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
