gpu: ion: Fix bug in ion_system_heap map_user

When the requested mmap length was not an integer number of
chunks or the buffer, or if an offset was provided, a bug
would cause extra or incorrect pages of the buffer to be mapped.
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion_system_heap.c

@@ -292,18 +292,27 @@ int ion_system_heap_map_user(struct ion_heap *heap, struct ion_buffer *buffer,
 {
 	struct sg_table *table = buffer->priv_virt;
 	unsigned long addr = vma->vm_start;
-	unsigned long offset = vma->vm_pgoff;
+	unsigned long offset = vma->vm_pgoff * PAGE_SIZE;
 	struct scatterlist *sg;
 	int i;
 
 	for_each_sg(table->sgl, sg, table->nents, i) {
-		if (offset) {
-			offset--;
+		struct page *page = sg_page(sg);
+		unsigned long remainder = vma->vm_end - addr;
+		unsigned long len = sg_dma_len(sg);
+
+		if (offset >= sg_dma_len(sg)) {
+			offset -= sg_dma_len(sg);
 			continue;
+		} else if (offset) {
+			page += offset / PAGE_SIZE;
+			len = sg_dma_len(sg) - offset;
+			offset = 0;
 		}
-		remap_pfn_range(vma, addr, page_to_pfn(sg_page(sg)),
-				sg_dma_len(sg), vma->vm_page_prot);
-		addr += sg_dma_len(sg);
+		len = min(len, remainder);
+		remap_pfn_range(vma, addr, page_to_pfn(page), len,
+				vma->vm_page_prot);
+		addr += len;
 		if (addr >= vma->vm_end)
 			return 0;
 	}