linux-kernel - [PATCH 1/2] drm: Add might_fault to drm_modeset

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Message-ID: <20240710093120.732208-1-daniel.vetter@ffwll.ch>
Date: Wed, 10 Jul 2024 11:31:16 +0200
From: Daniel Vetter <daniel.vetter@...ll.ch>
To: DRI Development <dri-devel@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>
Cc: Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Daniel Vetter <daniel.vetter@...el.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Christian König <christian.koenig@....com>,
	linux-media@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org
Subject: [PATCH 1/2] drm: Add might_fault to drm_modeset_lock priming

We already teach lockdep that dma_resv nests within drm_modeset_lock,
but there's a lot more: All drm kms ioctl rely on being able to
put/get_user while holding modeset locks, so we really need a
might_fault in there too to complete the picture. Add it.

Motivated by a syzbot report that blew up on bcachefs doing an
unconditional console_lock way deep in the locking hierarchy, and
lockdep only noticing the depency loop in a drm ioctl instead of much
earlier. This annotation will make sure such issues have a much harder
time escaping.

References: https://lore.kernel.org/dri-devel/00000000000073db8b061cd43496@google.com/
Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc: Maxime Ripard <mripard@...nel.org>
Cc: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Sumit Semwal <sumit.semwal@...aro.org>
Cc: "Christian König" <christian.koenig@....com>
Cc: linux-media@...r.kernel.org
Cc: linaro-mm-sig@...ts.linaro.org
---
 drivers/gpu/drm/drm_mode_config.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
index 568972258222..37d2e0a4ef4b 100644
--- a/drivers/gpu/drm/drm_mode_config.c
+++ b/drivers/gpu/drm/drm_mode_config.c
@@ -456,6 +456,8 @@ int drmm_mode_config_init(struct drm_device *dev)
 		if (ret == -EDEADLK)
 			ret = drm_modeset_backoff(&modeset_ctx);
 
+		might_fault();
+
 		ww_acquire_init(&resv_ctx, &reservation_ww_class);
 		ret = dma_resv_lock(&resv, &resv_ctx);
 		if (ret == -EDEADLK)
-- 
2.45.2