[PATCH 18/82] ext4: Refactor intentional wrap-around calculation

[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 unsigned wrap-around addition test to use
check_add_overflow(), retaining the result for later usage (which removes
the redundant open-coded addition). 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: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
Cc: linux-ext4@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/ext4/extents.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 01299b55a567..aa30b2c75959 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1920,6 +1920,7 @@ static unsigned int ext4_ext_check_overlap(struct ext4_sb_info *sbi,
 					   struct ext4_extent *newext,
 					   struct ext4_ext_path *path)
 {
+	ext4_lblk_t sum;
 	ext4_lblk_t b1, b2;
 	unsigned int depth, len1;
 	unsigned int ret = 0;
@@ -1943,14 +1944,14 @@ static unsigned int ext4_ext_check_overlap(struct ext4_sb_info *sbi,
 	}
 
 	/* check for wrap through zero on extent logical start block*/
-	if (b1 + len1 < b1) {
+	if (check_add_overflow(b1, len1, &sum)) {
 		len1 = EXT_MAX_BLOCKS - b1;
 		newext->ee_len = cpu_to_le16(len1);
 		ret = 1;
 	}
 
 	/* check for overlap */
-	if (b1 + len1 > b2) {
+	if (sum > b2) {
 		newext->ee_len = cpu_to_le16(b2 - b1);
 		ret = 1;
 	}
-- 
2.34.1