[PATCH 33/82] mm/vmalloc: 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: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 mm/vmalloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index d12a17fc0c17..7932ac99e9d3 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1223,6 +1223,7 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
 	unsigned long align, unsigned long vstart)
 {
 	unsigned long nva_start_addr;
+	unsigned long sum;
 
 	if (va->va_start > vstart)
 		nva_start_addr = ALIGN(va->va_start, align);
@@ -1230,11 +1231,11 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
 		nva_start_addr = ALIGN(vstart, align);
 
 	/* Can be overflowed due to big size or alignment. */
-	if (nva_start_addr + size < nva_start_addr ||
+	if (check_add_overflow(nva_start_addr, size, &sum) ||
 			nva_start_addr < vstart)
 		return false;
 
-	return (nva_start_addr + size <= va->va_end);
+	return (sum <= va->va_end);
 }
 
 /*
-- 
2.34.1