[PATCH 2/2] drm/i915/gem: Fix Virtual Memory mapping boundaries calculation

[Andi Shyti <andi.shyti@linux.intel.com>]

Calculating the size of the mapped area as the lesser value
between the requested size and the actual size does not consider
the partial mapping offset. This can cause page fault access.

Fix the calculation of the starting and ending addresses, the
total size is now deduced from the difference between the end and
start addresses.

Additionally, the calculations have been rewritten in a clearer
and more understandable form.

Fixes: c58305af1835 ("drm/i915: Use remap_io_mapping() to prefault all PTE in a single pass")
Reported-by: Jann Horn <jannh@google.com>
Co-developed-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: <stable@vger.kernel.org> # v4.9+
---
 drivers/gpu/drm/i915/gem/i915_gem_mman.c | 53 +++++++++++++++++++++---
 1 file changed, 47 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index ce10dd259812..cac6d4184506 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
@@ -290,6 +290,41 @@ static vm_fault_t vm_fault_cpu(struct vm_fault *vmf)
 	return i915_error_to_vmf_fault(err);
 }
 
+static void set_address_limits(struct vm_area_struct *area,
+			       struct i915_vma *vma,
+			       unsigned long obj_offset,
+			       unsigned long *start_vaddr,
+			       unsigned long *end_vaddr)
+{
+	unsigned long vm_start, vm_end, vma_size; /* user's memory parameters */
+	long start, end; /* memory boundaries */
+
+	/*
+	 * Let's move into the ">> PAGE_SHIFT"
+	 * domain to be sure not to lose bits
+	 */
+	vm_start = area->vm_start >> PAGE_SHIFT;
+	vm_end = area->vm_end >> PAGE_SHIFT;
+	vma_size = vma->size >> PAGE_SHIFT;
+
+	/*
+	 * Calculate the memory boundaries by considering the offset
+	 * provided by the user during memory mapping and the offset
+	 * provided for the partial mapping.
+	 */
+	start = vm_start;
+	start -= obj_offset;
+	start += vma->gtt_view.partial.offset;
+	end = start + vma_size;
+
+	start = max_t(long, start, vm_start);
+	end = min_t(long, end, vm_end);
+
+	/* Let's move back into the "<< PAGE_SHIFT" domain */
+	*start_vaddr = (unsigned long)start << PAGE_SHIFT;
+	*end_vaddr = (unsigned long)end << PAGE_SHIFT;
+}
+
 static vm_fault_t vm_fault_gtt(struct vm_fault *vmf)
 {
 #define MIN_CHUNK_PAGES (SZ_1M >> PAGE_SHIFT)
@@ -302,14 +337,18 @@ static vm_fault_t vm_fault_gtt(struct vm_fault *vmf)
 	struct i915_ggtt *ggtt = to_gt(i915)->ggtt;
 	bool write = area->vm_flags & VM_WRITE;
 	struct i915_gem_ww_ctx ww;
+	unsigned long obj_offset;
+	unsigned long start, end; /* memory boundaries */
 	intel_wakeref_t wakeref;
 	struct i915_vma *vma;
 	pgoff_t page_offset;
+	unsigned long pfn;
 	int srcu;
 	int ret;
 
-	/* We don't use vmf->pgoff since that has the fake offset */
+	obj_offset = area->vm_pgoff - drm_vma_node_start(&mmo->vma_node);
 	page_offset = (vmf->address - area->vm_start) >> PAGE_SHIFT;
+	page_offset += obj_offset;
 
 	trace_i915_gem_object_fault(obj, page_offset, true, write);
 
@@ -402,12 +441,14 @@ static vm_fault_t vm_fault_gtt(struct vm_fault *vmf)
 	if (ret)
 		goto err_unpin;
 
+	set_address_limits(area, vma, obj_offset, &start, &end);
+
+	pfn = (ggtt->gmadr.start + i915_ggtt_offset(vma)) >> PAGE_SHIFT;
+	pfn += (start - area->vm_start) >> PAGE_SHIFT;
+	pfn += obj_offset - vma->gtt_view.partial.offset;
+
 	/* Finally, remap it using the new GTT offset */
-	ret = remap_io_mapping(area,
-			       area->vm_start + (vma->gtt_view.partial.offset << PAGE_SHIFT),
-			       (ggtt->gmadr.start + i915_ggtt_offset(vma)) >> PAGE_SHIFT,
-			       min_t(u64, vma->size, area->vm_end - area->vm_start),
-			       &ggtt->iomap);
+	ret = remap_io_mapping(area, start, pfn, end - start, &ggtt->iomap);
 	if (ret)
 		goto err_fence;
 
-- 
2.45.2