[PATCH 47/82] dm verity: Refactor intentional wrap-around test

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Alasdair Kergon <agk@xxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: dm-devel@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/md/dm-switch.c        | 2 +-
 drivers/md/dm-verity-target.c | 2 +-
 drivers/md/dm-writecache.c    | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-switch.c b/drivers/md/dm-switch.c
index dfd9fb52a6f3..9053d7e65603 100644
--- a/drivers/md/dm-switch.c
+++ b/drivers/md/dm-switch.c
@@ -410,7 +410,7 @@ static int process_set_region_mappings(struct switch_ctx *sctx,
 				       cycle_length - 1, region_index);
 				return -EINVAL;
 			}
-			if (unlikely(region_index + num_write < region_index) ||
+			if (unlikely(add_would_overflow(region_index, num_write)) ||
 			    unlikely(region_index + num_write >= sctx->nr_regions)) {
 				DMWARN("invalid set_region_mappings region number: %lu + %lu >= %lu",
 				       region_index, num_write, sctx->nr_regions);
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index 14e58ae70521..f2676c8c83c0 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -1392,7 +1392,7 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		v->hash_level_block[i] = hash_position;
 		s = (v->data_blocks + ((sector_t)1 << ((i + 1) * v->hash_per_block_bits)) - 1)
 					>> ((i + 1) * v->hash_per_block_bits);
-		if (hash_position + s < hash_position) {
+		if (add_would_overflow(hash_position, s)) {
 			ti->error = "Hash device offset overflow";
 			r = -E2BIG;
 			goto bad;
diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index 074cb785eafc..45e54edd24aa 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -2631,7 +2631,7 @@ static int writecache_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	offset = (offset + wc->block_size - 1) & ~(size_t)(wc->block_size - 1);
 	data_size = wc->n_blocks * (size_t)wc->block_size;
 	if (!offset || (data_size / wc->block_size != wc->n_blocks) ||
-	    (offset + data_size < offset))
+	    (add_would_overflow(offset, data_size)))
 		goto overflow;
 	if (offset + data_size > wc->memory_map_size) {
 		ti->error = "Memory area is too small";
-- 
2.34.1