without the
+ key. Use allow_discards_keyed instead on new volumes.
+
+allow_discards_keyed
+ Like allow_discards, but marks a discarded block with a keyed
+ checksum of the sector number, HMAC_key(salt || sector), instead of
+ the constant filler tag, so it can't be forged without the
+ integrity key.
+
+ Not compatible with volumes that already have discarded blocks
+ marked the old way; only use on a freshly formatted volume.
+
fix_padding
Use a smaller padding of the tag area that is more
space-efficient. If this option is not present, large padding is
diff --git a/drivers/md/dm-integrity.c b/drivers/md/dm-integrity.c
index 06e805902..03514eb27 100644
--- a/drivers/md/dm-integrity.c
+++ b/drivers/md/dm-integrity.c
@@ -91,6 +91,7 @@ struct superblock {
#define SB_FLAG_FIXED_PADDING 0x8
#define SB_FLAG_FIXED_HMAC 0x10
#define SB_FLAG_INLINE 0x20
+#define SB_FLAG_DISCARD_KEYED 0x40
#define JOURNAL_ENTRY_ROUNDUP 8
@@ -277,6 +278,7 @@ struct dm_integrity_c {
bool recalculate_flag;
bool reset_recalculate_flag;
bool discard;
+ bool discard_keyed;
bool fix_padding;
bool fix_hmac;
bool legacy_recalculate;
@@ -1414,7 +1416,8 @@ 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;
+ bool discard_keyed = ic->sb->flags & cpu_to_le32(SB_FLAG_DISCARD_KEYED);
+ unsigned char mismatch_filler = !ic->discard || discard_keyed;
do {
unsigned char *data, *dp;
@@ -1466,7 +1469,7 @@ static int dm_integrity_rw_tag(struct dm_integrity_c *ic, unsigned char *tag, se
}
hash_offset = 0;
mismatch_hash = 0;
- mismatch_filler = !ic->discard;
+ mismatch_filler = !ic->discard || discard_keyed;
}
}
}
@@ -1645,7 +1648,8 @@ 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 +1678,12 @@ 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 +1704,8 @@ 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 +1714,7 @@ 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 +1728,12 @@ 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 +1746,10 @@ 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 +1773,40 @@ 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_discard_fill_tags(struct dm_integrity_c *ic, struct ahash_request **ahash_req,
+ unsigned char *checksums, sector_t *sector,
+ unsigned int blocks)
+{
+ unsigned int i;
+
+ for (i = 0; i < blocks; i++) {
+ integrity_discard_checksum(ic, ahash_req, *sector, checksums + i * ic->tag_size);
+ *sector += ic->sectors_per_block;
+ }
}
static void *integrity_kmap(struct dm_integrity_c *ic, struct page *p)
@@ -1795,6 +1835,29 @@ static void *integrity_identity(struct dm_integrity_c *ic, void *data)
return virt_to_page(data);
}
+static int integrity_recheck_verify_tag(struct dm_integrity_io *dio, char *checksum,
+ char *on_disk_tag, sector_t logical_sector)
+{
+ struct dm_integrity_c *ic = dio->ic;
+ int r;
+
+ if (!(ic->sb->flags & cpu_to_le32(SB_FLAG_DISCARD_KEYED)))
+ return 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 (unlikely(r))
+ return r;
+
+ r = crypto_memneq(on_disk_tag, checksum, ic->tag_size);
+ if (unlikely(r)) {
+ integrity_discard_checksum(ic, &dio->ahash_req, logical_sector, checksum);
+ r = crypto_memneq(on_disk_tag, checksum, ic->tag_size);
+ }
+ return r;
+}
+
static noinline void integrity_recheck(struct dm_integrity_io *dio, char *checksum)
{
struct bio *bio = dm_bio_from_per_bio_data(dio, sizeof(struct dm_integrity_io));
@@ -1820,6 +1883,7 @@ 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 +1911,8 @@ 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 = integrity_recheck_verify_tag(dio, checksum, on_disk_tag,
+ logical_sector);
if (r) {
if (r > 0) {
DMERR_LIMIT("%pg: Checksum failed at sector 0x%llx",
@@ -1914,13 +1978,19 @@ 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;
+ sector_t sector = dio->range.logical_sector;
+ bool discard_keyed = ic->sb->flags & cpu_to_le32(SB_FLAG_DISCARD_KEYED);
- memset(checksums, DISCARD_FILLER, max_size);
+ if (!discard_keyed)
+ memset(checksums, DISCARD_FILLER, max_size);
while (bi_size) {
unsigned int this_step_blocks = bi_size >> (SECTOR_SHIFT + ic->sb->log2_sectors_per_block);
this_step_blocks = min(this_step_blocks, max_blocks);
+ if (discard_keyed)
+ integrity_discard_fill_tags(ic, &dio->ahash_req, checksums,
+ §or, this_step_blocks);
r = dm_integrity_rw_tag(ic, checksums, &dio->metadata_block, &dio->metadata_offset,
this_step_blocks * ic->tag_size, TAG_WRITE);
if (unlikely(r)) {
@@ -3795,7 +3865,7 @@ static void dm_integrity_resume(struct dm_target *ti)
ic->wrote_to_journal = false;
- flags = ic->sb->flags & cpu_to_le32(SB_FLAG_RECALCULATING);
+ flags = ic->sb->flags & cpu_to_le32(SB_FLAG_RECALCULATING | SB_FLAG_DISCARD_KEYED);
r = sync_rw_sb(ic, REQ_OP_READ);
if (r)
dm_integrity_io_error(ic, "reading superblock", r);
@@ -3955,6 +4025,7 @@ static void dm_integrity_status(struct dm_target *ti, status_type_t type,
arg_count += !!ic->journal_mac_alg.alg_string;
arg_count += (ic->sb->flags & cpu_to_le32(SB_FLAG_FIXED_PADDING)) != 0;
arg_count += (ic->sb->flags & cpu_to_le32(SB_FLAG_FIXED_HMAC)) != 0;
+ arg_count += (ic->sb->flags & cpu_to_le32(SB_FLAG_DISCARD_KEYED)) != 0;
arg_count += ic->legacy_recalculate;
DMEMIT("%s %llu %u %c %u", ic->dev->name, ic->start,
ic->tag_size, ic->mode, arg_count);
@@ -3968,6 +4039,8 @@ static void dm_integrity_status(struct dm_target *ti, status_type_t type,
DMEMIT(" reset_recalculate");
if (ic->discard)
DMEMIT(" allow_discards");
+ if ((ic->sb->flags & cpu_to_le32(SB_FLAG_DISCARD_KEYED)) != 0)
+ DMEMIT(" allow_discards_keyed");
if (ic->mode != 'I')
DMEMIT(" interleave_sectors:%u", 1U << ic->sb->log2_interleave_sectors);
DMEMIT(" buffer_sectors:%u", 1U << ic->log2_buffer_sectors);
@@ -4017,6 +4090,8 @@ static void dm_integrity_status(struct dm_target *ti, status_type_t type,
DMEMIT(",recalculate=%c", (ic->sb->flags & cpu_to_le32(SB_FLAG_RECALCULATING)) ?
'y' : 'n');
DMEMIT(",allow_discards=%c", ic->discard ? 'y' : 'n');
+ DMEMIT(",allow_discards_keyed=%c",
+ ((ic->sb->flags & cpu_to_le32(SB_FLAG_DISCARD_KEYED)) != 0) ? 'y' : 'n');
DMEMIT(",fix_padding=%c",
((ic->sb->flags & cpu_to_le32(SB_FLAG_FIXED_PADDING)) != 0) ? 'y' : 'n');
DMEMIT(",fix_hmac=%c",
@@ -4178,6 +4253,9 @@ static int initialize_superblock(struct dm_integrity_c *ic,
get_random_bytes(ic->sb->salt, SALT_SIZE);
}
+ if (ic->discard_keyed)
+ ic->sb->flags |= cpu_to_le32(SB_FLAG_DISCARD_KEYED);
+
if (!ic->meta_dev) {
if (ic->fix_padding)
ic->sb->flags |= cpu_to_le32(SB_FLAG_FIXED_PADDING);
@@ -4835,6 +4913,9 @@ static int dm_integrity_ctr(struct dm_target *ti, unsigned int argc, char **argv
ic->reset_recalculate_flag = true;
} else if (!strcmp(opt_string, "allow_discards")) {
ic->discard = true;
+ } else if (!strcmp(opt_string, "allow_discards_keyed")) {
+ ic->discard = true;
+ ic->discard_keyed = true;
} else if (!strcmp(opt_string, "fix_padding")) {
ic->fix_padding = true;
} else if (!strcmp(opt_string, "fix_hmac")) {
@@ -5212,6 +5293,9 @@ static int dm_integrity_ctr(struct dm_target *ti, unsigned int argc, char **argv
ic->sb->recalc_sector = cpu_to_le64(0);
}
+ if (ic->discard_keyed)
+ ic->sb->flags |= cpu_to_le32(SB_FLAG_DISCARD_KEYED);
+
if (ic->internal_hash) {
ic->recalc_wq = alloc_workqueue("dm-integrity-recalc",
WQ_MEM_RECLAIM | WQ_PERCPU, 1);
@@ -5430,7 +5514,7 @@ static void dm_integrity_dtr(struct dm_target *ti)
static struct target_type integrity_target = {
.name = "integrity",
- .version = {1, 14, 0},
+ .version = {1, 15, 0},
.module = THIS_MODULE,
.features = DM_TARGET_SINGLETON | DM_TARGET_INTEGRITY,
.ctr = dm_integrity_ctr,
--
2.53.0