Re: [PATCH v3] seccomp: Add find_notification helper

[Nathan Chancellor <natechancellor@gmail.com>]

On Mon, Jun 01, 2020 at 04:25:32AM -0700, Sargun Dhillon wrote:
> This adds a helper which can iterate through a seccomp_filter to
> find a notification matching an ID. It removes several replicated
> chunks of code.
> 
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> Reviewed-by: Tycho Andersen <tycho@tycho.ws>
> Cc: Matt Denton <mpdenton@google.com>
> Cc: Kees Cook <keescook@google.com>,
> Cc: Jann Horn <jannh@google.com>,
> Cc: Robert Sesek <rsesek@google.com>,
> Cc: Chris Palmer <palmer@google.com>
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Tycho Andersen <tycho@tycho.ws>
> ---
>  kernel/seccomp.c | 55 ++++++++++++++++++++++++------------------------
>  1 file changed, 28 insertions(+), 27 deletions(-)
> 
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 55a6184f5990..cc6b47173a95 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -41,6 +41,7 @@
>  #include <linux/tracehook.h>
>  #include <linux/uaccess.h>
>  #include <linux/anon_inodes.h>
> +#include <linux/lockdep.h>
>  
>  enum notify_state {
>  	SECCOMP_NOTIFY_INIT,
> @@ -1021,10 +1022,27 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +/* must be called with notif_lock held */
> +static inline struct seccomp_knotif *
> +find_notification(struct seccomp_filter *filter, u64 id)
> +{
> +	struct seccomp_knotif *cur;
> +
> +	lockdep_assert_held(&filter->notify_lock);
> +
> +	list_for_each_entry(cur, &filter->notif->notifications, list) {
> +		if (cur->id == id)
> +			return cur;
> +	}
> +
> +	return NULL;
> +}
> +
> +
>  static long seccomp_notify_recv(struct seccomp_filter *filter,
>  				void __user *buf)
>  {
> -	struct seccomp_knotif *knotif = NULL, *cur;
> +	struct seccomp_knotif *knotif, *cur;
>  	struct seccomp_notif unotif;
>  	ssize_t ret;
>  
> @@ -1078,15 +1096,8 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
>  		 * may have died when we released the lock, so we need to make
>  		 * sure it's still around.
>  		 */
> -		knotif = NULL;
>  		mutex_lock(&filter->notify_lock);
> -		list_for_each_entry(cur, &filter->notif->notifications, list) {
> -			if (cur->id == unotif.id) {
> -				knotif = cur;
> -				break;
> -			}
> -		}
> -
> +		knotif = find_notification(filter, unotif.id);
>  		if (knotif) {
>  			knotif->state = SECCOMP_NOTIFY_INIT;
>  			up(&filter->notif->request);
> @@ -1101,7 +1112,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
>  				void __user *buf)
>  {
>  	struct seccomp_notif_resp resp = {};
> -	struct seccomp_knotif *knotif = NULL, *cur;
> +	struct seccomp_knotif *knotif;
>  	long ret;
>  
>  	if (copy_from_user(&resp, buf, sizeof(resp)))
> @@ -1118,13 +1129,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
>  	if (ret < 0)
>  		return ret;
>  
> -	list_for_each_entry(cur, &filter->notif->notifications, list) {
> -		if (cur->id == resp.id) {
> -			knotif = cur;
> -			break;
> -		}
> -	}
> -
> +	knotif = find_notification(filter, resp.id);
>  	if (!knotif) {
>  		ret = -ENOENT;
>  		goto out;
> @@ -1150,7 +1155,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
>  static long seccomp_notify_id_valid(struct seccomp_filter *filter,
>  				    void __user *buf)
>  {
> -	struct seccomp_knotif *knotif = NULL;

I don't know that this should have been removed, clang now warns:

kernel/seccomp.c:1063:2: warning: variable 'knotif' is used uninitialized whenever 'for' loop exits because its condition is false [-Wsometimes-uninitialized]
        list_for_each_entry(cur, &filter->notif->notifications, list) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/list.h:602:7: note: expanded from macro 'list_for_each_entry'
             &pos->member != (head);                                    \
             ^~~~~~~~~~~~~~~~~~~~~~
kernel/seccomp.c:1075:7: note: uninitialized use occurs here
        if (!knotif) {
             ^~~~~~
kernel/seccomp.c:1063:2: note: remove the condition if it is always true
        list_for_each_entry(cur, &filter->notif->notifications, list) {
        ^
include/linux/list.h:602:7: note: expanded from macro 'list_for_each_entry'
             &pos->member != (head);                                    \
             ^
kernel/seccomp.c:1045:31: note: initialize the variable 'knotif' to silence this warning
        struct seccomp_knotif *knotif, *cur;
                                     ^
                                      = NULL
1 warning generated.

> +	struct seccomp_knotif *knotif;
>  	u64 id;
>  	long ret;
>  
> @@ -1161,16 +1166,12 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
>  	if (ret < 0)
>  		return ret;
>  
> -	ret = -ENOENT;
> -	list_for_each_entry(knotif, &filter->notif->notifications, list) {
> -		if (knotif->id == id) {
> -			if (knotif->state == SECCOMP_NOTIFY_SENT)
> -				ret = 0;
> -			goto out;
> -		}
> -	}
> +	knotif = find_notification(filter, id);
> +	if (knotif && knotif->state == SECCOMP_NOTIFY_SENT)
> +		ret = 0;
> +	else
> +		ret = -ENOENT;
>  
> -out:
>  	mutex_unlock(&filter->notify_lock);
>  	return ret;
>  }
> -- 
> 2.25.1
> 

Cheers,
Nathan