Messages in this thread | | | From | Alexander Mihalicyn <> | Date | Wed, 14 Jul 2021 20:42:39 +0300 | Subject | Re: [PATCH 0/2] shm: omit forced shm destroy if task IPC namespace was changed |
| |
Hi Eric,
I've sent the RFC patch "shm: extend forced shm destroy to support objects from", please take a look once time permits.
A few words about design: 1. In the exit_shm() function I'm using a temporary list to collect shm's before destroying because I can't take semaphore (which protects ns idr structure) under spinlock (which protects list on task) 2. I've to take IPC ns refcounter in exit_shm() to prevent possible race with shm_exit_ns(). 3. I haven't added list_del_init(&shp->shm_clist) into shmat() syscall because I can't understand why this is safe. But maybe I've to think more about that. 4. I need to remove the extra "ns" argument (todo).
Thanks, Alex
On Mon, Jul 12, 2021 at 10:27 PM Alexander Mihalicyn <alexander@mihalicyn.com> wrote: > > Hi Eric, > > On Mon, Jul 12, 2021 at 10:18 PM Eric W. Biederman > <ebiederm@xmission.com> wrote: > > > > Alexander Mihalicyn <alexander@mihalicyn.com> writes: > > > > > Hello Manfred, > > > > > > On Sun, Jul 11, 2021 at 2:47 PM Manfred Spraul <manfred@colorfullife.com> wrote: > > >> > > >> Hi Alex, > > >> > > >> > > >> Am Sonntag, 11. Juli 2021 schrieb Alexander Mihalicyn <alexander@mihalicyn.com>: > > >> > > > >> > Hi, Manfred, > > >> > > > >> > On Sun, Jul 11, 2021 at 12:13 PM Manfred Spraul > > >> > <manfred@colorfullife.com> wrote: > > >> > > > > >> > > Hi, > > >> > > > > >> > > > > >> > > Am Samstag, 10. Juli 2021 schrieb Alexander Mihalicyn <alexander@mihalicyn.com>: > > >> > >> > > >> > >> > > >> > >> Now, using setns() syscall, we can construct situation when on > > >> > >> task->sysvshm.shm_clist list > > >> > >> we have shm items from several (!) IPC namespaces. > > >> > >> > > >> > >> > > >> > > Does this imply that locking ist affected as well? According to the initial patch, accesses to shm_clist are protected by "the" IPC shm namespace rwsem. This can't work if the list contains objects from several namespaces. > > >> > > > >> > Of course, you are right. I've to rework this part -> I can add check into > > >> > static int newseg(struct ipc_namespace *ns, struct ipc_params *params) > > >> > function and before adding new shm into task list check that list is empty OR > > >> > an item which is present on the list from the same namespace as > > >> > current->nsproxy->ipc_ns. > > >> > > > >> Ok. (Sorry, I have only smartphone internet, thus I could not check > > >> the patch fully) > > >> > > >> > >> I've proposed a change which keeps the old behaviour of setns() but > > >> > >> fixes double free. > > >> > >> > > >> > > Assuming that locking works, I would consider this as a namespace design question: Do we want to support that a task contains shm objects from several ipc namespaces? > > >> > > > >> > This depends on what we mean by "task contains shm objects from > > >> > several ipc namespaces". There are two meanings: > > >> > > > >> > 1. Task has attached shm object from different ipc namespaces > > >> > > > >> > We already support that by design. When we doing a change of namespace > > >> > using unshare(CLONE_NEWIPC) even with > > >> > sysctl shm_rmid_forced=1 we not detach all ipc's from task! > > >> > > >> OK. Thus shm and sem have different behavior anyways. > > >> > > >> > > > >> > 2. Task task->sysvshm.shm_clist list has items from different IPC namespaces. > > >> > > > >> > I'm not sure, do we need that or not. But I'm ready to prepare a patch > > >> > for any of the options which we choose: > > >> > a) just add exit_shm(current)+shm_init_task(current); > > >> > b) prepare PATCHv2 with appropriate check in the newseg() to prevent > > >> > adding new items from different namespace to the list > > >> > c) rework algorithm so we can safely have items from different > > >> > namespaces in task->sysvshm.shm_clist > > >> > > > >> Before you write something, let's wait what the others say. I don't > > >> qualify AS shm expert > > >> > > >> a) is user space visible, without any good excuse > > > > > > yes, but maybe we decide that this is not so critical? > > > We need more people here :) > > > > It is barely visible. You have to do something very silly > > to see this happening. It is probably ok, but the work to > > verify that nothing cares so that we can safely backport > > the change is probably much more work than just updating > > the list to handle shmid's for multiple namespaces. > > > > > > >> c) is probably highest amount of Changes > > > > > > yep. but ok, I will prepare patches fast. > > > > Given that this is a bug I think c) is the safest option. > > > > A couple of suggestions. > > 1) We can replace the test "shm_creator != NULL" with > > "list_empty(&shp->shm_clist)" and remove shm_creator. > > > > Along with replacing "shm_creator = NULL" with > > "list_del_init(&shp->shm_clist)". > > > > 2) We can update shmat to do "list_del_init(&shp->shm_clist)" > > upon shmat. The last unmap will still shm_destroy the > > shm segment as ns->shm_rmid_forced is set. > > > > For a multi-threaded process I think this will nicely clean up > > the clist, and make it clear that the clist only cares about > > those segments that have been created but never attached. > > > > 3) Put a non-reference counted struct ipc_namespace in struct > > shmid_kernel, and use it to remove the namespace parameter > > from shm_destroy. > > Thanks for your detailed suggestions! ;) > I will prepare a patch tomorrow for kernel + test what's happening with > CRIU and will prepare a fix for it. > > > > > I think that is enough to fix this bug with no changes in semantics, > > no additional memory consumed, and an implementation that is easier > > to read and perhaps a little faster. > > > > Eric > > Regards, > Alex
| |