[PATCH 29/82] rds: Refactor intentional wrap-around calculation

[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 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
unsigned wrap-around sanitizer[2] 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: Santosh Shilimkar <santosh.shilimkar@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-rdma@xxxxxxxxxxxxxxx
Cc: rds-devel@xxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 net/rds/info.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/rds/info.c b/net/rds/info.c
index b6b46a8214a0..87b35d07ce04 100644
--- a/net/rds/info.c
+++ b/net/rds/info.c
@@ -163,6 +163,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, char __user *optval,
 	unsigned long nr_pages = 0;
 	unsigned long start;
 	rds_info_func func;
+	unsigned long sum;
 	struct page **pages = NULL;
 	int ret;
 	int len;
@@ -175,7 +176,8 @@ int rds_info_getsockopt(struct socket *sock, int optname, char __user *optval,
 
 	/* check for all kinds of wrapping and the like */
 	start = (unsigned long)optval;
-	if (len < 0 || len > INT_MAX - PAGE_SIZE + 1 || start + len < start) {
+	if (len < 0 || len > INT_MAX - PAGE_SIZE + 1 ||
+	    check_add_overflow(start, len, &sum)) {
 		ret = -EINVAL;
 		goto out;
 	}
@@ -184,7 +186,7 @@ int rds_info_getsockopt(struct socket *sock, int optname, char __user *optval,
 	if (len == 0)
 		goto call_func;
 
-	nr_pages = (PAGE_ALIGN(start + len) - (start & PAGE_MASK))
+	nr_pages = (PAGE_ALIGN(sum) - (start & PAGE_MASK))
 			>> PAGE_SHIFT;
 
 	pages = kmalloc_array(nr_pages, sizeof(struct page *), GFP_KERNEL);
-- 
2.34.1