linux-kernel - [PATCH 53/82] fs: Refactor intentional wrap-around test

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Message-Id: <20240123002814.1396804-53-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:27:28 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	linux-fsdevel@...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 53/82] fs: 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: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>
Cc: linux-fsdevel@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/remap_range.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/remap_range.c b/fs/remap_range.c
index f8c1120b8311..15e91bf2c5e3 100644
--- a/fs/remap_range.c
+++ b/fs/remap_range.c
@@ -45,7 +45,7 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in,
 		return -EINVAL;

 	/* Ensure offsets don't wrap. */
-	if (pos_in + count < pos_in || pos_out + count < pos_out)
+	if (add_would_overflow(pos_in, count) || add_would_overflow(pos_out, count))
 		return -EINVAL;

 	size_in = i_size_read(inode_in);
-- 
2.34.1