Re: [PATCH 1/3] ipc/shm: introduce shmctl(SHM_STAT_ANY)

[Michal Hocko]

On Thu 15-02-18 08:24:56, Davidlohr Bueso wrote:
> There is a permission discrepancy when consulting shm ipc
> object metadata between /proc/sysvipc/shm (0444) and the
> SHM_STAT shmctl command. The later does permission checks
> for the object vs S_IRUGO. As such there can be cases where
> EACCESS is returned via syscall but the info is displayed
> anyways in the procfs files.
> 
> While this might have security implications via info leaking
> (albeit no writing to the shm metadata), this behavior goes
> way back and showing all the objects regardless of the
> permissions was most likely an overlook - so we are stuck
> with it. Furthermore, modifying either the syscall or the
> procfs file can cause userspace programs to break (ie ipcs).
> Some applications require getting the procfs info (without
> root privileges) and can be rather slow in comparison with
> a syscall -- up to 500x in some reported cases.
> 
> This patch introduces a new SHM_STAT_ANY command such that
> the shm ipc object permissions are ignored, and only audited
> instead. In addition, I've left the lsm security hook checks
> in place, as if some policy can block the call, then the user
> has no other choice than just parsing the procfs file.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>

Thanks for taking this over Davidlohr! The patch looks reasonable to me.
I am not very much familiar with the security modules (audit) part to
be honest but it matches my expectations.

Feel free to add my:
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  include/uapi/linux/shm.h   |  5 +++--
>  ipc/shm.c                  | 23 ++++++++++++++++++-----
>  security/selinux/hooks.c   |  1 +
>  security/smack/smack_lsm.c |  1 +
>  4 files changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
> index 4de12a39b075..dde1344f047c 100644
> --- a/include/uapi/linux/shm.h
> +++ b/include/uapi/linux/shm.h
> @@ -83,8 +83,9 @@ struct shmid_ds {
>  #define SHM_UNLOCK 	12
>  
>  /* ipcs ctl commands */
> -#define SHM_STAT 	13
> -#define SHM_INFO 	14
> +#define SHM_STAT	13
> +#define SHM_INFO	14
> +#define SHM_STAT_ANY    15
>  
>  /* Obsolete, used only for backwards compatibility */
>  struct	shminfo {
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 4643865e9171..60827d9c3716 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -915,14 +915,14 @@ static int shmctl_stat(struct ipc_namespace *ns, int shmid,
>  	memset(tbuf, 0, sizeof(*tbuf));
>  
>  	rcu_read_lock();
> -	if (cmd == SHM_STAT) {
> +	if (cmd == SHM_STAT || cmd == SHM_STAT_ANY) {
>  		shp = shm_obtain_object(ns, shmid);
>  		if (IS_ERR(shp)) {
>  			err = PTR_ERR(shp);
>  			goto out_unlock;
>  		}
>  		id = shp->shm_perm.id;
> -	} else {
> +	} else { /* IPC_STAT */
>  		shp = shm_obtain_object_check(ns, shmid);
>  		if (IS_ERR(shp)) {
>  			err = PTR_ERR(shp);
> @@ -930,9 +930,20 @@ static int shmctl_stat(struct ipc_namespace *ns, int shmid,
>  		}
>  	}
>  
> -	err = -EACCES;
> -	if (ipcperms(ns, &shp->shm_perm, S_IRUGO))
> -		goto out_unlock;
> +	/*
> +	 * Semantically SHM_STAT_ANY ought to be identical to
> +	 * that functionality provided by the /proc/sysvipc/
> +	 * interface. As such, only audit these calls and
> +	 * do not do traditional S_IRUGO permission checks on
> +	 * the ipc object.
> +	 */
> +	if (cmd == SHM_STAT_ANY)
> +		audit_ipc_obj(&shp->shm_perm);
> +	else {
> +		err = -EACCES;
> +		if (ipcperms(ns, &shp->shm_perm, S_IRUGO))
> +			goto out_unlock;
> +	}
>  
>  	err = security_shm_shmctl(shp, cmd);
>  	if (err)
> @@ -1072,6 +1083,7 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
>  		return err;
>  	}
>  	case SHM_STAT:
> +	case SHM_STAT_ANY:
>  	case IPC_STAT: {
>  		err = shmctl_stat(ns, shmid, cmd, &sem64);
>  		if (err < 0)
> @@ -1245,6 +1257,7 @@ COMPAT_SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, void __user *, uptr)
>  		return err;
>  	}
>  	case IPC_STAT:
> +	case SHM_STAT_ANY:
>  	case SHM_STAT:
>  		err = shmctl_stat(ns, shmid, cmd, &sem64);
>  		if (err < 0)
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 35ef1e9045e8..373dceede50d 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -5734,6 +5734,7 @@ static int selinux_shm_shmctl(struct shmid_kernel *shp, int cmd)
>  				    SECCLASS_SYSTEM, SYSTEM__IPC_INFO, NULL);
>  	case IPC_STAT:
>  	case SHM_STAT:
> +	case SHM_STAT_ANY:
>  		perms = SHM__GETATTR | SHM__ASSOCIATE;
>  		break;
>  	case IPC_SET:
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 03fdecba93bb..51d22b03b0ae 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -3034,6 +3034,7 @@ static int smack_shm_shmctl(struct shmid_kernel *shp, int cmd)
>  	switch (cmd) {
>  	case IPC_STAT:
>  	case SHM_STAT:
> +	case SHM_STAT_ANY:
>  		may = MAY_READ;
>  		break;
>  	case IPC_SET:
> -- 
> 2.13.6

-- 
Michal Hocko
SUSE Labs