linux-kernel - [PATCH 51/82] ext4: Refactor intentional wrap-around test

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Message-Id: <20240123002814.1396804-51-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:27:26 -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 51/82] ext4: Refactor intentional wrap-around test

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@....edu>
Cc: Andreas Dilger <adilger.kernel@...ger.ca>
Cc: linux-ext4@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.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