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Message-ID: <20110705142659.GA18290@peqn>
Date: Tue, 5 Jul 2011 09:26:59 -0500
From: Serge Hallyn <serge.hallyn@...onical.com>
To: Vasiliy Kulikov <segoon@...nwall.com>
Cc: akpm@...ux-foundation.org, daniel.lezcano@...e.fr,
ebiederm@...ssion.com, mingo@...e.hu, oleg@...hat.com,
rdunlap@...otime.net, tj@...nel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] shm: handle separate PID namespaces case
Quoting Vasiliy Kulikov (segoon@...nwall.com):
> shm_try_destroy_orphaned() and shm_try_destroy_current() didn't handle
> the case of separate PID namespaces, but a single IPC namespace. If
> there are tasks with the same PID values using the same shmem object,
> the wrong destroy decision could be reached.
>
> On shm segment creation store the pointer to the creator task in
> shmid_kernel->shm_creator field and zero it on task exit. Then
> use the ->shm_creator insread of ->shm_cprid in both functions.
> As shmid_kernel object is already locked at this stage, no additional
> locking is needed.
Thanks, Vasiliy. Sounds like the Documentation/ file could stand some
clarification.
A concern below, though:
> Signed-off-by: Vasiliy Kulikov <segoon@...nwall.com>
> ---
> include/linux/shm.h | 3 +++
> ipc/shm.c | 27 ++++++++++++++++++++-------
> 2 files changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/shm.h b/include/linux/shm.h
> index b030a4e..12d2234 100644
> --- a/include/linux/shm.h
> +++ b/include/linux/shm.h
> @@ -95,6 +95,9 @@ struct shmid_kernel /* private to the kernel */
> pid_t shm_cprid;
> pid_t shm_lprid;
> struct user_struct *mlock_user;
> +
> + /* The task created the shm object. NULL if the task is dead. */
> + struct task_struct *shm_creator;
> };
>
> /* shm_mode upper byte flags */
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 22006f1..3baae98 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -239,7 +239,23 @@ static int shm_try_destroy_current(int id, void *p, void *data)
> if (IS_ERR(shp))
> return 0;
>
> - if (shp->shm_cprid != task_tgid_vnr(current)) {
> + if (shp->shm_creator != current) {
> + shm_unlock(shp);
> + return 0;
> + }
> +
> + /*
> + * Mark it as orphaned to destroy the segment when
> + * kernel.shm_forced_rmid is changed.
> + * It is noop if the following shm_may_destroy() returns true.
> + */
> + shp->shm_creator = NULL;
This function, shm_try_destroy_current(), only gets called by shm_exit()
if the shm_forced_rmid is set, right? So something funky can happen if
first shm_forced_rmid is 0 and some get created and the creating tasks
exits, then shm_forced_rmid gets set to one, and the task pointer gets
reused?
Using a struct pid may still be the best bet. It's much lighter-weight
than a task struct, so keeping a ref shouldn't much matter. It'll
avoid this wraparound issue (assuming I'm not imagining that issue).
Struct pid is namespace-safe, and you can still do your simple, quick
pointer comparison.
> + /*
> + * Don't even try to destroy it. If shm_forced_rmid=0 and IPC_RMID
> + * is not set, it shouldn't be deleted here.
> + */
> + if (!ns->shm_forced_rmid) {
> shm_unlock(shp);
> return 0;
> }
> @@ -255,7 +271,6 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data)
> {
> struct ipc_namespace *ns = data;
> struct shmid_kernel *shp = shm_lock(ns, id);
> - struct task_struct *task;
>
> if (IS_ERR(shp))
> return 0;
> @@ -263,11 +278,8 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data)
> /*
> * We want to destroy segments without users and with already
> * exit'ed originating process.
> - *
> - * XXX: the originating process may exist in another pid namespace.
> */
> - task = find_task_by_vpid(shp->shm_cprid);
> - if (task != NULL) {
> + if (shp->shm_creator != NULL) {
> shm_unlock(shp);
> return 0;
> }
> @@ -295,7 +307,7 @@ void exit_shm(struct task_struct *task)
> if (!nsp)
> return;
> ns = nsp->ipc_ns;
> - if (!ns || !ns->shm_forced_rmid)
> + if (!ns)
> return;
>
> /* Destroy all already created segments, but not mapped yet */
> @@ -494,6 +506,7 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
> shp->shm_segsz = size;
> shp->shm_nattch = 0;
> shp->shm_file = file;
> + shp->shm_creator = current;
> /*
> * shmid gets reported as "inode#" in /proc/pid/maps.
> * proc-ps tools use this. Changing this will break them.
> --
> 1.7.0.4
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