linux-hardening - [PATCH 42/82] bcachefs: Refactor intentional wrap-around test

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Date: Mon, 22 Jan 2024 16:27:17 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Brian Foster <bfoster@...hat.com>,
	linux-bcachefs@...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 42/82] bcachefs: 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: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Brian Foster <bfoster@...hat.com>
Cc: linux-bcachefs@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/bcachefs/bkey.c  | 4 ++--
 fs/bcachefs/fs.c    | 2 +-
 fs/bcachefs/quota.c | 2 +-
 fs/bcachefs/util.c  | 2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/bcachefs/bkey.c b/fs/bcachefs/bkey.c
index 76e79a15ba08..c68f1cfd579e 100644
--- a/fs/bcachefs/bkey.c
+++ b/fs/bcachefs/bkey.c
@@ -448,7 +448,7 @@ static bool bkey_format_has_too_big_fields(const struct bkey_format *f)
 			: 0;
 		u64 field_offset = le64_to_cpu(f->field_offset[i]);
 
-		if (packed_max + field_offset < packed_max ||
+		if (add_would_overflow(packed_max, field_offset) ||
 		    packed_max + field_offset > unpacked_max)
 			return true;
 	}
@@ -664,7 +664,7 @@ int bch2_bkey_format_invalid(struct bch_fs *c,
 				: 0;
 			u64 field_offset = le64_to_cpu(f->field_offset[i]);
 
-			if (packed_max + field_offset < packed_max ||
+			if (add_would_overflow(packed_max, field_offset) ||
 			    packed_max + field_offset > unpacked_max) {
 				prt_printf(err, "field %u too large: %llu + %llu > %llu",
 					   i, packed_max, field_offset, unpacked_max);
diff --git a/fs/bcachefs/fs.c b/fs/bcachefs/fs.c
index ec419b8e2c43..00a606171656 100644
--- a/fs/bcachefs/fs.c
+++ b/fs/bcachefs/fs.c
@@ -901,7 +901,7 @@ static int bch2_fiemap(struct inode *vinode, struct fiemap_extent_info *info,
 	if (ret)
 		return ret;
 
-	if (start + len < start)
+	if (add_would_overflow(start, len))
 		return -EINVAL;
 
 	start >>= 9;
diff --git a/fs/bcachefs/quota.c b/fs/bcachefs/quota.c
index e68b34eab90a..1738b1fc1c75 100644
--- a/fs/bcachefs/quota.c
+++ b/fs/bcachefs/quota.c
@@ -392,7 +392,7 @@ static void __bch2_quota_transfer(struct bch_memquota *src_q,
 				  enum quota_counters counter, s64 v)
 {
 	BUG_ON(v > src_q->c[counter].v);
-	BUG_ON(v + dst_q->c[counter].v < v);
+	BUG_ON(add_would_overflow(v, dst_q->c[counter].v));
 
 	src_q->c[counter].v -= v;
 	dst_q->c[counter].v += v;
diff --git a/fs/bcachefs/util.c b/fs/bcachefs/util.c
index a135136adeee..2200c81edbd2 100644
--- a/fs/bcachefs/util.c
+++ b/fs/bcachefs/util.c
@@ -148,7 +148,7 @@ static int __bch2_strtou64_h(const char *cp, u64 *res)
 		return -ERANGE;
 
 	f_n = div_u64(f_n * b, f_d);
-	if (v + f_n < v)
+	if (add_would_overflow(v, f_n))
 		return -ERANGE;
 	v += f_n;
 
-- 
2.34.1