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

[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@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: linux-ext4@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 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

[PATCH 51/82] ext4: 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: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: linux-ext4@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/ext4/block_validity.c | 2 +-
 fs/ext4/resize.c         | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
index 6fe3c941b565..85f859979d2f 100644
--- a/fs/ext4/block_validity.c
+++ b/fs/ext4/block_validity.c
@@ -302,7 +302,7 @@ int ext4_sb_block_valid(struct super_block *sb, struct inode *inode,
 	int ret = 1;
 
 	if ((start_blk <= le32_to_cpu(sbi->s_es->s_first_data_block)) ||
-	    (start_blk + count < start_blk) ||
+	    (add_would_overflow(start_blk, count)) ||
 	    (start_blk + count > ext4_blocks_count(sbi->s_es)))
 		return 0;
 
diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index 4d4a5a32e310..fb8d3745d031 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -1871,7 +1871,7 @@ int ext4_group_extend(struct super_block *sb, struct ext4_super_block *es,
 
 	add = EXT4_BLOCKS_PER_GROUP(sb) - last;
 
-	if (o_blocks_count + add < o_blocks_count) {
+	if (add_would_overflow(o_blocks_count, add)) {
 		ext4_warning(sb, "blocks_count overflow");
 		return -EINVAL;
 	}
-- 
2.34.1