[PATCH] dma-remap: fix dma_common_find_pages() page lookup for offsets

[Andrei-Edward Popa <andrei.popa105@yahoo.com>]

dma_common_find_pages() previously assumed that the CPU virtual address
always pointed to the start of a DMA-coherent allocation. This fails when
memory is allocated via dma_alloc_attrs() without DMA_ATTR_FORCE_CONTIGUOUS
and then subdivided into smaller blocks using a gen_pool, relevant only
when an IOMMU is enabled.

In such cases, userspace may request a mapping via dma_mmap_attrs()
for a CPU address that is offset inside the original allocation. The
previous code could return the wrong struct page pointer.

Example scenario:

  - Allocate a large DMA buffer with dma_alloc_attrs() (non-contiguous)
  - Create a gen_pool using this buffer
  - Take sub-allocations from the gen_pool
  - Map the sub-allocations to userspace via dma_mmap_attrs()
  - dma_common_find_pages() must return the correct struct page for the offset

This patch computes the page index relative to the vm_struct backing
the DMA allocation:

    page_index = (vaddr - area->addr) >> PAGE_SHIFT

Bounds checks ensure the CPU address is within the vm_struct and
page_index < area->nr_pages.

This ensures correct behavior for sub-allocated regions from gen_pool,
without affecting allocations starting at the base address or allocations
with DMA_ATTR_FORCE_CONTIGUOUS.

Signed-off-by: Andrei-Edward Popa <andrei.popa105@yahoo.com>
---
 kernel/dma/remap.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/remap.c b/kernel/dma/remap.c
index b7c1c0c92d0c..d27477e32ed8 100644
--- a/kernel/dma/remap.c
+++ b/kernel/dma/remap.c
@@ -9,12 +9,25 @@
 struct page **dma_common_find_pages(void *cpu_addr)
 {
 	struct vm_struct *area = find_vm_area(cpu_addr);
+	unsigned long vaddr, area_vaddr;
+	size_t page_index;
 
 	if (!area || !(area->flags & VM_DMA_COHERENT))
 		return NULL;
 	WARN(area->flags != VM_DMA_COHERENT,
 	     "unexpected flags in area: %p\n", cpu_addr);
-	return area->pages;
+
+	vaddr = (unsigned long)cpu_addr;
+	area_vaddr = (unsigned long)area->addr;
+	if (unlikely(vaddr < area_vaddr ||
+		     vaddr >= area_vaddr + area->size))
+		return NULL;
+
+	page_index = (vaddr - area_vaddr) >> PAGE_SHIFT;
+	if (unlikely(page_index >= area->nr_pages))
+		return NULL;
+
+	return &area->pages[page_index];
 }
 
 /*
-- 
2.52.0