linux-kernel - Re: [PATCH] drm: move lease init after validation in drm_lease

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Date:   Thu, 21 Dec 2017 09:50:15 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Keith Packard <keithp@...thp.com>
Cc:     linux-kernel@...r.kernel.org, Dave Airlie <airlied@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm: move lease init after validation in drm_lease_create

On Wed, Dec 20, 2017 at 10:54:24PM -0800, Keith Packard wrote:
> Patch bd36d3bab2e3d08f80766c86487090dbceed4651 fixed a deadlock in the
> failure path of drm_lease_create. This made the partially initialized
> lease object visible for a short window of time.
> 
> To avoid having the lessee state appear transiently, I've rearranged
> the code so that the lessor fields are not filled in until the
> parameters are all validated and the function will succeed.
> 
> Signed-off-by: Keith Packard <keithp@...thp.com>

Good catch, sorry I missed that. And registering the object as the very
last thing is definitely the safe pattern.

Applied, thanks for your patch.
-Daniel

> ---
>  drivers/gpu/drm/drm_lease.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> index 59849f02e2ad..1402c0e71b03 100644
> --- a/drivers/gpu/drm/drm_lease.c
> +++ b/drivers/gpu/drm/drm_lease.c
> @@ -220,17 +220,6 @@ static struct drm_master *drm_lease_create(struct drm_master *lessor, struct idr
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
>  
> -	/* Insert the new lessee into the tree */
> -	id = idr_alloc(&(drm_lease_owner(lessor)->lessee_idr), lessee, 1, 0, GFP_KERNEL);
> -	if (id < 0) {
> -		error = id;
> -		goto out_lessee;
> -	}
> -
> -	lessee->lessee_id = id;
> -	lessee->lessor = drm_master_get(lessor);
> -	list_add_tail(&lessee->lessee_list, &lessor->lessees);
> -
>  	idr_for_each_entry(leases, entry, object) {
>  		error = 0;
>  		if (!idr_find(&dev->mode_config.crtc_idr, object))
> @@ -246,6 +235,17 @@ static struct drm_master *drm_lease_create(struct drm_master *lessor, struct idr
>  		}
>  	}
>  
> +	/* Insert the new lessee into the tree */
> +	id = idr_alloc(&(drm_lease_owner(lessor)->lessee_idr), lessee, 1, 0, GFP_KERNEL);
> +	if (id < 0) {
> +		error = id;
> +		goto out_lessee;
> +	}
> +
> +	lessee->lessee_id = id;
> +	lessee->lessor = drm_master_get(lessor);
> +	list_add_tail(&lessee->lessee_list, &lessor->lessees);
> +
>  	/* Move the leases over */
>  	lessee->leases = *leases;
>  	DRM_DEBUG_LEASE("new lessee %d %p, lessor %d %p\n", lessee->lessee_id, lessee, lessor->lessee_id, lessor);
> -- 
> 2.15.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch