Re: [PATCH] device_cgroup: Roll back to original exceptions after copy failure

[Aristeu Rozanski <aris@redhat.com>]

On Tue, Oct 25, 2022 at 07:31:01PM +0800, Wang Weiyang wrote:
> When add the 'a *:* rwm' entry to devcgroup A's whitelist, at first A's
> exceptions will be cleaned and A's behavior is changed to
> DEVCG_DEFAULT_ALLOW. Then parent's exceptions will be copyed to A's
> whitelist. If copy failure occurs, just return leaving A to grant
> permissions to all devices. And A may grant more permissions than
> parent.
> 
> Backup A's whitelist and recover original exceptions after copy
> failure.
> 
> Fixes: 4cef7299b478 ("device_cgroup: add proper checking when changing default behavior")
> Signed-off-by: Wang Weiyang <wangweiyang2@huawei.com>
> ---
>  security/device_cgroup.c | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index a9f8c63a96d1..bef2b9285fb3 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -82,6 +82,17 @@ static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
>  	return -ENOMEM;
>  }
>  
> +static void dev_exceptions_move(struct list_head *dest, struct list_head *orig)
> +{
> +	struct dev_exception_item *ex, *tmp;
> +
> +	lockdep_assert_held(&devcgroup_mutex);
> +
> +	list_for_each_entry_safe(ex, tmp, orig, list) {
> +		list_move_tail(&ex->list, dest);
> +	}
> +}
> +
>  /*
>   * called under devcgroup_mutex
>   */
> @@ -604,11 +615,13 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
>  	int count, rc = 0;
>  	struct dev_exception_item ex;
>  	struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
> +	struct dev_cgroup tmp_devcgrp;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  
>  	memset(&ex, 0, sizeof(ex));
> +	memset(&tmp_devcgrp, 0, sizeof(tmp_devcgrp));
>  	b = buffer;
>  
>  	switch (*b) {
> @@ -620,15 +633,27 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
>  
>  			if (!may_allow_all(parent))
>  				return -EPERM;
> -			dev_exception_clean(devcgroup);
> -			devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
> -			if (!parent)
> +			if (!parent) {
> +				devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
> +				dev_exception_clean(devcgroup);
>  				break;
> +			}
>  
> +			INIT_LIST_HEAD(&tmp_devcgrp.exceptions);
> +			rc = dev_exceptions_copy(&tmp_devcgrp.exceptions,
> +						 &devcgroup->exceptions);
> +			if (rc)
> +				return rc;
> +			dev_exception_clean(devcgroup);
>  			rc = dev_exceptions_copy(&devcgroup->exceptions,
>  						 &parent->exceptions);
> -			if (rc)
> +			if (rc) {
> +				dev_exceptions_move(&devcgroup->exceptions,
> +						    &tmp_devcgrp.exceptions);
>  				return rc;
> +			}
> +			devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
> +			dev_exception_clean(&tmp_devcgrp);
>  			break;
>  		case DEVCG_DENY:
>  			if (css_has_online_children(&devcgroup->css))

Reviewed-by: Aristeu Rozanski <aris@redhat.com>

-- 
Aristeu