[PATCH 06/76] drm/i915: Prevent writing into a read-only object via a GGTT mmap

Chris Wilson chris at chris-wilson.co.uk
Sat Jun 2 09:37:09 UTC 2018

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 at chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Matthew Auld <matthew.william.auld at 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 62974e8862d5..4b488f06dd6b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2011,6 +2011,10 @@ 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;
@@ -2297,10 +2301,17 @@ 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);
 	return ret;

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