[PATCH 82/82] mqueue: 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: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: Alexey Gladkov <legion@xxxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 ipc/mqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 5eea4dc0509e..7ef9d325183a 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -366,7 +366,7 @@ static struct inode *mqueue_get_inode(struct super_block *sb,
 			min_t(unsigned int, info->attr.mq_maxmsg, MQ_PRIO_MAX) *
 			sizeof(struct posix_msg_tree_node);
 		mq_bytes = info->attr.mq_maxmsg * info->attr.mq_msgsize;
-		if (mq_bytes + mq_treesize < mq_bytes)
+		if (add_would_overflow(mq_bytes, mq_treesize))
 			goto out_inode;
 		mq_bytes += mq_treesize;
 		info->ucounts = get_ucounts(current_ucounts());
-- 
2.34.1