dma-buf: heaps: Fix off-by-one in CMA heap fault handler

Until VM_DONTEXPAND was added in commit 1c1914d ("dma-buf: heaps:
Don't track CMA dma-buf pages under RssFile") it was possible to obtain
a mapping larger than the buffer size via mremap and bypass the overflow
check in dma_buf_mmap_internal. When using such a mapping to attempt to
fault past the end of the buffer, the CMA heap fault handler also checks
the fault offset against the buffer size, but gets the boundary wrong by
1. Fix the boundary check so that we don't read off the end of the pages
array and insert an arbitrary page in the mapping.

Reported-by: Xingyu Jin <xingyuj@google.com>
Fixes: a5d2d29 ("dma-buf: heaps: Move heap-helper logic into the cma_heap implementation")
Cc: stable@vger.kernel.org # Applicable >= 5.10. Needs adjustments only for 5.10.
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240830192627.2546033-1-tjmercier@google.com

Author: T.J. Mercier

drivers/dma-buf/heaps/cma_heap.c

@@ -165,7 +165,7 @@ static vm_fault_t cma_heap_vm_fault(struct vm_fault *vmf)
 	struct vm_area_struct *vma = vmf->vma;
 	struct cma_heap_buffer *buffer = vma->vm_private_data;
 
-	if (vmf->pgoff > buffer->pagecount)
+	if (vmf->pgoff >= buffer->pagecount)
 		return VM_FAULT_SIGBUS;
 
 	return vmf_insert_pfn(vma, vmf->address, page_to_pfn(buffer->pages[vmf->pgoff]));