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 <torva...@linux-foundation.org>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: "Liam R. Howlett" <liam.howl...@oracle.com>
Cc: Mark Brown <broo...@kernel.org>
Cc: Mike Kravetz <mike.krav...@oracle.com>
Cc: Vasily Averin <vasily.ave...@linux.dev>
Cc: Alexander Mikhalitsyn <alexan...@mihalicyn.com>
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
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
