linux-kernel - [PATCH 14/82] smb: client: Refactor intentional wrap-around calculation

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Message-Id: <20240123002814.1396804-14-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:26:49 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Steve French <sfrench@...ba.org>,
	Paulo Alcantara <pc@...guebit.com>,
	Ronnie Sahlberg <lsahlber@...hat.com>,
	Shyam Prasad N <sprasad@...rosoft.com>,
	Tom Talpey <tom@...pey.com>,
	linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.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 14/82] smb: client: 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 pointer 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 sanitizer 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: Steve French <sfrench@...ba.org>
Cc: Paulo Alcantara <pc@...guebit.com>
Cc: Ronnie Sahlberg <lsahlber@...hat.com>
Cc: Shyam Prasad N <sprasad@...rosoft.com>
Cc: Tom Talpey <tom@...pey.com>
Cc: linux-cifs@...r.kernel.org
Cc: samba-technical@...ts.samba.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/smb/client/readdir.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/readdir.c b/fs/smb/client/readdir.c
index 94255401b38d..7715297359ab 100644
--- a/fs/smb/client/readdir.c
+++ b/fs/smb/client/readdir.c
@@ -467,12 +467,13 @@ static char *nxt_dir_entry(char *old_entry, char *end_of_smb, int level)
 				pfData->FileNameLength;
 	} else {
 		u32 next_offset = le32_to_cpu(pDirInfo->NextEntryOffset);
+		char *sum;
 
-		if (old_entry + next_offset < old_entry) {
+		if (check_add_overflow(old_entry, next_offset, &sum)) {
 			cifs_dbg(VFS, "Invalid offset %u\n", next_offset);
 			return NULL;
 		}
-		new_entry = old_entry + next_offset;
+		new_entry = sum;
 	}
 	cifs_dbg(FYI, "new entry %p old entry %p\n", new_entry, old_entry);
 	/* validate that new_entry is not past end of SMB */
-- 
2.34.1