A long story behind all of that...
Some time ago I met kernel crash after CRIU restore procedure, fortunately, it was CRIU restore, so, I had dump files and could do restore many times and crash reproduced easily. After some investigation I've constructed the minimal reproducer. It was found that it's use-after-free and it happens only if sysctl kernel.shm_rmid_forced = 1.
The key of the problem is that the exit_shm() function not handles shp's object destroy when task->sysvshm.shm_clist contains items from different IPC namespaces. In most cases this list will contain only items from one IPC namespace.
Why this list may contain object from different namespaces? Function exit_shm() designed to clean up this list always when process leaves IPC namespace. But we made a mistake a long time ago and not add exit_shm() call into setns() syscall procedures. 1st second idea was just to add this call to setns() syscall but it's obviously changes semantics of setns() syscall and that's userspace-visible change. So, I gave up this idea.
First real attempt to address the issue was just to omit forced destroy if we meet shp object not from current task IPC namespace [1]. But that was not the best idea because task->sysvshm.shm_clist was protected by rwsem which belongs to current task IPC namespace. It means that list corruption may occur.
Second approach is just extend exit_shm() to properly handle shp's from different IPC namespaces [2]. This is really non-trivial thing, I've put a lot of effort into that but not believed that it's possible to make it fully safe, clean and clear.
Thanks to the efforts of Manfred Spraul working and elegant solution was designed. Thanks a lot, Manfred!
Eric also suggested the way to address the issue in ("[RFC][PATCH] shm: In shm_exit destroy all created and never attached segments") Eric's idea was to maintain a list of shm_clists one per IPC namespace, use lock-less lists. But there is some extra memory consumption-related concerns.
Alternative solution which was suggested by me was implemented in ("shm: reset shm_clist on setns but omit forced shm destroy") Idea is pretty simple, we add exit_shm() syscall to setns() but DO NOT destroy shm segments even if sysctl kernel.shm_rmid_forced = 1, we just clean up the task->sysvshm.shm_clist list. This chages semantics of setns() syscall a little bit but in comparision to "naive" solution when we just add exit_shm() without any special exclusions this looks like a safer option.
[1] https://lkml.org/lkml/2021/7/6/1108 [2] https://lkml.org/lkml/2021/7/14/736
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 Signed-off-by: Alexander Mikhalitsyn alexander.mikhalitsyn@virtuozzo.com
Alexander Mikhalitsyn (2): ipc: WARN if trying to remove ipc object which is absent shm: extend forced shm destroy to support objects from several IPC nses
include/linux/ipc_namespace.h | 15 +++ include/linux/sched/task.h | 2 +- include/linux/shm.h | 2 +- ipc/shm.c | 170 +++++++++++++++++++++++++--------- ipc/util.c | 6 +- 5 files changed, 145 insertions(+), 50 deletions(-)