Re: [PATCH 2/2] shm: extend forced shm destroy to support objects from several IPC nses

On 10/30/21 06:26, Eric W. Biederman wrote:

Alexander Mikhalitsyn alexander.mikhalitsyn@virtuozzo.com writes:

...

Currently, exit_shm function not designed to work properly when task->sysvshm.shm_clist holds shm objects from different IPC namespaces.

This is a real pain when sysctl kernel.shm_rmid_forced = 1, because it leads to use-after-free (reproducer exists).

That particular patch is attempt to fix the problem by extending exit_shm mechanism to handle shm's destroy from several IPC ns'es.

To achieve that we do several things:

1. add namespace (non-refcounted) pointer to the struct shmid_kernel
2. during new shm object creation (newseg()/shmget syscall) we initialize

this pointer by current task IPC ns 3. exit_shm() fully reworked such that it traverses over all shp's in task->sysvshm.shm_clist and gets IPC namespace not from current task as it was before but from shp's object itself, then call shm_destroy(shp, ns).

Note. We need to be really careful here, because as it was said before (1), our pointer to IPC ns non-refcnt'ed. To be on the safe side we using special helper get_ipc_ns_not_zero() which allows to get IPC ns refcounter only if IPC ns not in the "state of destruction".

...

Q/A

Q: Why we can access shp->ns memory using non-refcounted pointer? A: Because shp object lifetime is always shorther than IPC namespace lifetime, so, if we get shp object from the task->sysvshm.shm_clist while holding task_lock(task) nobody can steal our namespace.

Not true. A struct shmid_kernel can outlive the namespace in which it was created. I you look at do_shm_rmid which is called when the namespace is destroyed for every shmid_kernel in the namespace that if the struct shmid_kernel still has users only ipc_set_key_private is called. The struct shmid_kernel continues to exist.

No, shm_nattach is always 0 when a namespace is destroyed.

Thus it is impossible that shmid_kernel continues to exist.

Let's check all shm_nattach modifications:

1) do_shmat:

shp->shm_nattach++;

sfd->ns = get_ipc_ns(ns);

shp->shm_nattach--;

pairs with

shm_release()

put_ipc_ns()

2) shm_open()

only shp->shm_nattach++

shm_open unconditionally accesses shm_file_data, i.e. sfd must be valid, there must be a reference to the namespace

pairs with shm_close()

only shp->shm_nattach--;

shm_close unconditionally accesses shm_file_data, i.e. sfd must be valid, there must be a reference to the namespace

As shm_open()/close "nests" inside do_shmat: there is always a get_ipc_ns().

Or, much simpler: Check shm_open() and shm_close():

These two functions address a shm segment by namespace and  ID, not by a shm pointer. Thus _if_ it is possible that shm_nattach is > 0 at namespace destruction, then there would be far more issues.

Or: Attached is a log file, a test application, and a patch that adds pr_info statements.

The namespace is destroyed immediately when no segments are mapped, the destruction is delayed until exit() if there are mapped segments.

...

Q: Does this patch change semantics of unshare/setns/clone syscalls? A: Not. It's just fixes non-covered case when process may leave IPC namespace without getting task->sysvshm.shm_clist list cleaned up.

Just reading through exit_shm the code is not currently safe.

At a minimum do_shm_rmid needs to set the shp->ns to NULL. Otherwise the struct shmid_kernel can contain a namespace pointer after the namespace exits. Which results in a different use after free.

No [unless there are additional bugs]

Beyond that there is dropping the task lock. The code holds a reference to the namespace which means that the code does not need to worry about free_ipcs. References from mappings are still possible.

Which means that the code could see: exit_shm() task_lock() shp = ...;

task_unlock()
                                  shm_close()
                                      down_write(&shm_ids(ns).rwsem);
                                      ...
                                      shm_destroy(shp);
                                      up_write(&shm_ids(ns).rwsem);
down_write(&shm_ids(ns)->rwsem);
shm_lock_by_ptr(shp);	/* use after free */

I am trying to imagine how to close that race with the current code structure. Maybe something could be done by looking at shm_nattach count and making it safe to look at that count under the task_lock.

There is no race. Before dropping task_lock, a reference to both the namespace and the shp pointer is obtained.

Thus neither one can disappear.

But even then because shmid_kernel is still in the hash table it could be mapped and unmapped in the window when task_lock was dropped.

We have ipc_valid_object(), i.e. perm->deleted. If set, then the pointer and the spinlock are valid, even though the rest is already destroyed.

ipc_rmid() just sets deleted, the (rcu delayed) kfree is done via ipc_rcu_putref().

Alternatively shmctl(id, IPC_RMID) can be called in when task_lock is dropped. Much less code is involved than mapping and unmapping so it is much more likely to win the race.

I don't see how that race can be closed.

Am I missing something?

Eric

...

Fixes: ab602f79915 ("shm: make exit_shm work proportional to task activity")

Cc: "Eric W. Biederman" ebiederm@xmission.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Davidlohr Bueso dave@stgolabs.net Cc: Greg KH gregkh@linuxfoundation.org Cc: Andrei Vagin avagin@gmail.com Cc: Pavel Tikhomirov ptikhomirov@virtuozzo.com Cc: Vasily Averin vvs@virtuozzo.com Cc: Manfred Spraul manfred@colorfullife.com Cc: Alexander Mikhalitsyn alexander@mihalicyn.com Cc: stable@vger.kernel.org Co-developed-by: Manfred Spraul manfred@colorfullife.com Signed-off-by: Manfred Spraul manfred@colorfullife.com Signed-off-by: Alexander Mikhalitsyn alexander.mikhalitsyn@virtuozzo.com

Should/can I mark that I have tested the code?

I would drop one change and one comment is incorrect, otherwise no findings. See the attached 0002 patch

Tested-by: Manfred Spraul manfred@colorfullife.com

...

---

include/linux/ipc_namespace.h | 15 +++ include/linux/sched/task.h | 2 +- include/linux/shm.h | 2 +- ipc/shm.c | 170 +++++++++++++++++++++++++--------- 4 files changed, 142 insertions(+), 47 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h index 05e22770af51..b75395ec8d52 100644 --- a/include/linux/ipc_namespace.h +++ b/include/linux/ipc_namespace.h @@ -131,6 +131,16 @@ static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns) return ns; } +static inline struct ipc_namespace *get_ipc_ns_not_zero(struct ipc_namespace *ns) +{

• if (ns) {

• if (refcount_inc_not_zero(&ns->ns.count))
  

• 	return ns;
  

• }
• return NULL;

+}

• extern void put_ipc_ns(struct ipc_namespace *ns); #else static inline struct ipc_namespace *copy_ipcs(unsigned long flags,

@@ -147,6 +157,11 @@ static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns) return ns; } +static inline struct ipc_namespace *get_ipc_ns_not_zero(struct ipc_namespace *ns) +{

• return ns;

+}

• static inline void put_ipc_ns(struct ipc_namespace *ns) { }

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index ef02be869cf2..bfdf84dab4be 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -157,7 +157,7 @@ static inline struct vm_struct *task_stack_vm_area(const struct task_struct *t)

• Protects ->fs, ->files, ->mm, ->group_info, ->comm, keyring
• subscriptions and synchronises with wait4(). Also used in procfs. Also
• pins the final release of task.io_context. Also protects ->cpuset and

• • ->cgroup.subsys[]. And ->vfork_done.

• • ->cgroup.subsys[]. And ->vfork_done. And ->sysvshm.shm_clist.
  • Nests both inside and outside of read_lock(&tasklist_lock).
  • It must not be nested with write_lock_irq(&tasklist_lock),

diff --git a/include/linux/shm.h b/include/linux/shm.h index d8e69aed3d32..709f6d0451c0 100644 --- a/include/linux/shm.h +++ b/include/linux/shm.h @@ -11,7 +11,7 @@ struct file; #ifdef CONFIG_SYSVIPC struct sysv_shm {

• struct list_head shm_clist;

• struct list_head shm_clist; };

This is a whitespace only change. We can drop it.

...

long do_shmat(int shmid, char __user *shmaddr, int shmflg, unsigned long *addr, diff --git a/ipc/shm.c b/ipc/shm.c index 748933e376ca..29667e17b12a 100644 --- a/ipc/shm.c +++ b/ipc/shm.c @@ -62,9 +62,18 @@ struct shmid_kernel /* private to the kernel */ struct pid *shm_lprid; struct ucounts *mlock_ucounts;

• /* The task created the shm object. NULL if the task is dead. */

• /*

• * The task created the shm object, for looking up
  

• * task->sysvshm.shm_clist_lock
  

• */
  

  struct task_struct *shm_creator;

• struct list_head shm_clist; /* list by creator */

• /*

• * list by creator. shm_clist_lock required for read/write
  

• * if list_empty(), then the creator is dead already
  

• */
  

shm_clist_lock was replaced by task_lock(->shm_creator).

...

• struct list_head shm_clist;
• struct ipc_namespace *ns; } __randomize_layout;

/* shm_mode upper byte flags */ @@ -115,6 +124,7 @@ static void do_shm_rmid(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp) struct shmid_kernel *shp; shp = container_of(ipcp, struct shmid_kernel, shm_perm);

• WARN_ON(ns != shp->ns);

if (shp->shm_nattch) { shp->shm_perm.mode |= SHM_DEST; @@ -225,10 +235,36 @@ static void shm_rcu_free(struct rcu_head *head) kfree(shp); } -static inline void shm_rmid(struct ipc_namespace *ns, struct shmid_kernel *s) +/*

• • It has to be called with shp locked.
• • It must be called before ipc_rmid()
• */

+static inline void shm_clist_rm(struct shmid_kernel *shp) {

• list_del(&s->shm_clist);
• ipc_rmid(&shm_ids(ns), &s->shm_perm);

• struct task_struct *creator;
• /*

• * A concurrent exit_shm may do a list_del_init() as well.
  

• * Just do nothing if exit_shm already did the work
  

• */
  

• if (list_empty(&shp->shm_clist))

• return;
  

• /*

• * shp->shm_creator is guaranteed to be valid *only*
  

• * if shp->shm_clist is not empty.
  

• */
  

• creator = shp->shm_creator;
• task_lock(creator);
• list_del_init(&shp->shm_clist);
• task_unlock(creator);

Lock ordering rwsem ipc_lock task_lock

correct.

...

+}

+static inline void shm_rmid(struct shmid_kernel *s) +{

• shm_clist_rm(s);
• ipc_rmid(&shm_ids(s->ns), &s->shm_perm); }

@@ -283,7 +319,7 @@ static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp) shm_file = shp->shm_file; shp->shm_file = NULL; ns->shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT;

• shm_rmid(ns, shp);

• shm_rmid(shp); shm_unlock(shp); if (!is_file_hugepages(shm_file)) shmem_lock(shm_file, 0, shp->mlock_ucounts);

@@ -306,10 +342,10 @@ static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)

• 2. sysctl kernel.shm_rmid_forced is set to 1.

*/ -static bool shm_may_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp) +static bool shm_may_destroy(struct shmid_kernel *shp) { return (shp->shm_nattch == 0) &&

•       (ns->shm_rmid_forced ||
  

•       (shp->ns->shm_rmid_forced ||
  

  (shp->shm_perm.mode & SHM_DEST)); }

@@ -340,7 +376,7 @@ static void shm_close(struct vm_area_struct *vma) ipc_update_pid(&shp->shm_lprid, task_tgid(current)); shp->shm_dtim = ktime_get_real_seconds(); shp->shm_nattch--;

• if (shm_may_destroy(ns, shp))

• if (shm_may_destroy(shp)) shm_destroy(ns, shp); else shm_unlock(shp);

@@ -361,10 +397,10 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data) * * As shp->* are changed under rwsem, it's safe to skip shp locking. */

• if (shp->shm_creator != NULL)

• if (!list_empty(&shp->shm_clist)) return 0;

• if (shm_may_destroy(ns, shp)) {

• if (shm_may_destroy(shp)) { shm_lock_by_ptr(shp); shm_destroy(ns, shp); }

@@ -382,48 +418,87 @@ void shm_destroy_orphaned(struct ipc_namespace *ns) /* Locking assumes this will only be called with task == current */ void exit_shm(struct task_struct *task) {

• struct ipc_namespace *ns = task->nsproxy->ipc_ns;
• struct shmid_kernel *shp, *n;

• for (;;) {

• struct shmid_kernel *shp;
  

• struct ipc_namespace *ns;
  

• if (list_empty(&task->sysvshm.shm_clist))

• return;
  

• task_lock(task);
  

• if (list_empty(&task->sysvshm.shm_clist)) {
  

• 	task_unlock(task);
  

• 	break;
  

• }
  

• shp = list_first_entry(&task->sysvshm.shm_clist, struct shmid_kernel,
  

• 		shm_clist);
  

• /* 1) unlink */
  

• list_del_init(&shp->shm_clist);
  

• /*

• * If kernel.shm_rmid_forced is not set then only keep track of
  

• * which shmids are orphaned, so that a later set of the sysctl
  

• * can clean them up.
  

• */
  

• if (!ns->shm_rmid_forced) {

• down_read(&shm_ids(ns).rwsem);
  

• list_for_each_entry(shp, &task->sysvshm.shm_clist, shm_clist)
  

• 	shp->shm_creator = NULL;
  

  /*

•  * Only under read lock but we are only called on current
  

•  * so no entry on the list will be shared.
  

•  * 2) Get pointer to the ipc namespace. It is worth to say
  

•  * that this pointer is guaranteed to be valid because
  

•  * shp lifetime is always shorter than namespace lifetime
  

•  * in which shp lives.
  

•  * We taken task_lock it means that shp won't be freed.
  

  */

• list_del(&task->sysvshm.shm_clist);
  

• up_read(&shm_ids(ns).rwsem);
  

• return;
  

• }

• ns = shp->ns;
  

• /*

• * Destroy all already created segments, that were not yet mapped,
  

• * and mark any mapped as orphan to cover the sysctl toggling.
  

• * Destroy is skipped if shm_may_destroy() returns false.
  

• */
  

• down_write(&shm_ids(ns).rwsem);
• list_for_each_entry_safe(shp, n, &task->sysvshm.shm_clist, shm_clist) {

• shp->shm_creator = NULL;
  

• /*
  

•  * 3) If kernel.shm_rmid_forced is not set then only keep track of
  

•  * which shmids are orphaned, so that a later set of the sysctl
  

•  * can clean them up.
  

•  */
  

• if (!ns->shm_rmid_forced) {
  

• 	task_unlock(task);
  

• 	continue;
  

• }
  

• if (shm_may_destroy(ns, shp)) {
  

• /*
  

•  * 4) get a reference to the namespace.
  

•  *    The refcount could be already 0. If it is 0, then
  

•  *    the shm objects will be free by free_ipc_work().
  

•  */
  

• ns = get_ipc_ns_not_zero(ns);
  

• if (ns) {
  

• 	/*
  

• 	 * 5) get a reference to the shp itself.
  

• 	 *   This cannot fail: shm_clist_rm() is called before
  

• 	 *   ipc_rmid(), thus the refcount cannot be 0.
  

• 	 */
  

• 	WARN_ON(!ipc_rcu_getref(&shp->shm_perm));
  

• }
  

• task_unlock(task);
  

<<<<<<<<< BOOM >>>>>>>

I don't see anything that prevents another task from calling shm_destroy(ns, shp) here and freeing it before this task can take the rwsem for writing.

shm_destroy() can be called. But due to the ipc_rcu_getref(), the structure will remain valid.

...

• if (ns) {
  

• 	down_write(&shm_ids(ns).rwsem);
  shm_lock_by_ptr(shp);
  

• 	shm_destroy(ns, shp);
  

• 	/*
  

• 	 * rcu_read_lock was implicitly taken in
  

• 	 * shm_lock_by_ptr, it's safe to call
  

• 	 * ipc_rcu_putref here
  

• 	 */
  

• 	ipc_rcu_putref(&shp->shm_perm, shm_rcu_free);
  

• 	if (ipc_valid_object(&shp->shm_perm)) {
  

And this will return false if there was a shm_destroy().

...

• 		if (shm_may_destroy(shp))
  

• 			shm_destroy(ns, shp);
  

• 		else
  

• 			shm_unlock(shp);
  

• 	} else {
  

• 		/*
  

• 		 * Someone else deleted the shp from namespace
  

• 		 * idr/kht while we have waited.
  

• 		 * Just unlock and continue.
  

• 		 */
  

-> just do a NOP if shm_destroy() was alread performed.

Actually, the same design is used by find_alloc_undo() in ipc/sem.c.

...

• 		shm_unlock(shp);
  

• 	}
  

• 	up_write(&shm_ids(ns).rwsem);
  

• 	put_ipc_ns(ns); /* paired with get_ipc_ns_not_zero */
  

  } }

• /* Remove the list head from any segments still attached. */
• list_del(&task->sysvshm.shm_clist);
• up_write(&shm_ids(ns).rwsem); }

static vm_fault_t shm_fault(struct vm_fault *vmf) @@ -680,7 +755,11 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params) if (error < 0) goto no_id;

• shp->ns = ns;
• task_lock(current); list_add(&shp->shm_clist, &current->sysvshm.shm_clist);
• task_unlock(current);

/* * shmid gets reported as "inode#" in /proc/pid/maps. @@ -1573,7 +1652,8 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, down_write(&shm_ids(ns).rwsem); shp = shm_lock(ns, shmid); shp->shm_nattch--;

• if (shm_may_destroy(ns, shp))

• if (shm_may_destroy(shp)) shm_destroy(ns, shp); else shm_unlock(shp);