[PATCH 15/82] dma-buf: 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
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: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: "Christian König" <christian.koenig@xxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/dma-buf/dma-buf.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 8fe5aa67b167..3743c63a9b59 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1458,6 +1458,8 @@ EXPORT_SYMBOL_NS_GPL(dma_buf_end_cpu_access, DMA_BUF);
 int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		 unsigned long pgoff)
 {
+	unsigned long sum;
+
 	if (WARN_ON(!dmabuf || !vma))
 		return -EINVAL;
 
@@ -1466,12 +1468,11 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		return -EINVAL;
 
 	/* check for offset overflow */
-	if (pgoff + vma_pages(vma) < pgoff)
+	if (check_add_overflow(pgoff, vma_pages(vma), &sum))
 		return -EOVERFLOW;
 
 	/* check for overflowing the buffer's size */
-	if (pgoff + vma_pages(vma) >
-	    dmabuf->size >> PAGE_SHIFT)
+	if (sum > dmabuf->size >> PAGE_SHIFT)
 		return -EINVAL;
 
 	/* readjust the vma */
-- 
2.34.1