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SubjectRe: [PATCH 1/3] ipc/shm: introduce shmctl(SHM_STAT_ANY)
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@suse.de>

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@suse.com>

> ---
> 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

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