drm/i915: Implement vm_ops->access for gdb access into mmaps

gdb uses ptrace() to peek and poke bytes of the target's address space.
The driver must implement an vm_ops->access() handler or else gdb will
be unable to inspect the pointer and report it as out-of-bounds.
Worse than useless as it causes immediate suspicion of the valid GTT
pointer, distracting the poor programmer trying to find his bug.

v2: Write-protect readonly objects (Matthew).

Testcase: igt/gem_mmap_gtt/ptrace
Testcase: igt/gem_mmap_offset/ptrace
Suggested-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Maciej Patelczyk <maciej.patelczyk@intel.com>
Cc: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200501145120.18830-1-chris@chris-wilson.co.uk

1 files changed, 34 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index b39c24dae64e..70f5f82da288 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
@@ -396,6 +396,38 @@ err:
 	return i915_error_to_vmf_fault(ret);
 }
 
+static int
+vm_access(struct vm_area_struct *area, unsigned long addr,
+	  void *buf, int len, int write)
+{
+	struct i915_mmap_offset *mmo = area->vm_private_data;
+	struct drm_i915_gem_object *obj = mmo->obj;
+	void *vaddr;
+
+	if (i915_gem_object_is_readonly(obj) && write)
+		return -EACCES;
+
+	addr -= area->vm_start;
+	if (addr >= obj->base.size)
+		return -EINVAL;
+
+	/* As this is primarily for debugging, let's focus on simplicity */
+	vaddr = i915_gem_object_pin_map(obj, I915_MAP_FORCE_WC);
+	if (IS_ERR(vaddr))
+		return PTR_ERR(vaddr);
+
+	if (write) {
+		memcpy(vaddr + addr, buf, len);
+		__i915_gem_object_flush_map(obj, addr, len);
+	} else {
+		memcpy(buf, vaddr + addr, len);
+	}
+
+	i915_gem_object_unpin_map(obj);
+
+	return len;
+}
+
 void __i915_gem_object_release_mmap_gtt(struct drm_i915_gem_object *obj)
 {
 	struct i915_vma *vma;
@@ -745,12 +777,14 @@ static void vm_close(struct vm_area_struct *vma)
 
 static const struct vm_operations_struct vm_ops_gtt = {
 	.fault = vm_fault_gtt,
+	.access = vm_access,
 	.open = vm_open,
 	.close = vm_close,
 };
 
 static const struct vm_operations_struct vm_ops_cpu = {
 	.fault = vm_fault_cpu,
+	.access = vm_access,
 	.open = vm_open,
 	.close = vm_close,
 };