linux-hardening - [PATCH 54/82] hpfs: Refactor intentional wrap-around test

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Date: Mon, 22 Jan 2024 16:27:29 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 54/82] hpfs: 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: Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/hpfs/alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/hpfs/alloc.c b/fs/hpfs/alloc.c
index 66617b1557c6..e9c7cc6033b5 100644
--- a/fs/hpfs/alloc.c
+++ b/fs/hpfs/alloc.c
@@ -99,7 +99,7 @@ static int chk_if_allocated(struct super_block *s, secno sec, char *msg)

 int hpfs_chk_sectors(struct super_block *s, secno start, int len, char *msg)
 {
-	if (start + len < start || start < 0x12 ||
+	if (add_would_overflow(start, len) || start < 0x12 ||
 	    start + len > hpfs_sb(s)->sb_fs_size) {
 	    	hpfs_error(s, "sector(s) '%s' badly placed at %08x", msg, start);
 		return 1;
-- 
2.34.1