Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 17 Jun 2021 19:12:02 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, daniel@ffwll.ch,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        emil.l.velikov@gmail.com, Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [PATCH v2 2/2] drm: Protect drm_master pointers in drm_lease.c
Message-ID: <YMuCYqLafn5sGcFo@phenom.ffwll.local>
Mail-Followup-To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        emil.l.velikov@gmail.com
References: <20210615023645.6535-1-desmondcheongzx@gmail.com>
 <20210615023645.6535-3-desmondcheongzx@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210615023645.6535-3-desmondcheongzx@gmail.com>
Precedence: bulk

On Tue, Jun 15, 2021 at 10:36:45AM +0800, Desmond Cheong Zhi Xi wrote:
> This patch ensures that the device's master mutex is acquired before
> accessing pointers to struct drm_master that are subsequently
> dereferenced. Without the mutex, the struct drm_master may be freed
> concurrently by another process calling drm_setmaster_ioctl(). This
> could then lead to use-after-free errors.
> 
> Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
> ---
>  drivers/gpu/drm/drm_lease.c | 58 +++++++++++++++++++++++++++----------
>  1 file changed, 43 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> index da4f085fc09e..3e6f689236e5 100644
> --- a/drivers/gpu/drm/drm_lease.c
> +++ b/drivers/gpu/drm/drm_lease.c
> @@ -107,10 +107,16 @@ static bool _drm_has_leased(struct drm_master *master, int id)
>   */
>  bool _drm_lease_held(struct drm_file *file_priv, int id)
>  {
> +	bool ret;
> +
>  	if (!file_priv || !file_priv->master)
>  		return true;
>  
> -	return _drm_lease_held_master(file_priv->master, id);
> +	mutex_lock(&file_priv->master->dev->master_mutex);

So maybe we have a bug somewhere, and the kerneldoc isn't 100% clear, but
I thought file_priv->master is invariant over the lifetime of file_priv.
So we don't need a lock to check anything here.

It's the drm_device->master derefence that gets us into trouble. Well also
file_priv->is_owner is protected by dev->master_mutex.

So I think with your previous patch all the access here in drm_lease.c is
ok and already protected? Or am I missing something?

Thanks, Daniel


> +	ret = _drm_lease_held_master(file_priv->master, id);
> +	mutex_unlock(&file_priv->master->dev->master_mutex);
> +
> +	return ret;
>  }
>  
>  /**
> @@ -132,10 +138,12 @@ bool drm_lease_held(struct drm_file *file_priv, int id)
>  	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
>  		return true;
>  
> +	mutex_lock(&file_priv->master->dev->master_mutex);
>  	master = file_priv->master;
>  	mutex_lock(&master->dev->mode_config.idr_mutex);
>  	ret = _drm_lease_held_master(master, id);
>  	mutex_unlock(&master->dev->mode_config.idr_mutex);
> +	mutex_unlock(&file_priv->master->dev->master_mutex);
>  	return ret;
>  }
>  
> @@ -158,6 +166,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
>  	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
>  		return crtcs_in;
>  
> +	mutex_lock(&file_priv->master->dev->master_mutex);
>  	master = file_priv->master;
>  	dev = master->dev;
>  
> @@ -177,6 +186,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
>  		count_in++;
>  	}
>  	mutex_unlock(&master->dev->mode_config.idr_mutex);
> +	mutex_unlock(&file_priv->master->dev->master_mutex);
>  	return crtcs_out;
>  }
>  
> @@ -490,7 +500,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	size_t object_count;
>  	int ret = 0;
>  	struct idr leases;
> -	struct drm_master *lessor = lessor_priv->master;
> +	struct drm_master *lessor;
>  	struct drm_master *lessee = NULL;
>  	struct file *lessee_file = NULL;
>  	struct file *lessor_file = lessor_priv->filp;
> @@ -502,12 +512,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> -	/* Do not allow sub-leases */
> -	if (lessor->lessor) {
> -		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
> -		return -EINVAL;
> -	}
> -
>  	/* need some objects */
>  	if (cl->object_count == 0) {
>  		DRM_DEBUG_LEASE("no objects in lease\n");
> @@ -519,12 +523,23 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  		return -EINVAL;
>  	}
>  
> +	mutex_lock(&dev->master_mutex);
> +	lessor = lessor_priv->master;
> +	/* Do not allow sub-leases */
> +	if (lessor->lessor) {
> +		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
> +		ret = -EINVAL;
> +		goto unlock;
> +	}
> +
>  	object_count = cl->object_count;
>  
>  	object_ids = memdup_user(u64_to_user_ptr(cl->object_ids),
>  			array_size(object_count, sizeof(__u32)));
> -	if (IS_ERR(object_ids))
> -		return PTR_ERR(object_ids);
> +	if (IS_ERR(object_ids)) {
> +		ret = PTR_ERR(object_ids);
> +		goto unlock;
> +	}
>  
>  	idr_init(&leases);
>  
> @@ -535,14 +550,15 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	if (ret) {
>  		DRM_DEBUG_LEASE("lease object lookup failed: %i\n", ret);
>  		idr_destroy(&leases);
> -		return ret;
> +		goto unlock;
>  	}
>  
>  	/* Allocate a file descriptor for the lease */
>  	fd = get_unused_fd_flags(cl->flags & (O_CLOEXEC | O_NONBLOCK));
>  	if (fd < 0) {
>  		idr_destroy(&leases);
> -		return fd;
> +		ret = fd;
> +		goto unlock;
>  	}
>  
>  	DRM_DEBUG_LEASE("Creating lease\n");
> @@ -578,6 +594,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  	/* Hook up the fd */
>  	fd_install(fd, lessee_file);
>  
> +	mutex_unlock(&dev->master_mutex);
>  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
>  	return 0;
>  
> @@ -587,6 +604,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>  out_leases:
>  	put_unused_fd(fd);
>  
> +unlock:
> +	mutex_unlock(&dev->master_mutex);
>  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
>  	return ret;
>  }
> @@ -609,7 +628,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  	struct drm_mode_list_lessees *arg = data;
>  	__u32 __user *lessee_ids = (__u32 __user *) (uintptr_t) (arg->lessees_ptr);
>  	__u32 count_lessees = arg->count_lessees;
> -	struct drm_master *lessor = lessor_priv->master, *lessee;
> +	struct drm_master *lessor, *lessee;
>  	int count;
>  	int ret = 0;
>  
> @@ -620,6 +639,8 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> +	mutex_lock(&dev->master_mutex);
> +	lessor = lessor_priv->master;
>  	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
> @@ -643,6 +664,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>  		arg->count_lessees = count;
>  
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> +	mutex_unlock(&dev->master_mutex);
>  
>  	return ret;
>  }
> @@ -662,7 +684,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  	struct drm_mode_get_lease *arg = data;
>  	__u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
>  	__u32 count_objects = arg->count_objects;
> -	struct drm_master *lessee = lessee_priv->master;
> +	struct drm_master *lessee;
>  	struct idr *object_idr;
>  	int count;
>  	void *entry;
> @@ -676,6 +698,8 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> +	mutex_lock(&dev->master_mutex);
> +	lessee = lessee_priv->master;
>  	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>  
>  	mutex_lock(&dev->mode_config.idr_mutex);
> @@ -703,6 +727,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>  		arg->count_objects = count;
>  
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> +	mutex_unlock(&dev->master_mutex);
>  
>  	return ret;
>  }
> @@ -721,7 +746,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  				void *data, struct drm_file *lessor_priv)
>  {
>  	struct drm_mode_revoke_lease *arg = data;
> -	struct drm_master *lessor = lessor_priv->master;
> +	struct drm_master *lessor;
>  	struct drm_master *lessee;
>  	int ret = 0;
>  
> @@ -731,8 +756,10 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return -EOPNOTSUPP;
>  
> +	mutex_lock(&dev->master_mutex);
>  	mutex_lock(&dev->mode_config.idr_mutex);
>  
> +	lessor = lessor_priv->master;
>  	lessee = _drm_find_lessee(lessor, arg->lessee_id);
>  
>  	/* No such lessee */
> @@ -751,6 +778,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>  
>  fail:
>  	mutex_unlock(&dev->mode_config.idr_mutex);
> +	mutex_unlock(&dev->master_mutex);
>  
>  	return ret;
>  }
> -- 
> 2.25.1
> 

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