Messages in this thread | | | Date | Sat, 23 Aug 2014 17:32:22 +0200 | From | Oleg Nesterov <> | Subject | Re: + prctl-pr_set_mm-introduce-pr_set_mm_map-operation-v3.patch added to -mm tree |
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On 08/23, Cyrill Gorcunov wrote: > > On Sat, Aug 23, 2014 at 03:30:01PM +0200, Oleg Nesterov wrote: > > > > On 08/23, Oleg Nesterov wrote: > > > > > > On 08/23, Cyrill Gorcunov wrote: > > > > > > > Looks like I need > > > > to use cred_guard_mutex instead of task_lock here, no? > > > > > > Please don't. First of all, it can't help because proc_pid_auxv() doesn't hold > > > this lock. It does mm_access() which drops this lock after return. And to remind, > > > we are going to remove mm_access/lock_trace from sys_read() paths in proc. > > > > Besides, it can't help anyway. cred_guard_mutex is per-process (not per-thread), > > suppose that a vfork()'ed child does prctl() while another thread reads the > > parent's /proc/pid/auxv. > > Then either I need to use some other lock (not sure which one) either leave it > completely unlocked mentionin in the man page such lockless behaviour. Thoughts?
Personally I think "lockless" is the best choice (not sure man page should know about this detail). I mean, I think that we do not care if proc_pid_auxv() prints garbage if it races with ptctl().
Otherwise we have to use mmap_sem in proc_pid_auxv(), doesn't look nice.
> | > Stricktly speaking yes, but don't forget we might need to update > | > exe::file as well which requires lock to be taken. > | > | For reading? I see prctl_set_mm_exe_file_locked() in this patch, probably > | this function was added by another patch. But, if this function calls > | set_mm_exe_file() (I guess it does?) then down_read() is not enough? > | set_mm_exe_file() can race with itself. > > yes, for reading, look in set_mm_exe_file we lookup for vma which should > be not present when we change the link, and yes, because of read-only lock > this call can race but only one caller success there because we allow > to change exe link only once.
Ah, I forgot about MMF_EXE_FILE_CHANGED, thanks for correcting me.
(btw I think this check must die too, but this is off-topic and I was wrong anyway).
OK, but I still think down_read(mmap_sem) is not enough. get_mm_exe_file() can do get_file() after prctl() paths do the final fput().
Or please look at tomoyo_get_exe(). Another thread can play with mm->exe_file fput().
Plus I am a bit worrried about inode_permission() under mmap_sem... but this is probably fine. Although you can never know which locks a creative filesystem/security module can take ;) But probably this is fine.
> | But for what? Ignoring the (I think buggy) check in do_shmat() ->start_stack > | is simply unused, we only report it via /proc/. The same for, say, mm->start_code. > > that't the good question if this check in do_shmat is buggy or not, why do > you think it's a bug there?
Please see the patch I sent.
> Oleg, letme summarize all the concerns maybe there would be a way to handle > them gracefully > > 1) How code flows for now (with all fixes on top of current Andrew's queued patches) > > - obtain struct prctl_mm_map from userspace > - copy saved_auxv from userspace > - down-read mmap_sem > - validate all the data passed from userspace
I won't argue, but at least mmap_min/max_addr do not need mmap_sem.
> - we need a reference to stack-vma for RLIMIT_STACK check (this is doable, > as you said, but until we drop the RLIMIT_STACK from do_shmat I would > prefer it to be here)
OK, I won't argue, but I think this is pointless and misleading.
And btw, where do you see RLIMIT_STACK in do_shmat() ?
> - we need to be sure that start_brk, brk,
probably yes, simply because the kernel actually uses this members.
> arg_start, arg_end, env_start, env_end > really point to existing VMAs, strictly speaking the probgram can unmap > all own VMAs except executable one and continue running without problem > but this is not that practical I think and at first iteration I prefer > more severe tests here on VMAs
But, again, for what? There are only used to report this info via /proc/.
> - setup new mm::exe_file (we need to be sure the old exe_file is unmapped > so mmap_sem read-lock is needed)
See above.
> Oleg, check please if I undersnad you correctly, you propose > > - drop off mmap_sem completely
No, no, I didn't, we obviously can't do this.
> - don't verify for RLIMIT_STACK
Yes, and more "don't verify". But again, I won't really argue. Just in my opinion almost all these checks looks misleading, confusing, and unnecessary.
Please think about those who will try to understand this code. A little comment like "this is not needed but we all are paranoid in openvz" could make it a bit more understandable ;)
> - drop off task_lock when updating mm::saved_auxv but still invent > how to prevent update/read race
Personally I think we can simply ignore this race.
But let me repeat, I won't argue with any approach as long as I think it is fine correctness wise.
Oleg.
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