| Message ID | 20131118172844.GA10005@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 11/18, Oleg Nesterov wrote: > > On 11/15, Eric W. Biederman wrote: > > > > I don't understand that one. Having a preforked thread with the proper > > environment that can act like kthreadd in terms of spawning user mode > > helpers works and is simple. > > Can't we ask ->child_reaper to create the non-daemonized kernel thread > with the "right" ->nsproxy, ->fs, etc? > > IOW. Please the the "patch" below. It is obviously incomplete and wrong, > and it can be more clear/clean. And probably we need another API. Just > to explain what I mean. Or, perhaps UMH_IN_MY_NS should only work if ->child_reaper explicitly does, say, prctl(PR_SPAWN_UMH_IN_NS_HELPER) which forks the non-daemonized kernel kthread_worker thread, I dunno. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, 18 Nov 2013 19:02:59 +0100 Oleg Nesterov <oleg@redhat.com> wrote: > On 11/18, Oleg Nesterov wrote: > > > > On 11/15, Eric W. Biederman wrote: > > > > > > I don't understand that one. Having a preforked thread with the proper > > > environment that can act like kthreadd in terms of spawning user mode > > > helpers works and is simple. > > > > Can't we ask ->child_reaper to create the non-daemonized kernel thread > > with the "right" ->nsproxy, ->fs, etc? > > > > IOW. Please the the "patch" below. It is obviously incomplete and wrong, > > and it can be more clear/clean. And probably we need another API. Just > > to explain what I mean. > > Or, perhaps UMH_IN_MY_NS should only work if ->child_reaper explicitly > does, say, prctl(PR_SPAWN_UMH_IN_NS_HELPER) which forks the non-daemonized > kernel kthread_worker thread, I dunno. > > Oleg. > Neat idea. So is it always the case that tasks in a container have the same namespace settings and capabilities as the child_reaper? We'll still have the basic problem for nfsd that we'll need to keep track of what the child_reaper is when nfsd is started, but I think that's not too hard to solve.
On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > On 11/15, Eric W. Biederman wrote: > > > > I don't understand that one. Having a preforked thread with the > > proper > > environment that can act like kthreadd in terms of spawning user > > mode > > helpers works and is simple. Forgive me replying to such an old thread but ... After realizing workqueues can't be used to pre-create threads to run usermode helpers I've returned to look at this. > > Can't we ask ->child_reaper to create the non-daemonized kernel thread > with the "right" ->nsproxy, ->fs, etc? Eric, do you think this approach would be sufficient too? Probably wouldn't be quite right for user namespaces but should provide what's needed for other cases? It certainly has the advantage of not having to maintain a plague of processes waiting around to execute helpers. > > IOW. Please the the "patch" below. It is obviously incomplete and > wrong, > and it can be more clear/clean. And probably we need another API. Just > to explain what I mean. > > With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec > from the caller's namespace. > > Oleg. > --- > > --- a/include/linux/kmod.h > +++ b/include/linux/kmod.h > @@ -24,6 +24,7 @@ > #include <linux/errno.h> > #include <linux/compiler.h> > #include <linux/workqueue.h> > +#include <linux/task_work.h> > #include <linux/sysctl.h> > > #define KMOD_PATH_LEN 256 > @@ -53,8 +54,14 @@ struct file; > #define UMH_WAIT_PROC 2 /* wait for the process to > complete */ > #define UMH_KILLABLE 4 /* wait for EXEC/PROC killable > */ > > +// FIXME: IMH_* is not actually a mask > +#define UMH_IN_MY_NS 8 > + > struct subprocess_info { > - struct work_struct work; > + union { > + struct work_struct work; > + struct callback_head twork; > + }; > struct completion *complete; > char *path; > char **argv; > --- a/kernel/kmod.c > +++ b/kernel/kmod.c > @@ -541,7 +541,6 @@ struct subprocess_info > *call_usermodehelper_setup(char *path, char **argv, > if (!sub_info) > goto out; > > - INIT_WORK(&sub_info->work, __call_usermodehelper); > sub_info->path = path; > sub_info->argv = argv; > sub_info->envp = envp; > @@ -554,6 +553,24 @@ struct subprocess_info > *call_usermodehelper_setup(char *path, char **argv, > } > EXPORT_SYMBOL(call_usermodehelper_setup); > > +static int call_call_usermodehelper(void *twork) > +{ > + struct subprocess_info *sub_info = > + container_of(twork, struct subprocess_info, twork); > + > + __call_usermodehelper(&sub_info->work); > + do_exit(0); > + > +} > + > +static void fork_umh_helper(struct callback_head *twork) > +{ > + if (current->flags & PF_EXITING) > + return; // WRONG, FIXME > + > + kernel_thread(call_call_usermodehelper, twork, SIGCHLD); > +} > + > /** > * call_usermodehelper_exec - start a usermode application > * @sub_info: information about the subprocessa > @@ -570,6 +587,10 @@ int call_usermodehelper_exec(struct > subprocess_info *sub_info, int wait) > { > DECLARE_COMPLETION_ONSTACK(done); > int retval = 0; > + bool in_my_ns; > + > + in_my_ns = wait & UMH_IN_MY_NS; > + wait &= ~UMH_IN_MY_NS; > > if (!sub_info->path) { > call_usermodehelper_freeinfo(sub_info); > @@ -594,7 +615,21 @@ int call_usermodehelper_exec(struct > subprocess_info *sub_info, int wait) > sub_info->complete = &done; > sub_info->wait = wait; > > - queue_work(khelper_wq, &sub_info->work); > + if (likely(!in_my_ns)) { > + INIT_WORK(&sub_info->work, __call_usermodehelper); > + queue_work(khelper_wq, &sub_info->work); > + } else { > + // RACY, WRONG, ETC > + struct task_struct *my_init = > task_active_pid_ns(current)->child_reaper; > + > + init_task_work(&sub_info->twork, fork_umh_helper); > + task_work_add(my_init, &sub_info->twork, false); > + > + // until we have task_work_add_interruptibel() > + do_send_sig_info(SIGCHLD, SEND_SIG_FORCED, my_init, > false); > + > + } > + > if (wait == UMH_NO_WAIT) /* task has freed sub_info */ > goto unlock; > > > -- > To unsubscribe from this list: send the line "unsubscribe linux > -fsdevel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Ccing The containers list because a related discussion is happening there and somehow this thread has never made it there. Ian Kent <raven@themaw.net> writes: > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: >> On 11/15, Eric W. Biederman wrote: >> > >> > I don't understand that one. Having a preforked thread with the >> > proper >> > environment that can act like kthreadd in terms of spawning user >> > mode >> > helpers works and is simple. > > Forgive me replying to such an old thread but ... > > After realizing workqueues can't be used to pre-create threads to run > usermode helpers I've returned to look at this. If someone can wind up with a good implementation I will be happy. >> Can't we ask ->child_reaper to create the non-daemonized kernel thread >> with the "right" ->nsproxy, ->fs, etc? > > Eric, do you think this approach would be sufficient too? > > Probably wouldn't be quite right for user namespaces but should provide > what's needed for other cases? > > It certainly has the advantage of not having to maintain a plague of > processes waiting around to execute helpers. That certainly sounds attractive. Especially for the case of everyone who wants to set a core pattern in a container. I am fuzzy on all of the details right now, but what I do remember is that in the kernel the user mode helper concepts when they attempted to scrub a processes environment were quite error prone until we managed to get kthreadd(pid 2) on the scene which always had a clean environment. If we are going to tie this kind of thing to the pid namespace I recommend simplying denying it if you are in a user namespace without an approrpriate pid namespace. AKA simply not allowing thigns to be setup if current->pid_ns->user_ns != current->user_ns. That still leaves things a little hand-wavy but I hope that helps conceptually. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 2016/02/18 11:57, Eric W. Biederman wrote: > > Ccing The containers list because a related discussion is happening there > and somehow this thread has never made it there. > > Ian Kent <raven@themaw.net> writes: > >> On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: >>> On 11/15, Eric W. Biederman wrote: >>>> >>>> I don't understand that one. Having a preforked thread with the >>>> proper >>>> environment that can act like kthreadd in terms of spawning user >>>> mode >>>> helpers works and is simple. >> >> Forgive me replying to such an old thread but ... >> >> After realizing workqueues can't be used to pre-create threads to run >> usermode helpers I've returned to look at this. > > If someone can wind up with a good implementation I will be happy. > >>> Can't we ask ->child_reaper to create the non-daemonized kernel thread >>> with the "right" ->nsproxy, ->fs, etc? >> >> Eric, do you think this approach would be sufficient too? >> >> Probably wouldn't be quite right for user namespaces but should provide >> what's needed for other cases? >> >> It certainly has the advantage of not having to maintain a plague of >> processes waiting around to execute helpers. > > That certainly sounds attractive. Especially for the case of everyone > who wants to set a core pattern in a container. > > I am fuzzy on all of the details right now, but what I do remember is > that in the kernel the user mode helper concepts when they attempted to > scrub a processes environment were quite error prone until we managed to > get kthreadd(pid 2) on the scene which always had a clean environment. > > If we are going to tie this kind of thing to the pid namespace I > recommend simplying denying it if you are in a user namespace without > an approrpriate pid namespace. AKA simply not allowing thigns to be setup > if current->pid_ns->user_ns != current->user_ns. > Can't be handled by simple capability like CAP_SYS_USERMODEHELPER ? User_ns check seems not to allow core-dump-cather in host will not work if user_ns is used. Thanks, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, 2016-02-18 at 12:43 +0900, Kamezawa Hiroyuki wrote: > On 2016/02/18 11:57, Eric W. Biederman wrote: > > > > Ccing The containers list because a related discussion is happening > > there > > and somehow this thread has never made it there. > > > > Ian Kent <raven@themaw.net> writes: > > > > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > > > > On 11/15, Eric W. Biederman wrote: > > > > > > > > > > I don't understand that one. Having a preforked thread with > > > > > the > > > > > proper > > > > > environment that can act like kthreadd in terms of spawning > > > > > user > > > > > mode > > > > > helpers works and is simple. > > > > > > Forgive me replying to such an old thread but ... > > > > > > After realizing workqueues can't be used to pre-create threads to > > > run > > > usermode helpers I've returned to look at this. > > > > If someone can wind up with a good implementation I will be happy. > > > > > > Can't we ask ->child_reaper to create the non-daemonized kernel > > > > thread > > > > with the "right" ->nsproxy, ->fs, etc? > > > > > > Eric, do you think this approach would be sufficient too? > > > > > > Probably wouldn't be quite right for user namespaces but should > > > provide > > > what's needed for other cases? > > > > > > It certainly has the advantage of not having to maintain a plague > > > of > > > processes waiting around to execute helpers. > > > > That certainly sounds attractive. Especially for the case of > > everyone > > who wants to set a core pattern in a container. > > > > I am fuzzy on all of the details right now, but what I do remember > > is > > that in the kernel the user mode helper concepts when they attempted > > to > > scrub a processes environment were quite error prone until we > > managed to > > get kthreadd(pid 2) on the scene which always had a clean > > environment. > > > > If we are going to tie this kind of thing to the pid namespace I > > recommend simplying denying it if you are in a user namespace > > without > > an approrpriate pid namespace. AKA simply not allowing thigns to be > > setup > > if current->pid_ns->user_ns != current->user_ns. > > > Can't be handled by simple capability like CAP_SYS_USERMODEHELPER ? > > User_ns check seems not to allow core-dump-cather in host will not > work if user_ns is used. I don't think so but I'm not sure. The approach I was talking about assumes the init process of the caller (say within a container, corresponding to ->child_reaper) is an appropriate template for umh thread execution. But I don't think that covers the case where unshare has created different namespaces, like a mount namespace for example. The current workqueue sub system can't be used to pre-create a thread to be used for umh execution so, either is needs changes or yet another mechanism needs to be implemented. For uses other than core dumping capturing a reference to the struct pid of the environment init process and using that as an execution template should be sufficient and takes care of environment existence problems with some extra checks, not to mention eliminating the need for a potentially huge number of kernel threads needing to be created to provide execution templates. Where to store this and how to access it when needed is another problem. Not sure a usermode helper capability is the right thing either as I thought one important use of user namespaces was to allow unprivileged users to perform operations they otherwise can't. Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible .... Still an appropriate execution template would be needed and IIUC we can't trust getting that from within a user created namespace environment. > > Thanks, > -Kame > -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, 2016-02-18 at 14:36 +0800, Ian Kent wrote: > On Thu, 2016-02-18 at 12:43 +0900, Kamezawa Hiroyuki wrote: > > On 2016/02/18 11:57, Eric W. Biederman wrote: > > > > > > Ccing The containers list because a related discussion is > > > happening > > > there > > > and somehow this thread has never made it there. > > > > > > Ian Kent <raven@themaw.net> writes: > > > > > > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > > > > > On 11/15, Eric W. Biederman wrote: > > > > > > > > > > > > I don't understand that one. Having a preforked thread with > > > > > > the > > > > > > proper > > > > > > environment that can act like kthreadd in terms of spawning > > > > > > user > > > > > > mode > > > > > > helpers works and is simple. > > > > > > > > Forgive me replying to such an old thread but ... > > > > > > > > After realizing workqueues can't be used to pre-create threads > > > > to > > > > run > > > > usermode helpers I've returned to look at this. > > > > > > If someone can wind up with a good implementation I will be happy. > > > > > > > > Can't we ask ->child_reaper to create the non-daemonized > > > > > kernel > > > > > thread > > > > > with the "right" ->nsproxy, ->fs, etc? > > > > > > > > Eric, do you think this approach would be sufficient too? > > > > > > > > Probably wouldn't be quite right for user namespaces but should > > > > provide > > > > what's needed for other cases? > > > > > > > > It certainly has the advantage of not having to maintain a > > > > plague > > > > of > > > > processes waiting around to execute helpers. > > > > > > That certainly sounds attractive. Especially for the case of > > > everyone > > > who wants to set a core pattern in a container. > > > > > > I am fuzzy on all of the details right now, but what I do remember > > > is > > > that in the kernel the user mode helper concepts when they > > > attempted > > > to > > > scrub a processes environment were quite error prone until we > > > managed to > > > get kthreadd(pid 2) on the scene which always had a clean > > > environment. > > > > > > If we are going to tie this kind of thing to the pid namespace I > > > recommend simplying denying it if you are in a user namespace > > > without > > > an approrpriate pid namespace. AKA simply not allowing thigns to > > > be > > > setup > > > if current->pid_ns->user_ns != current->user_ns. > > > > > Can't be handled by simple capability like CAP_SYS_USERMODEHELPER ? > > > > User_ns check seems not to allow core-dump-cather in host will not > > work if user_ns is used. > > I don't think so but I'm not sure. > > The approach I was talking about assumes the init process of the > caller > (say within a container, corresponding to ->child_reaper) is an > appropriate template for umh thread execution. > > But I don't think that covers the case where unshare has created > different namespaces, like a mount namespace for example. > > The current workqueue sub system can't be used to pre-create a thread > to > be used for umh execution so, either is needs changes or yet another > mechanism needs to be implemented. > > For uses other than core dumping capturing a reference to the struct > pid > of the environment init process and using that as an execution > template > should be sufficient and takes care of environment existence problems > with some extra checks, not to mention eliminating the need for a > potentially huge number of kernel threads needing to be created to > provide execution templates. > > Where to store this and how to access it when needed is another > problem. > > Not sure a usermode helper capability is the right thing either as I > thought one important use of user namespaces was to allow unprivileged > users to perform operations they otherwise can't. > > Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible .... > > Still an appropriate execution template would be needed and IIUC we > can't trust getting that from within a user created namespace > environment. Perhaps, if a struct cred could be captured at some appropriate time that could be used to cater for user namespaces. Eric, do you think that would be possible to do without allowing users to circumvent security? > > > > > Thanks, > > -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Ian Kent <raven@themaw.net> writes: > On Thu, 2016-02-18 at 14:36 +0800, Ian Kent wrote: >> On Thu, 2016-02-18 at 12:43 +0900, Kamezawa Hiroyuki wrote: >> > On 2016/02/18 11:57, Eric W. Biederman wrote: >> > > >> > > Ccing The containers list because a related discussion is >> > > happening >> > > there >> > > and somehow this thread has never made it there. >> > > >> > > Ian Kent <raven@themaw.net> writes: >> > > >> > > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: >> > > > > On 11/15, Eric W. Biederman wrote: >> > > > > > >> > > > > > I don't understand that one. Having a preforked thread with >> > > > > > the >> > > > > > proper >> > > > > > environment that can act like kthreadd in terms of spawning >> > > > > > user >> > > > > > mode >> > > > > > helpers works and is simple. >> > > > >> > > > Forgive me replying to such an old thread but ... >> > > > >> > > > After realizing workqueues can't be used to pre-create threads >> > > > to >> > > > run >> > > > usermode helpers I've returned to look at this. >> > > >> > > If someone can wind up with a good implementation I will be happy. >> > > >> > > > > Can't we ask ->child_reaper to create the non-daemonized >> > > > > kernel >> > > > > thread >> > > > > with the "right" ->nsproxy, ->fs, etc? >> > > > >> > > > Eric, do you think this approach would be sufficient too? >> > > > >> > > > Probably wouldn't be quite right for user namespaces but should >> > > > provide >> > > > what's needed for other cases? >> > > > >> > > > It certainly has the advantage of not having to maintain a >> > > > plague >> > > > of >> > > > processes waiting around to execute helpers. >> > > >> > > That certainly sounds attractive. Especially for the case of >> > > everyone >> > > who wants to set a core pattern in a container. >> > > >> > > I am fuzzy on all of the details right now, but what I do remember >> > > is >> > > that in the kernel the user mode helper concepts when they >> > > attempted >> > > to >> > > scrub a processes environment were quite error prone until we >> > > managed to >> > > get kthreadd(pid 2) on the scene which always had a clean >> > > environment. >> > > >> > > If we are going to tie this kind of thing to the pid namespace I >> > > recommend simplying denying it if you are in a user namespace >> > > without >> > > an approrpriate pid namespace. AKA simply not allowing thigns to >> > > be >> > > setup >> > > if current->pid_ns->user_ns != current->user_ns. >> > > >> > Can't be handled by simple capability like CAP_SYS_USERMODEHELPER ? I wasn't talking about a capability I was talking about how to identify where the user mode helper lives. >> > User_ns check seems not to allow core-dump-cather in host will not >> > work if user_ns is used. The bottom line is all of this approaches non-sense if user namespaces are not used. If you just have a pid namespace or a mount namespace (or perhaps both) and your fire off a new fangled user mode helper you get a deep problem. The user space process started to handle your core dump or your nfs callback will have a full set of capabilities (because it is still in the root user namespace). With a full set of capabilities and perhaps a little luck there is no containment. The imperfect solution that currently exists for the core dump helper is to provide enough information to the user space application that it can query and find out the context of the core dumping application and keep everything in that application sandbox if it so desires. I expect something similar could be done for other user mode helper style callbacks. To make starting the user space application other than how we do today needs a good argument that you are you can allow a lesser privileged process set things up and that it can be exploited to gain privielge. >> I don't think so but I'm not sure. >> >> The approach I was talking about assumes the init process of the >> caller >> (say within a container, corresponding to ->child_reaper) is an >> appropriate template for umh thread execution. >> >> But I don't think that covers the case where unshare has created >> different namespaces, like a mount namespace for example. >> >> The current workqueue sub system can't be used to pre-create a thread >> to >> be used for umh execution so, either is needs changes or yet another >> mechanism needs to be implemented. >> >> For uses other than core dumping capturing a reference to the struct >> pid >> of the environment init process and using that as an execution >> template >> should be sufficient and takes care of environment existence problems >> with some extra checks, not to mention eliminating the need for a >> potentially huge number of kernel threads needing to be created to >> provide execution templates. >> >> Where to store this and how to access it when needed is another >> problem. >> >> Not sure a usermode helper capability is the right thing either as I >> thought one important use of user namespaces was to allow unprivileged >> users to perform operations they otherwise can't. >> >> Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible .... >> >> Still an appropriate execution template would be needed and IIUC we >> can't trust getting that from within a user created namespace >> environment. > > Perhaps, if a struct cred could be captured at some appropriate time > that could be used to cater for user namespaces. > > Eric, do you think that would be possible to do without allowing users > to circumvent security? The general problem with capturing less than a full process is that we always mess it up and forget to capture something important. In a lot of ways this is a very simpilar problem to setting up an at job or a cron job. You build a script you test it then you tell at to run it at a certain time and it fails, because your working environment did not include something important that was in your actuall environment. Unfortunately in this case the failures we are talking about are container escapes and privilege escalation, so we do need to tread carefully. We might be able to safely define the context as the context of the currently running init process (Which we can identifiy with a struct pid). Justifying that looks a little trickier but doable. After a mechanism is picked it simply becomes a case of making certain your permission checks for starting something are in sync with your mechanism. Personally I am a fan of the don't be clever and capture a kernel thread approach as it is very easy to see you what if any exploitation opportunities there are. The justifications for something more clever is trickier. Of course we do something that from this perspective would be considered ``clever'' today with kthreadd and user mode helpers. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 2016/02/19 5:45, Eric W. Biederman wrote: > Personally I am a fan of the don't be clever and capture a kernel thread > approach as it is very easy to see you what if any exploitation > opportunities there are. The justifications for something more clever > is trickier. Of course we do something that from this perspective would > be considered ``clever'' today with kthreadd and user mode helpers. > I read old discussion....let me allow clarification to create a helper kernel thread to run usermodehelper with using kthreadd. 0) define a trigger to create an independent usermodehelper environment for a container. Option A) at creating some namespace (pid, uid, etc...) Option B) at creating a new nsproxy Option C).at a new systemcall is called or some sysctl, make_private_usermode_helper() or some, It's expected this should be triggered by init process of a container with some capability. And scope of the effect should be defined. pid namespace ? nsporxy ? or new namespace ? 1) create a helper thread. task = kthread_create(kthread_work_fn, ?, ?, "usermodehelper") switch task's nsproxy to current.(swtich_task_namespaces()) switch task's cgroups to current (cgroup_attach_task_all()) switch task's cred to current. copy task's capability from current (and any other ?) wake_up_process() And create a link between kthread_wq and container. 2) modify call_usermodehelper() to use kthread_worker .... It seems the problem is which object container private user mode helper should be tied to. Regards, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, 2016-02-18 at 14:45 -0600, Eric W. Biederman wrote: > Ian Kent <raven@themaw.net> writes: > > > On Thu, 2016-02-18 at 14:36 +0800, Ian Kent wrote: > > > On Thu, 2016-02-18 at 12:43 +0900, Kamezawa Hiroyuki wrote: > > > > On 2016/02/18 11:57, Eric W. Biederman wrote: > > > > > > > > > > Ccing The containers list because a related discussion is > > > > > happening > > > > > there > > > > > and somehow this thread has never made it there. > > > > > > > > > > Ian Kent <raven@themaw.net> writes: > > > > > > > > > > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > > > > > > > On 11/15, Eric W. Biederman wrote: > > > > > > > > > > > > > > > > I don't understand that one. Having a preforked thread > > > > > > > > with > > > > > > > > the > > > > > > > > proper > > > > > > > > environment that can act like kthreadd in terms of > > > > > > > > spawning > > > > > > > > user > > > > > > > > mode > > > > > > > > helpers works and is simple. > > > > > > > > > > > > Forgive me replying to such an old thread but ... > > > > > > > > > > > > After realizing workqueues can't be used to pre-create > > > > > > threads > > > > > > to > > > > > > run > > > > > > usermode helpers I've returned to look at this. > > > > > > > > > > If someone can wind up with a good implementation I will be > > > > > happy. > > > > > > > > > > > > Can't we ask ->child_reaper to create the non-daemonized > > > > > > > kernel > > > > > > > thread > > > > > > > with the "right" ->nsproxy, ->fs, etc? > > > > > > > > > > > > Eric, do you think this approach would be sufficient too? > > > > > > > > > > > > Probably wouldn't be quite right for user namespaces but > > > > > > should > > > > > > provide > > > > > > what's needed for other cases? > > > > > > > > > > > > It certainly has the advantage of not having to maintain a > > > > > > plague > > > > > > of > > > > > > processes waiting around to execute helpers. > > > > > > > > > > That certainly sounds attractive. Especially for the case of > > > > > everyone > > > > > who wants to set a core pattern in a container. > > > > > > > > > > I am fuzzy on all of the details right now, but what I do > > > > > remember > > > > > is > > > > > that in the kernel the user mode helper concepts when they > > > > > attempted > > > > > to > > > > > scrub a processes environment were quite error prone until we > > > > > managed to > > > > > get kthreadd(pid 2) on the scene which always had a clean > > > > > environment. > > > > > > > > > > If we are going to tie this kind of thing to the pid namespace > > > > > I > > > > > recommend simplying denying it if you are in a user namespace > > > > > without > > > > > an approrpriate pid namespace. AKA simply not allowing thigns > > > > > to > > > > > be > > > > > setup > > > > > if current->pid_ns->user_ns != current->user_ns. > > > > > > > > > Can't be handled by simple capability like > > > > CAP_SYS_USERMODEHELPER ? > > I wasn't talking about a capability I was talking about how to > identify > where the user mode helper lives. > > > > > User_ns check seems not to allow core-dump-cather in host will > > > > not > > > > work if user_ns is used. > > The bottom line is all of this approaches non-sense if user namespaces > are not used. If you just have a pid namespace or a mount namespace > (or > perhaps both) and your fire off a new fangled user mode helper you get > a > deep problem. The user space process started to handle your core dump > or > your nfs callback will have a full set of capabilities (because it is > still in the root user namespace). With a full set of capabilities > and perhaps a little luck there is no containment. > > The imperfect solution that currently exists for the core dump helper > is to provide enough information to the user space application that > it can query and find out the context of the core dumping application > and keep everything in that application sandbox if it so desires. > I expect something similar could be done for other user mode helper > style callbacks. > > To make starting the user space application other than how we do today > needs a good argument that you are you can allow a lesser privileged > process set things up and that it can be exploited to gain privielge. > > > > I don't think so but I'm not sure. > > > > > > The approach I was talking about assumes the init process of the > > > caller > > > (say within a container, corresponding to ->child_reaper) is an > > > appropriate template for umh thread execution. > > > > > > But I don't think that covers the case where unshare has created > > > different namespaces, like a mount namespace for example. > > > > > > The current workqueue sub system can't be used to pre-create a > > > thread > > > to > > > be used for umh execution so, either is needs changes or yet > > > another > > > mechanism needs to be implemented. > > > > > > For uses other than core dumping capturing a reference to the > > > struct > > > pid > > > of the environment init process and using that as an execution > > > template > > > should be sufficient and takes care of environment existence > > > problems > > > with some extra checks, not to mention eliminating the need for a > > > potentially huge number of kernel threads needing to be created to > > > provide execution templates. > > > > > > Where to store this and how to access it when needed is another > > > problem. > > > > > > Not sure a usermode helper capability is the right thing either as > > > I > > > thought one important use of user namespaces was to allow > > > unprivileged > > > users to perform operations they otherwise can't. > > > > > > Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible .... > > > > > > Still an appropriate execution template would be needed and IIUC > > > we > > > can't trust getting that from within a user created namespace > > > environment. > > > > Perhaps, if a struct cred could be captured at some appropriate time > > that could be used to cater for user namespaces. > > > > Eric, do you think that would be possible to do without allowing > > users > > to circumvent security? > > The general problem with capturing less than a full process is that > we always mess it up and forget to capture something important. > > In a lot of ways this is a very simpilar problem to setting up an at > job > or a cron job. You build a script you test it then you tell at to run > it at a certain time and it fails, because your working environment > did > not include something important that was in your actuall environment. > > Unfortunately in this case the failures we are talking about are > container escapes and privilege escalation, so we do need to tread > carefully. > > We might be able to safely define the context as the context of the > currently running init process (Which we can identifiy with a struct > pid). Justifying that looks a little trickier but doable. Right, that seems like a fairly straight forward thing to implement based on Olegs' example patch. I'll put together a series based on that approach. Keep in mind that the patches in my previous posts for sub-system usage are definitely wrong but I can use them (and they will be only an initial example of how to use the mechanism) to verify that contained execution happens. They will need to change. I was thinking that also capturing a struct cred (although I need to look more at the relationship between the process cred, and the nsproxy locations) at a particular time combined with a double fork and exec could allow inclusion of user namespace. Perhaps at only one level deep, ie. only allowing the first user namesapec created from init or from container and not user namespaces created from within a user namespace (if I can work out how to identify that case). Again when these are captured and how to get at them when needed is going to be a challenge. > > After a mechanism is picked it simply becomes a case of making certain > your permission checks for starting something are in sync with your > mechanism. Hopefully yourself and others can help with that, ;) > > Personally I am a fan of the don't be clever and capture a kernel > thread > approach as it is very easy to see you what if any exploitation > opportunities there are. The justifications for something more clever > is trickier. Of course we do something that from this perspective > would > be considered ``clever'' today with kthreadd and user mode helpers. Indeed, a good policy, but it seems the choice of the init process context (of a given container) is fairly straight forward and much of the tricky stuff and a good measure of checks may already be done in thread creation and exec code. As you have pointed out before this is a very difficult problem to deal with ..... Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 2016-02-19 at 12:08 +0900, Kamezawa Hiroyuki wrote: > On 2016/02/19 5:45, Eric W. Biederman wrote: > > Personally I am a fan of the don't be clever and capture a kernel > > thread > > approach as it is very easy to see you what if any exploitation > > opportunities there are. The justifications for something more > > clever > > is trickier. Of course we do something that from this perspective > > would > > be considered ``clever'' today with kthreadd and user mode helpers. > > > > I read old discussion....let me allow clarification to create a > helper kernel thread > to run usermodehelper with using kthreadd. > > 0) define a trigger to create an independent usermodehelper > environment for a container. > Option A) at creating some namespace (pid, uid, etc...) > Option B) at creating a new nsproxy > Option C).at a new systemcall is called or some sysctl, > make_private_usermode_helper() or some, > > It's expected this should be triggered by init process of a > container with some capability. > And scope of the effect should be defined. pid namespace ? nsporxy ? > or new namespace ? > > 1) create a helper thread. > task = kthread_create(kthread_work_fn, ?, ?, "usermodehelper") > switch task's nsproxy to current.(swtich_task_namespaces()) > switch task's cgroups to current (cgroup_attach_task_all()) > switch task's cred to current. > copy task's capability from current > (and any other ?) > wake_up_process() > > And create a link between kthread_wq and container. Not sure I quite understand this but I thought the difficulty with this approach previously (even though the approach was very much incomplete) was knowing that all the "moving parts" would not allow vulnerabilities. And it looks like this would require a kernel thread for each instance. So for a thousand containers that each mount an NFS mount that means, at least, 1000 additional kernel threads. Might be able to sell that, if we were lucky, but from an system administration POV it's horrible. There's also the question of existence (aka. lifetime) to deal with since the thread above needs to be created at a time other than the usermode helper callback. What happens for SIGKILL on a container? > 2) modify call_usermodehelper() to use kthread_worker > .... > > It seems the problem is which object container private user mode > helper should be tied to. > > Regards, > -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 2016/02/19 14:37, Ian Kent wrote: > On Fri, 2016-02-19 at 12:08 +0900, Kamezawa Hiroyuki wrote: >> On 2016/02/19 5:45, Eric W. Biederman wrote: >>> Personally I am a fan of the don't be clever and capture a kernel >>> thread >>> approach as it is very easy to see you what if any exploitation >>> opportunities there are. The justifications for something more >>> clever >>> is trickier. Of course we do something that from this perspective >>> would >>> be considered ``clever'' today with kthreadd and user mode helpers. >>> >> >> I read old discussion....let me allow clarification to create a >> helper kernel thread >> to run usermodehelper with using kthreadd. >> >> 0) define a trigger to create an independent usermodehelper >> environment for a container. >> Option A) at creating some namespace (pid, uid, etc...) >> Option B) at creating a new nsproxy >> Option C).at a new systemcall is called or some sysctl, >> make_private_usermode_helper() or some, >> >> It's expected this should be triggered by init process of a >> container with some capability. >> And scope of the effect should be defined. pid namespace ? nsporxy ? >> or new namespace ? >> >> 1) create a helper thread. >> task = kthread_create(kthread_work_fn, ?, ?, "usermodehelper") >> switch task's nsproxy to current.(swtich_task_namespaces()) >> switch task's cgroups to current (cgroup_attach_task_all()) >> switch task's cred to current. >> copy task's capability from current >> (and any other ?) >> wake_up_process() >> >> And create a link between kthread_wq and container. > > Not sure I quite understand this but I thought the difficulty with this > approach previously (even though the approach was very much incomplete) > was knowing that all the "moving parts" would not allow vulnerabilities. > Ok, that was discussed. > And it looks like this would require a kernel thread for each instance. > So for a thousand containers that each mount an NFS mount that means, at > least, 1000 additional kernel threads. Might be able to sell that, if we > were lucky, but from an system administration POV it's horrible. > I agree. > There's also the question of existence (aka. lifetime) to deal with > since the thread above needs to be created at a time other than the > usermode helper callback. > > What happens for SIGKILL on a container? > It depends on how the helper kthread is tied to a container related object. If kthread is linked with some namespace, we can kill it when a namespace goes away. So, with your opinion, - a helper thread should be spawned on demand - the lifetime of it should be clear. It will be good to have as same life time as the container. I wonder there is no solution for "moving part" problem other than calling do_fork() or copy_process() with container's init process context if we do all in the kernel. Is that possible ? Thanks, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 2016-02-19 at 18:30 +0900, Kamezawa Hiroyuki wrote: > On 2016/02/19 14:37, Ian Kent wrote: > > On Fri, 2016-02-19 at 12:08 +0900, Kamezawa Hiroyuki wrote: > > > On 2016/02/19 5:45, Eric W. Biederman wrote: > > > > Personally I am a fan of the don't be clever and capture a > > > > kernel > > > > thread > > > > approach as it is very easy to see you what if any exploitation > > > > opportunities there are. The justifications for something more > > > > clever > > > > is trickier. Of course we do something that from this > > > > perspective > > > > would > > > > be considered ``clever'' today with kthreadd and user mode > > > > helpers. > > > > > > > > > > I read old discussion....let me allow clarification to create a > > > helper kernel thread > > > to run usermodehelper with using kthreadd. > > > > > > 0) define a trigger to create an independent usermodehelper > > > environment for a container. > > > Option A) at creating some namespace (pid, uid, etc...) > > > Option B) at creating a new nsproxy > > > Option C).at a new systemcall is called or some sysctl, > > > make_private_usermode_helper() or some, > > > > > > It's expected this should be triggered by init process of a > > > container with some capability. > > > And scope of the effect should be defined. pid namespace ? > > > nsporxy ? > > > or new namespace ? > > > > > > 1) create a helper thread. > > > task = kthread_create(kthread_work_fn, ?, ?, "usermodehelper") > > > switch task's nsproxy to current.(swtich_task_namespaces()) > > > switch task's cgroups to current (cgroup_attach_task_all()) > > > switch task's cred to current. > > > copy task's capability from current > > > (and any other ?) > > > wake_up_process() > > > > > > And create a link between kthread_wq and container. > > > > Not sure I quite understand this but I thought the difficulty with > > this > > approach previously (even though the approach was very much > > incomplete) > > was knowing that all the "moving parts" would not allow > > vulnerabilities. > > > Ok, that was discussed. > > > And it looks like this would require a kernel thread for each > > instance. > > So for a thousand containers that each mount an NFS mount that > > means, at > > least, 1000 additional kernel threads. Might be able to sell that, > > if we > > were lucky, but from an system administration POV it's horrible. > > > I agree. > > > There's also the question of existence (aka. lifetime) to deal with > > since the thread above needs to be created at a time other than the > > usermode helper callback. > > > > What happens for SIGKILL on a container? > > First understand that the fork and workqueue code is not something I've needed to look at in the past so it's still quite new to me even now. > It depends on how the helper kthread is tied to a container related > object. > If kthread is linked with some namespace, we can kill it when a > namespace > goes away. I don't know how to do that so without knowing any better I assume it could be difficult and complicated but, of course, I don't know. > > So, with your opinion, > - a helper thread should be spawned on demand > - the lifetime of it should be clear. It will be good to have as > same life time as the container. This was always what I believed to be the best way to do it but ... Not sure you've seen the other threads on this by me so let me provide some history. I started out posting a series (totally untested, an RFC only) in the hope of finding a way to do this. After a few iterations that lead to the conclusion that a kernel thread would need to be created to provide context for subsequent helper execution (for every distinct context), much the same as we have here, and that the init process of the required context would probably be sufficient for this, required as the environment of the thread requesting helper execution itself could be used subvert execution. I ended up accepting that even if I could work out what needed to be captured and work out what needed to be done to switch to the namspace(s) and other bits that would be high maintenance as it would be fairly complicated and subsystems may be added or changed over time. Also I had assumed a singlethread workqueue would create a single thread for helper execution which was wrong. After realizing what I had was far from what's needed I went back and started reviewing the previous threads. That lead me to following a link Oleg had posted to this thread where I finally saw his suggestion about using ->child_reaper as the execution template. That really got my attention because of its simplicity and that's why I want to give that a try now and see where it leads. However user namespaces do sound like a problem even with this. Having finally got a simple test scenario I see now that the palaces I use to capture the information used to run the helper is also wrong but that's less important than getting an execution method that works, is safe, and is as simple as it can be. > > I wonder there is no solution for "moving part" problem other than > calling > do_fork() or copy_process() with container's init process context if > we do all in the kernel. Not sure I understand this but I believe that ultimately there will be the equivalent of a fork (perhaps two) and exec (we need to exec the helper anyway) no matter how this is done. For example, IIUC, a fork must be done to change pid namespace but a template like the container init process would already have that pid namespace in cases other than possibly user namespaces. I hope I understood what you were asking and haven't needlessly rambled on, ;) Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 2016-02-19 at 13:14 +0800, Ian Kent wrote: > On Thu, 2016-02-18 at 14:45 -0600, Eric W. Biederman wrote: > > Ian Kent <raven@themaw.net> writes: > > > > > On Thu, 2016-02-18 at 14:36 +0800, Ian Kent wrote: > > > > On Thu, 2016-02-18 at 12:43 +0900, Kamezawa Hiroyuki wrote: > > > > > On 2016/02/18 11:57, Eric W. Biederman wrote: > > > > > > > > > > > > Ccing The containers list because a related discussion is > > > > > > happening > > > > > > there > > > > > > and somehow this thread has never made it there. > > > > > > > > > > > > Ian Kent <raven@themaw.net> writes: > > > > > > > > > > > > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > > > > > > > > On 11/15, Eric W. Biederman wrote: > > > > > > > > > > > > > > > > > > I don't understand that one. Having a preforked > > > > > > > > > thread > > > > > > > > > with > > > > > > > > > the > > > > > > > > > proper > > > > > > > > > environment that can act like kthreadd in terms of > > > > > > > > > spawning > > > > > > > > > user > > > > > > > > > mode > > > > > > > > > helpers works and is simple. > > > > > > > > > > > > > > Forgive me replying to such an old thread but ... > > > > > > > > > > > > > > After realizing workqueues can't be used to pre-create > > > > > > > threads > > > > > > > to > > > > > > > run > > > > > > > usermode helpers I've returned to look at this. > > > > > > > > > > > > If someone can wind up with a good implementation I will be > > > > > > happy. > > > > > > > > > > > > > > Can't we ask ->child_reaper to create the non-daemonized > > > > > > > > kernel > > > > > > > > thread > > > > > > > > with the "right" ->nsproxy, ->fs, etc? > > > > > > > > > > > > > > Eric, do you think this approach would be sufficient too? > > > > > > > > > > > > > > Probably wouldn't be quite right for user namespaces but > > > > > > > should > > > > > > > provide > > > > > > > what's needed for other cases? > > > > > > > > > > > > > > It certainly has the advantage of not having to maintain a > > > > > > > plague > > > > > > > of > > > > > > > processes waiting around to execute helpers. > > > > > > > > > > > > That certainly sounds attractive. Especially for the case > > > > > > of > > > > > > everyone > > > > > > who wants to set a core pattern in a container. > > > > > > > > > > > > I am fuzzy on all of the details right now, but what I do > > > > > > remember > > > > > > is > > > > > > that in the kernel the user mode helper concepts when they > > > > > > attempted > > > > > > to > > > > > > scrub a processes environment were quite error prone until > > > > > > we > > > > > > managed to > > > > > > get kthreadd(pid 2) on the scene which always had a clean > > > > > > environment. > > > > > > > > > > > > If we are going to tie this kind of thing to the pid > > > > > > namespace > > > > > > I > > > > > > recommend simplying denying it if you are in a user > > > > > > namespace > > > > > > without > > > > > > an approrpriate pid namespace. AKA simply not allowing > > > > > > thigns > > > > > > to > > > > > > be > > > > > > setup > > > > > > if current->pid_ns->user_ns != current->user_ns. > > > > > > > > > > > Can't be handled by simple capability like > > > > > CAP_SYS_USERMODEHELPER ? > > > > I wasn't talking about a capability I was talking about how to > > identify > > where the user mode helper lives. > > > > > > > User_ns check seems not to allow core-dump-cather in host will > > > > > not > > > > > work if user_ns is used. > > > > The bottom line is all of this approaches non-sense if user > > namespaces > > are not used. If you just have a pid namespace or a mount namespace > > (or > > perhaps both) and your fire off a new fangled user mode helper you > > get > > a > > deep problem. The user space process started to handle your core > > dump > > or > > your nfs callback will have a full set of capabilities (because it > > is > > still in the root user namespace). With a full set of capabilities > > and perhaps a little luck there is no containment. > > > > The imperfect solution that currently exists for the core dump > > helper > > is to provide enough information to the user space application that > > it can query and find out the context of the core dumping > > application > > and keep everything in that application sandbox if it so desires. > > I expect something similar could be done for other user mode helper > > style callbacks. > > > > To make starting the user space application other than how we do > > today > > needs a good argument that you are you can allow a lesser privileged > > process set things up and that it can be exploited to gain > > privielge. > > > > > > I don't think so but I'm not sure. > > > > > > > > The approach I was talking about assumes the init process of the > > > > caller > > > > (say within a container, corresponding to ->child_reaper) is an > > > > appropriate template for umh thread execution. > > > > > > > > But I don't think that covers the case where unshare has created > > > > different namespaces, like a mount namespace for example. > > > > > > > > The current workqueue sub system can't be used to pre-create a > > > > thread > > > > to > > > > be used for umh execution so, either is needs changes or yet > > > > another > > > > mechanism needs to be implemented. > > > > > > > > For uses other than core dumping capturing a reference to the > > > > struct > > > > pid > > > > of the environment init process and using that as an execution > > > > template > > > > should be sufficient and takes care of environment existence > > > > problems > > > > with some extra checks, not to mention eliminating the need for > > > > a > > > > potentially huge number of kernel threads needing to be created > > > > to > > > > provide execution templates. > > > > > > > > Where to store this and how to access it when needed is another > > > > problem. > > > > > > > > Not sure a usermode helper capability is the right thing either > > > > as > > > > I > > > > thought one important use of user namespaces was to allow > > > > unprivileged > > > > users to perform operations they otherwise can't. > > > > > > > > Maybe a CAP_SYS_USERNSCOREDUMP or similar would be sensible .... > > > > > > > > Still an appropriate execution template would be needed and IIUC > > > > we > > > > can't trust getting that from within a user created namespace > > > > environment. > > > > > > Perhaps, if a struct cred could be captured at some appropriate > > > time > > > that could be used to cater for user namespaces. > > > > > > Eric, do you think that would be possible to do without allowing > > > users > > > to circumvent security? > > > > The general problem with capturing less than a full process is that > > we always mess it up and forget to capture something important. > > > > In a lot of ways this is a very simpilar problem to setting up an at > > job > > or a cron job. You build a script you test it then you tell at to > > run > > it at a certain time and it fails, because your working environment > > did > > not include something important that was in your actuall > > environment. > > > > Unfortunately in this case the failures we are talking about are > > container escapes and privilege escalation, so we do need to tread > > carefully. > > > > We might be able to safely define the context as the context of the > > currently running init process (Which we can identifiy with a struct > > pid). Justifying that looks a little trickier but doable. > > Right, that seems like a fairly straight forward thing to implement > based on Olegs' example patch. > > I'll put together a series based on that approach. > > Keep in mind that the patches in my previous posts for sub-system > usage > are definitely wrong but I can use them (and they will be only an > initial example of how to use the mechanism) to verify that contained > execution happens. They will need to change. > > I was thinking that also capturing a struct cred (although I need to > look more at the relationship between the process cred, and the > nsproxy > locations) at a particular time combined with a double fork and exec > could allow inclusion of user namespace. > > Perhaps at only one level deep, ie. only allowing the first user > namesapec created from init or from container and not user namespaces > created from within a user namespace (if I can work out how to > identify > that case). You know, wrt. the mechanism Oleg suggested, I've been wondering if it's even necessary to capture process template information for execution. Isn't the main issue the execution of unknown arbitrary objects getting access to a privileged context? Then perhaps it is sufficient to require registration of an SHA hash (of some sort) for these objects by a suitably privileged process and only allow helper execution of valid objects. If that is sufficient then helper execution from within a container or user namespace could just use the callers environment itself. What else do we need to be wary of, any thoughts Eric? Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Feb 23, 2016 at 10:55:30AM +0800, Ian Kent wrote: > You know, wrt. the mechanism Oleg suggested, I've been wondering if it's > even necessary to capture process template information for execution. > > Isn't the main issue the execution of unknown arbitrary objects getting > access to a privileged context? > > Then perhaps it is sufficient to require registration of an SHA hash (of > some sort) for these objects by a suitably privileged process and only > allow helper execution of valid objects. That executable probably also depends on libraries, services, and tons of other miscellaneous stuff in its environment. The NFSv4 client idmapper, for example, may be doing ldap calls. Unless the helper is created with incredible care, I don't think that it's enough just to verify that you're executing the correct helper. --b. > > If that is sufficient then helper execution from within a container or > user namespace could just use the callers environment itself. > > What else do we need to be wary of, any thoughts Eric? > > Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, 2016-02-23 at 09:36 -0500, J. Bruce Fields wrote: > On Tue, Feb 23, 2016 at 10:55:30AM +0800, Ian Kent wrote: > > You know, wrt. the mechanism Oleg suggested, I've been wondering if > > it's > > even necessary to capture process template information for > > execution. > > > > Isn't the main issue the execution of unknown arbitrary objects > > getting > > access to a privileged context? > > > > Then perhaps it is sufficient to require registration of an SHA hash > > (of > > some sort) for these objects by a suitably privileged process and > > only > > allow helper execution of valid objects. > > That executable probably also depends on libraries, services, and tons > of other miscellaneous stuff in its environment. The NFSv4 client > idmapper, for example, may be doing ldap calls. Unless the helper is > created with incredible care, I don't think that it's enough just to > verify that you're executing the correct helper. Yeah, I was thinking the logistics of keeping something like this up to date would be hard but calculating this for every call would be too much overhead I think. > > --b. > > > > > If that is sufficient then helper execution from within a container > > or > > user namespace could just use the callers environment itself. > > > > What else do we need to be wary of, any thoughts Eric? > > > > Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > On 11/15, Eric W. Biederman wrote: > > > > I don't understand that one. Having a preforked thread with the > > proper > > environment that can act like kthreadd in terms of spawning user > > mode > > helpers works and is simple. > > Can't we ask ->child_reaper to create the non-daemonized kernel thread > with the "right" ->nsproxy, ->fs, etc? > > IOW. Please the the "patch" below. It is obviously incomplete and > wrong, > and it can be more clear/clean. And probably we need another API. Just > to explain what I mean. > > With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec > from the caller's namespace. Umm ... I don't think this can work. I don't think it can be assumed that the init process of a container will behave like an init process. If you try and do this with a Docker container that has /bin/bash as the init process signals never arrive and work doesn't start until some other signal arrives at which time it fails to create the kernel thread returning an error ERESTARTNOINTER (IIRC). In fact a number of other things relating to signalling processes to cleanly shutdown in a container suffer the same problem. I probably don't understand what's actually going on, this is just my impression of what I'm seeing. > > Oleg. > --- > > --- a/include/linux/kmod.h > +++ b/include/linux/kmod.h > @@ -24,6 +24,7 @@ > #include <linux/errno.h> > #include <linux/compiler.h> > #include <linux/workqueue.h> > +#include <linux/task_work.h> > #include <linux/sysctl.h> > > #define KMOD_PATH_LEN 256 > @@ -53,8 +54,14 @@ struct file; > #define UMH_WAIT_PROC 2 /* wait for the process to > complete */ > #define UMH_KILLABLE 4 /* wait for EXEC/PROC killable > */ > > +// FIXME: IMH_* is not actually a mask > +#define UMH_IN_MY_NS 8 > + > struct subprocess_info { > - struct work_struct work; > + union { > + struct work_struct work; > + struct callback_head twork; > + }; > struct completion *complete; > char *path; > char **argv; > --- a/kernel/kmod.c > +++ b/kernel/kmod.c > @@ -541,7 +541,6 @@ struct subprocess_info > *call_usermodehelper_setup(char *path, char **argv, > if (!sub_info) > goto out; > > - INIT_WORK(&sub_info->work, __call_usermodehelper); > sub_info->path = path; > sub_info->argv = argv; > sub_info->envp = envp; > @@ -554,6 +553,24 @@ struct subprocess_info > *call_usermodehelper_setup(char *path, char **argv, > } > EXPORT_SYMBOL(call_usermodehelper_setup); > > +static int call_call_usermodehelper(void *twork) > +{ > + struct subprocess_info *sub_info = > + container_of(twork, struct subprocess_info, twork); > + > + __call_usermodehelper(&sub_info->work); > + do_exit(0); > + > +} > + > +static void fork_umh_helper(struct callback_head *twork) > +{ > + if (current->flags & PF_EXITING) > + return; // WRONG, FIXME > + > + kernel_thread(call_call_usermodehelper, twork, SIGCHLD); > +} > + > /** > * call_usermodehelper_exec - start a usermode application > * @sub_info: information about the subprocessa > @@ -570,6 +587,10 @@ int call_usermodehelper_exec(struct > subprocess_info *sub_info, int wait) > { > DECLARE_COMPLETION_ONSTACK(done); > int retval = 0; > + bool in_my_ns; > + > + in_my_ns = wait & UMH_IN_MY_NS; > + wait &= ~UMH_IN_MY_NS; > > if (!sub_info->path) { > call_usermodehelper_freeinfo(sub_info); > @@ -594,7 +615,21 @@ int call_usermodehelper_exec(struct > subprocess_info *sub_info, int wait) > sub_info->complete = &done; > sub_info->wait = wait; > > - queue_work(khelper_wq, &sub_info->work); > + if (likely(!in_my_ns)) { > + INIT_WORK(&sub_info->work, __call_usermodehelper); > + queue_work(khelper_wq, &sub_info->work); > + } else { > + // RACY, WRONG, ETC > + struct task_struct *my_init = > task_active_pid_ns(current)->child_reaper; > + > + init_task_work(&sub_info->twork, fork_umh_helper); > + task_work_add(my_init, &sub_info->twork, false); > + > + // until we have task_work_add_interruptibel() > + do_send_sig_info(SIGCHLD, SEND_SIG_FORCED, my_init, > false); > + > + } > + > if (wait == UMH_NO_WAIT) /* task has freed sub_info */ > goto unlock; > > > -- > To unsubscribe from this list: send the line "unsubscribe linux > -fsdevel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Hi Ian, I can't really recall this old discussion, so I can be easily wrong... On 03/24, Ian Kent wrote: > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > > > > IOW. Please the the "patch" below. It is obviously incomplete and > > wrong, > > and it can be more clear/clean. And probably we need another API. Just > > to explain what I mean. I hope you didn't miss this part ;) In particular, we want to turn task_work_add(..., bool notify) into task_work_add(..., how_to_notify mask) and this "mask" should allow to force TIF_SIGPENDING. > > With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do exec > > from the caller's namespace. > > Umm ... I don't think this can work. > > I don't think it can be assumed that the init process of a container > will behave like an init process. > > If you try and do this with a Docker container that has /bin/bash as the > init process signals never arrive and work doesn't start until some > other signal arrives only if it blocks/ignores SIGCHLD? But this doesn't matter, see above and note the "until we have task_work_add_interruptibel()" in the pseudo-code I showed. > I probably don't understand what's actually going on, this is just my > impression of what I'm seeing. Or perhaps it is me who misunderstands your concerns. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 2016-03-25 at 02:28 +0100, Oleg Nesterov wrote: > Hi Ian, > > I can't really recall this old discussion, so I can be easily wrong... > > On 03/24, Ian Kent wrote: > > > > On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote: > > > > > > IOW. Please the the "patch" below. It is obviously incomplete and > > > wrong, > > > and it can be more clear/clean. And probably we need another API. > > > Just > > > to explain what I mean. > > I hope you didn't miss this part ;) Not at all. > > In particular, we want to turn task_work_add(..., bool notify) into > task_work_add(..., how_to_notify mask) and this "mask" should allow > to force TIF_SIGPENDING. The point of posting the reply was to try and get some advice as my understanding of the signalling subsystem is fairly poor. LOL, I'll have another look at the task_work_add() code and see if I can understand what your trying to tell me. > > > > With this patch call_usermodehelper(..., UMH_IN_MY_NS) should do > > > exec > > > from the caller's namespace. > > > > Umm ... I don't think this can work. > > > > I don't think it can be assumed that the init process of a container > > will behave like an init process. > > > > If you try and do this with a Docker container that has /bin/bash as > > the > > init process signals never arrive and work doesn't start until some > > other signal arrives > > only if it blocks/ignores SIGCHLD? But this doesn't matter, see above > and > note the "until we have task_work_add_interruptibel()" in the pseudo > -code > I showed. It seems, and this is not the only case I've encountered, that the init process in docker containers can be a problem when you want to capture and handle signals. I've seen this with /bin/bash and supervisord so far. I don't know if it is the docker container creation doing this or something else .... certainly I can catch signals within subordinate processes. The other thing that occurs to me is that just about anything in a container could be subverted so the definition of a privileged process which can be used as a template form execution is essentially undefined. Mmm ... maybe I've got that wrong too, ;) Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
--- a/include/linux/kmod.h +++ b/include/linux/kmod.h @@ -24,6 +24,7 @@ #include <linux/errno.h> #include <linux/compiler.h> #include <linux/workqueue.h> +#include <linux/task_work.h> #include <linux/sysctl.h> #define KMOD_PATH_LEN 256 @@ -53,8 +54,14 @@ struct file; #define UMH_WAIT_PROC 2 /* wait for the process to complete */ #define UMH_KILLABLE 4 /* wait for EXEC/PROC killable */ +// FIXME: IMH_* is not actually a mask +#define UMH_IN_MY_NS 8 + struct subprocess_info { - struct work_struct work; + union { + struct work_struct work; + struct callback_head twork; + }; struct completion *complete; char *path; char **argv; --- a/kernel/kmod.c +++ b/kernel/kmod.c @@ -541,7 +541,6 @@ struct subprocess_info *call_usermodehelper_setup(char *path, char **argv, if (!sub_info) goto out; - INIT_WORK(&sub_info->work, __call_usermodehelper); sub_info->path = path; sub_info->argv = argv; sub_info->envp = envp; @@ -554,6 +553,24 @@ struct subprocess_info *call_usermodehelper_setup(char *path, char **argv, } EXPORT_SYMBOL(call_usermodehelper_setup); +static int call_call_usermodehelper(void *twork) +{ + struct subprocess_info *sub_info = + container_of(twork, struct subprocess_info, twork); + + __call_usermodehelper(&sub_info->work); + do_exit(0); + +} + +static void fork_umh_helper(struct callback_head *twork) +{ + if (current->flags & PF_EXITING) + return; // WRONG, FIXME + + kernel_thread(call_call_usermodehelper, twork, SIGCHLD); +} + /** * call_usermodehelper_exec - start a usermode application * @sub_info: information about the subprocessa @@ -570,6 +587,10 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) { DECLARE_COMPLETION_ONSTACK(done); int retval = 0; + bool in_my_ns; + + in_my_ns = wait & UMH_IN_MY_NS; + wait &= ~UMH_IN_MY_NS; if (!sub_info->path) { call_usermodehelper_freeinfo(sub_info); @@ -594,7 +615,21 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) sub_info->complete = &done; sub_info->wait = wait; - queue_work(khelper_wq, &sub_info->work); + if (likely(!in_my_ns)) { + INIT_WORK(&sub_info->work, __call_usermodehelper); + queue_work(khelper_wq, &sub_info->work); + } else { + // RACY, WRONG, ETC + struct task_struct *my_init = task_active_pid_ns(current)->child_reaper; + + init_task_work(&sub_info->twork, fork_umh_helper); + task_work_add(my_init, &sub_info->twork, false); + + // until we have task_work_add_interruptibel() + do_send_sig_info(SIGCHLD, SEND_SIG_FORCED, my_init, false); + + } + if (wait == UMH_NO_WAIT) /* task has freed sub_info */ goto unlock;