From: ebiederm@xmission.com (Eric W. Biederman)
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: david@lang.hm, "H. Peter Anvin" <hpa@zytor.com>,
Alexey Dobriyan <adobriyan@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Pavel Emelyanov <xemul@parallels.com>,
Andrey Vagin <avagin@openvz.org>, Ingo Molnar <mingo@elte.hu>,
Thomas Gleixner <tglx@linutronix.de>,
Glauber Costa <glommer@parallels.com>,
Andi Kleen <andi@firstfloor.org>, Tejun Heo <tj@kernel.org>,
Matt Helsley <matthltc@us.ibm.com>,
Pekka Enberg <penberg@kernel.org>,
Eric Dumazet <eric.dumazet@gmail.com>,
Vasiliy Kulikov <segoon@openwall.com>,
Andrew Morton <akpm@linux-foundation.org>,
Valdis.Kletnieks@vt.edu
Subject: Re: [RFC] syscalls, x86: Add __NR_kcmp syscall
Date: Thu, 19 Jan 2012 19:16:02 -0800 [thread overview]
Message-ID: <m1ty3rukr1.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20120119065541.GA31379@moon> (Cyrill Gorcunov's message of "Thu, 19 Jan 2012 10:55:41 +0400")
Cyrill Gorcunov <gorcunov@gmail.com> writes:
> On Wed, Jan 18, 2012 at 03:29:50PM -0800, Eric W. Biederman wrote:
>> >
>> > It doesn't matter. Even if we take a list of objects the kernel either
>> > should return us some ordering info or find duplicates, in any case it
>> > makes things more complex i think. So we wanted to bring some minimum
>> > into kernel leaving the rest of work to user-space.
>>
>> Agreed a syscall does the duplication is probably not the way to go.
>>
>> A syscall that takes a huge list of objects would solve any security
>> concerns that we have with returning the object order to user space if
>> done carefully, but it would require a bunch of additional user space
>> and kernel memory.
>>
>
> yes, an it increase syscall time itself since we will have to provide
> this memory dynamically
I just did a back of the napkin calculation.
struct kobj_id {
pid_t pid;
size_t descriptor;
size_t first_idx;
void *kernel_ignore_this_pointer;
};
int find_kobject_dups(int type, struct kobj_id __user *ids, size_t count);
Looks pretty reasonable on a 64bit machine for 100,000 file
descriptors.
3 Meg of input data.
4 Meg of an internal rbtree that remembers the first entry where
we saw an item.
struct {
struct rb_node node;
void *key;
size_t idx;
};
And the code is very straight forward. Insert each pointer to a kernel
object we find into an rbtree, and return the index we find. Then
finally tear down the rbtree.
8Meg worst case does not seem like a lot of memory to me. Especially
since half of it is userspace memory.
A simple implementation plus a guarantee that we will never ever
leak information that we don't intend to seem very attractive to me.
>> Sometimes taking a data structure transforming it into a weird form for
>> a specific task and then transforming the data structure back to it's
>> original form is a useful way to go. So I think a general kernel object
>> deduplicating system call is an interesting plan B, but a straight
>> comparison function if we can make it work is a lot more flexible and
>> useful.
>>
>
> I hope the root-only restriction would resolve the potential security
> problem, since as I mentioned if I've hijacked the machine and already
> goot root -- mem order is not that interesting info I could obtain from
> such computer :)
We either need a full comparison operator or we don't. Root-only is a
solution just looking to get abused.
Eric
next prev parent reply other threads:[~2012-01-20 3:13 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-17 14:27 [RFC] syscalls, x86: Add __NR_kcmp syscall Cyrill Gorcunov
2012-01-17 14:38 ` Alexey Dobriyan
2012-01-17 14:44 ` Cyrill Gorcunov
2012-01-17 18:47 ` H. Peter Anvin
2012-01-17 21:15 ` Cyrill Gorcunov
2012-01-17 21:40 ` Eric W. Biederman
2012-01-18 5:07 ` Pavel Emelyanov
2012-01-17 21:35 ` Eric W. Biederman
2012-01-18 8:01 ` Cyrill Gorcunov
2012-01-18 9:12 ` KOSAKI Motohiro
2012-01-18 9:19 ` Pavel Emelyanov
2012-01-18 9:23 ` KOSAKI Motohiro
2012-01-18 11:57 ` Cyrill Gorcunov
2012-01-18 16:46 ` KOSAKI Motohiro
2012-01-18 17:20 ` Cyrill Gorcunov
2012-01-18 22:05 ` david
2012-01-18 22:49 ` Cyrill Gorcunov
2012-01-18 23:29 ` Eric W. Biederman
2012-01-19 6:55 ` Cyrill Gorcunov
2012-01-20 3:16 ` Eric W. Biederman [this message]
2012-01-20 8:40 ` Cyrill Gorcunov
2012-01-20 9:02 ` Cyrill Gorcunov
2012-01-20 14:51 ` H. Peter Anvin
2012-01-20 16:29 ` Cyrill Gorcunov
2012-01-20 16:57 ` H. Peter Anvin
2012-01-20 18:19 ` Cyrill Gorcunov
2012-01-20 18:22 ` Cyrill Gorcunov
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