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Message-Id: <20240123002814.1396804-18-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:26:53 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
"Theodore Ts'o" <tytso@....edu>,
Andreas Dilger <adilger.kernel@...ger.ca>,
linux-ext4@...r.kernel.org,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
Bill Wendling <morbo@...gle.com>,
Justin Stitt <justinstitt@...gle.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH 18/82] ext4: Refactor intentional wrap-around calculation
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@....edu>
Cc: Andreas Dilger <adilger.kernel@...ger.ca>
Cc: linux-ext4@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.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
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