drm/i915: Fix potential context UAFs

gem_context_register() makes the context visible to userspace, and which
point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
So we need to ensure that nothing uses the ctx ptr after this.  And we
need to ensure that adding the ctx to the xarray is the *last* thing
that gem_context_register() does with the ctx pointer.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Fixes: eb4dedae ("drm/i915/gem: Delay tracking the GEM context until it is registered")
Fixes: a4c1cdd3 ("drm/i915/gem: Delay context creation (v3)")
Fixes: 49bd54b3 ("drm/i915: Track all user contexts per client")
Cc: <stable@vger.kernel.org> # v5.10+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
[tursulin: Stable and fixes tags add/tidy.]
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230103234948.1218393-1-robdclark@gmail.com

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
 	init_contexts(&i915->gem.contexts);
 }
 
+/*
+ * Note that this implicitly consumes the ctx reference, by placing
+ * the ctx in the context_xa.
+ */
 static void gem_context_register(struct i915_gem_context *ctx,
 				 struct drm_i915_file_private *fpriv,
 				 u32 id)
@@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx,
 	snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
 		 current->comm, pid_nr(ctx->pid));
 
-	/* And finally expose ourselves to userspace via the idr */
-	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
-	WARN_ON(old);
-
 	spin_lock(&ctx->client->ctx_lock);
 	list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
 	spin_unlock(&ctx->client->ctx_lock);
@@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx,
 	spin_lock(&i915->gem.contexts.lock);
 	list_add_tail(&ctx->link, &i915->gem.contexts.list);
 	spin_unlock(&i915->gem.contexts.lock);
+
+	/* And finally expose ourselves to userspace via the idr */
+	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
+	WARN_ON(old);
 }
 
 int i915_gem_context_open(struct drm_i915_private *i915,
@@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv,
 	if (IS_ERR(ctx))
 		return ctx;
 
+	/*
+	 * One for the xarray and one for the caller.  We need to grab
+	 * the reference *prior* to making the ctx visble to userspace
+	 * in gem_context_register(), as at any point after that
+	 * userspace can try to race us with another thread destroying
+	 * the context under our feet.
+	 */
+	i915_gem_context_get(ctx);
+
 	gem_context_register(ctx, file_priv, id);
 
 	old = xa_erase(&file_priv->proto_context_xa, id);
 	GEM_BUG_ON(old != pc);
 	proto_context_close(file_priv->dev_priv, pc);
 
-	/* One for the xarray and one for the caller */
-	return i915_gem_context_get(ctx);
+	return ctx;
 }
 
 struct i915_gem_context *