[PATCH 52/82] f2fs: Refactor intentional wrap-around test

[Kees Cook]

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: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
Cc: Chao Yu <chao@xxxxxxxxxx>
Cc: linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/f2fs/file.c   | 2 +-
 fs/f2fs/verity.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index b58ab1157b7e..6360efb98f64 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2819,7 +2819,7 @@ static int f2fs_move_file_range(struct file *file_in, loff_t pos_in,
 	}
 
 	ret = -EINVAL;
-	if (pos_in + len > src->i_size || pos_in + len < pos_in)
+	if (pos_in + len > src->i_size || add_would_overflow(pos_in, len))
 		goto out_unlock;
 	if (len == 0)
 		olen = len = src->i_size - pos_in;
diff --git a/fs/f2fs/verity.c b/fs/f2fs/verity.c
index 4fc95f353a7a..b641cb8d75e8 100644
--- a/fs/f2fs/verity.c
+++ b/fs/f2fs/verity.c
@@ -237,7 +237,7 @@ static int f2fs_get_verity_descriptor(struct inode *inode, void *buf,
 	pos = le64_to_cpu(dloc.pos);
 
 	/* Get the descriptor */
-	if (pos + size < pos || pos + size > inode->i_sb->s_maxbytes ||
+	if (add_would_overflow(pos, size) || pos + size > inode->i_sb->s_maxbytes ||
 	    pos < f2fs_verity_metadata_pos(inode) || size > INT_MAX) {
 		f2fs_warn(F2FS_I_SB(inode), "invalid verity xattr");
 		f2fs_handle_error(F2FS_I_SB(inode),
-- 
2.34.1