[PATCH 3/5] drm/i915: Prevent writing into a read-only object via a GGTT mmap

[Chris Wilson <chris@chris-wilson.co.uk>]

If the user has created a read-only object, they should not be allowed
to circumvent the write protection by using a GGTT mmapping. Deny it.

Also most machines do not support read-only GGTT PTEs, so again we have
to reject attempted writes. Fortunately, this is known a priori, so we
can at least reject in the call to create the mmap with backup in the
fault handler. This is a little draconian as we could blatantly ignore
the write protection on the pages, but it is far simply to keep the
readonly object pure. (It is easier to lift a restriction than to impose
it later!)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 530d6d0109b4..e55278fadf9c 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2005,6 +2005,10 @@ int i915_gem_fault(struct vm_fault *vmf)
 	unsigned int flags;
 	int ret;
 
+	/* Sanity check that we allow writing into this object */
+	if (obj->gt_ro && (write || !ggtt->base.has_read_only))
+		return VM_FAULT_SIGBUS;
+
 	/* We don't use vmf->pgoff since that has the fake offset */
 	page_offset = (vmf->address - area->vm_start) >> PAGE_SHIFT;
 
@@ -2291,10 +2295,17 @@ i915_gem_mmap_gtt(struct drm_file *file,
 	if (!obj)
 		return -ENOENT;
 
+	/* If we will not be able to create the GGTT vma, reject it early. */
+	if (obj->gt_ro && !to_i915(dev)->ggtt.base.has_read_only) {
+		ret = -ENODEV;
+		goto out;
+	}
+
 	ret = i915_gem_object_create_mmap_offset(obj);
 	if (ret == 0)
 		*offset = drm_vma_node_offset_addr(&obj->base.vma_node);
 
+out:
 	i915_gem_object_put(obj);
 	return ret;
 }
-- 
2.17.1

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