linux-kernel - [PATCH] drm/cirrus: Fix NULL pointer dereference when registering the fbdev

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Date:	Tue,  9 Aug 2016 02:16:18 +0200
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
	dri-devel@...ts.freedesktop.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org,
	Boris Brezillon <boris.brezillon@...e-electrons.com>
Subject: [PATCH] drm/cirrus: Fix NULL pointer dereference when registering the fbdev

cirrus_modeset_init() is initializing/registering the emulated fbdev
and, since commit c61b93fe51b1 ("drm/atomic: Fix remaining places where
!funcs->best_encoder is valid"), DRM internals can access/test some of
the fields in mode_config->funcs as part of the fbdev registration
process.
Make sure dev->mode_config.funcs is properly set to avoid dereferencing
a NULL pointer.

Signed-off-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
Fixes: c61b93fe51b1 ("drm/atomic: Fix remaining places where !funcs->best_encoder is valid")
---
Hi Dave,

As discussed on IRC, I'm sending this patch in a proper format. That's
probably better to wait for Eric's feeback before applying it though.

Regards,

Boris
---
 drivers/gpu/drm/cirrus/cirrus_main.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/cirrus/cirrus_main.c b/drivers/gpu/drm/cirrus/cirrus_main.c
index 80446e2d3ab6..76bcb43e7c06 100644
--- a/drivers/gpu/drm/cirrus/cirrus_main.c
+++ b/drivers/gpu/drm/cirrus/cirrus_main.c
@@ -185,14 +185,23 @@ int cirrus_driver_load(struct drm_device *dev, unsigned long flags)
 		goto out;
 	}
 
+	/*
+	 * cirrus_modeset_init() is initializing/registering the emulated fbdev
+	 * and DRM internals can access/test some of the fields in
+	 * mode_config->funcs as part of the fbdev registration process.
+	 * Make sure dev->mode_config.funcs is properly set to avoid
+	 * dereferencing a NULL pointer.
+	 * FIXME: mode_config.funcs assignment should probably be done in
+	 * cirrus_modeset_init() (that's a common pattern seen in other DRM
+	 * drivers).
+	 */
+	dev->mode_config.funcs = &cirrus_mode_funcs;
 	r = cirrus_modeset_init(cdev);
 	if (r) {
 		dev_err(&dev->pdev->dev, "Fatal error during modeset init: %d\n", r);
 		goto out;
 	}
 
-	dev->mode_config.funcs = (void *)&cirrus_mode_funcs;
-
 	return 0;
 out:
 	cirrus_driver_unload(dev);
-- 
2.7.4