[PATCH 19/82] fs: Refactor intentional wrap-around calculation

[Kees Cook <keescook@chromium.org>]

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 unsigned 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 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@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/read_write.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index d4c036e82b6c..e24b94a8937d 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1417,6 +1417,7 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
 	struct inode *inode_out = file_inode(file_out);
 	uint64_t count = *req_count;
 	loff_t size_in;
+	loff_t sum_in, sum_out;
 	int ret;
 
 	ret = generic_file_rw_checks(file_in, file_out);
@@ -1451,7 +1452,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
 		return -ETXTBSY;
 
 	/* Ensure offsets don't wrap. */
-	if (pos_in + count < pos_in || pos_out + count < pos_out)
+	if (check_add_overflow(pos_in, count, &sum_in) ||
+	    check_add_overflow(pos_out, count, &sum_out))
 		return -EOVERFLOW;
 
 	/* Shorten the copy to EOF */
@@ -1467,8 +1469,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
 
 	/* Don't allow overlapped copying within the same file. */
 	if (inode_in == inode_out &&
-	    pos_out + count > pos_in &&
-	    pos_out < pos_in + count)
+	    sum_out > pos_in &&
+	    pos_out < sum_in)
 		return -EINVAL;
 
 	*req_count = count;
-- 
2.34.1