[PATCH] dm log: fix out-of-bounds write due to region_count overflow

[Junrui Luo]

The local variable region_count in create_log_context() is declared as
unsigned int (32-bit), but dm_sector_div_up() returns sector_t (64-bit).
When a device-mapper target has a sufficiently large ti->len with a small
region_size, the division result can exceed UINT_MAX. The truncated
value is then used to calculate bitset_size, causing clean_bits,
sync_bits, and recovering_bits to be allocated far smaller than needed
for the actual number of regions.

Subsequent log operations (log_set_bit, log_clear_bit, log_test_bit) use
region indices derived from the full untruncated region space, causing
out-of-bounds writes to kernel heap memory allocated by vmalloc.

This can be reproduced by creating a mirror target whose region_count
overflows 32 bits:

  dmsetup create bigzero --table '0 8589934594 zero'
  dmsetup create mymirror --table '0 8589934594 mirror \
    core 2 2 nosync 2 /dev/mapper/bigzero 0 \
    /dev/mapper/bigzero 0'

The status output confirms the truncation (sync_count=1 instead of
4294967297, because 0x100000001 was truncated to 1):

  $ dmsetup status mymirror
  0 8589934594 mirror 2 254:1 254:1 1/4294967297 ...

This leads to a kernel crash in core_in_sync:

  BUG: scheduling while atomic: (udev-worker)/9150/0x00000000
  RIP: 0010:core_in_sync+0x14/0x30 [dm_log]
  CR2: 0000000000000008
  Fixing recursive fault but reboot is needed!

Fix by widening the local region_count to sector_t and adding an
explicit overflow check before the value is assigned to lc->region_count.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
---
 drivers/md/dm-log.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-log.c b/drivers/md/dm-log.c
index 1aa6a4a7d232..d316757a328b 100644
--- a/drivers/md/dm-log.c
+++ b/drivers/md/dm-log.c
@@ -373,7 +373,7 @@ static int create_log_context(struct dm_dirty_log *log, struct dm_target *ti,
 
 	struct log_c *lc;
 	uint32_t region_size;
-	unsigned int region_count;
+	sector_t region_count;
 	size_t bitset_size, buf_size;
 	int r;
 	char dummy;
@@ -401,6 +401,10 @@ static int create_log_context(struct dm_dirty_log *log, struct dm_target *ti,
 	}
 
 	region_count = dm_sector_div_up(ti->len, region_size);
+	if (region_count > UINT_MAX) {
+		DMWARN("region count exceeds limit of %u", UINT_MAX);
+		return -EINVAL;
+	}
 
 	lc = kmalloc_obj(*lc);
 	if (!lc) {

---
base-commit: 0031c06807cfa8aa51a759ff8aa09e1aa48149af
change-id: 20260305-fixes-156b89003e60

Best regards,
-- 
Junrui Luo <moonafterrain@xxxxxxxxxxx>