[PATCH 34/82] ipc: 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: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Vasily Averin <vasily.averin@xxxxxxxxx>
Cc: Alexander Mikhalitsyn <alexander@xxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 ipc/shm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index a89f001a8bf0..227a1610628a 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1650,11 +1650,13 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 	}
 
 	if (addr && !(shmflg & SHM_REMAP)) {
+		unsigned long sum;
+
 		err = -EINVAL;
-		if (addr + size < addr)
+		if (check_add_overflow(addr, size, &sum))
 			goto invalid;
 
-		if (find_vma_intersection(current->mm, addr, addr + size))
+		if (find_vma_intersection(current->mm, addr, sum))
 			goto invalid;
 	}
 
-- 
2.34.1