From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
Andrew Morton <akpm@linux-foundation.org>,
Kees Cook <keescook@chromium.org>,
David Howells <dhowells@redhat.com>,
linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Rik van Riel <riel@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>,
Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Subject: Re: [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm
Date: Wed, 12 Aug 2015 13:39:27 -0500 [thread overview]
Message-ID: <87614k73mo.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20150812174847.GA6703@redhat.com> (Oleg Nesterov's message of "Wed, 12 Aug 2015 19:48:47 +0200")
Oleg Nesterov <oleg@redhat.com> writes:
> On 08/11, Eric W. Biederman wrote:
>>
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1866,13 +1866,17 @@ static int check_unshare_flags(unsigned long unshare_flags)
>> CLONE_NEWUSER|CLONE_NEWPID))
>> return -EINVAL;
>> /*
>> - * Not implemented, but pretend it works if there is nothing to
>> - * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
>> - * needs to unshare vm.
>> + * Not implemented, but pretend it works if there is nothing
>> + * to unshare. Note that unsharing the address space or the
>> + * signal handlers also need to unshare the signal queues (aka
>> + * CLONE_THREAD).
>> */
>> if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
>> - /* FIXME: get_task_mm() increments ->mm_users */
>> - if (atomic_read(¤t->mm->mm_users) > 1)
>> + if (!thread_group_empty(current))
>> + return -EINVAL;
>> + }
>> + if (unshare_flags & CLONE_VM) {
>> + if (!current_is_single_threaded())
>> return -EINVAL;
>> }
>
> OK, but then you can remove "| CLONE_VM" from the previous check...
As an optimization, but I don't think anything cares enough for the
optimization to be worth the confusion.
>> @@ -1941,16 +1945,16 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
>> if (unshare_flags & CLONE_NEWUSER)
>> unshare_flags |= CLONE_THREAD | CLONE_FS;
>> /*
>> - * If unsharing a thread from a thread group, must also unshare vm.
>> - */
>> - if (unshare_flags & CLONE_THREAD)
>> - unshare_flags |= CLONE_VM;
>
> OK,
>
>> /*
>> + * If unsharing a signal handlers, must also unshare the signal queues.
>> + */
>> + if (unshare_flags & CLONE_SIGHAND)
>> + unshare_flags |= CLONE_THREAD;
>
> This looks unnecessary, check_unshare_flags() checks "THREAD | SIGHAND".
> And to me the comment looks misleading although I won't argue.
I absolutely can not understand this code if we jump 5 steps ahead
and optimize out the individual dependencies, and try for a flattened
dependency tree instead. I can validate the individual dependencies
from first principles.
If we jump several steps ahead I can not validate the individual
dependencies.
It really is important to say if you want your own private struct
sighand_struct, you also need to have your own private struct
signal_struct.
> And in fact this doesn't look exactly right, or I am totally confused.
> Shouldn't we do
>
> if (unshare_flags & CLONE_SIGHAND)
> unshare_flags |= CLONE_VM;
Nope. The backward definitions of the flags in unshare has gotten you.
CLONE_SIGHAND means that you want a struct sighand_struct with a count
of 1. Nothing about a sighand_struct with a count of 1 implies or
requires mm_users == 1. clone can quite happily create those.
> ? Or change check_unshare_flags()...
>
> Otherwise suppose that a single threaded process does clone(VM | SIGHAND)
> and (say) child does sys_unshare(SIGHAND). This will wrongly succeed
> afaics.
Why would it be wrong to succeed in that case? struct sighand_struct
has a count of 1. unshare(CLONE_SIGHAND) requests a sighand_struct with
a count of 1.
I expect part of the confusion is the code in unshare has been wrongly
requiring an unshared vm to support a sighand_struct with a count of 1
since the day the code was merged.
Ugh. This patch has a bug where we don't check for sighand->count == 1.
clone(VM) ---> mm_users = 2 sighand->count == 1 signal->live == 1
clone(VM|SIGHAND) --> mm_users = 2 sighand->count == 2 signal->live == 1
unshare(SIGHAND) needs to guarantee that when it returns sighand->count == 1.
So unshare(SIGHAND) needs to test for sighand->count == 1.
Ugh. Untangling this ancient mess is a pain. One more pass at this
patch it seems.
Eric
next prev parent reply other threads:[~2015-08-12 18:46 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-28 17:15 [PATCH] user_ns: use correct check for single-threadedness Kees Cook
2015-07-28 18:02 ` Rik van Riel
2015-07-28 18:17 ` Eric W. Biederman
2015-07-28 20:55 ` Ricky Zhou
2015-07-28 21:01 ` Kees Cook
2015-08-05 18:13 ` Eric W. Biederman
2015-08-05 19:40 ` Kees Cook
2015-07-28 21:35 ` Andrew Morton
2015-07-28 21:50 ` Kees Cook
2015-07-28 22:11 ` Kirill A. Shutemov
2015-08-05 11:38 ` Ingo Molnar
2015-08-05 11:53 ` Kirill A. Shutemov
2015-08-05 13:13 ` Ricky Zhou
2015-08-05 17:23 ` Oleg Nesterov
2015-08-05 18:00 ` Eric W. Biederman
2015-08-05 18:52 ` Eric W. Biederman
2015-08-06 13:06 ` Oleg Nesterov
2015-08-06 13:44 ` Oleg Nesterov
2015-08-12 1:17 ` Eric W. Biederman
2015-08-12 14:40 ` Oleg Nesterov
2015-08-12 15:11 ` Eric W. Biederman
2015-08-12 1:22 ` [PATCH 0/2] userns: Creation logic fixes Eric W. Biederman
2015-08-12 1:24 ` [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm Eric W. Biederman
2015-08-12 17:48 ` Oleg Nesterov
2015-08-12 18:39 ` Eric W. Biederman [this message]
2015-08-13 12:55 ` Oleg Nesterov
2015-08-13 15:38 ` Eric W. Biederman
2015-08-13 16:17 ` Oleg Nesterov
2015-08-13 16:27 ` Eric W. Biederman
2015-08-13 16:50 ` Oleg Nesterov
2015-08-14 17:59 ` Oleg Nesterov
2015-08-12 19:59 ` [PATCH v2] " Eric W. Biederman
2015-08-13 12:57 ` Oleg Nesterov
2015-08-13 16:01 ` Eric W. Biederman
2015-08-13 16:30 ` Oleg Nesterov
2015-08-13 16:39 ` Eric W. Biederman
2015-08-12 1:25 ` [PATCH 2/2] userns,pidns: Force thread group sharing, not signal handler sharing Eric W. Biederman
2015-08-12 17:24 ` Oleg Nesterov
2015-08-12 6:29 ` [PATCH 0/2] userns: Creation logic fixes Kees Cook
2015-08-06 14:35 ` [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov
2015-08-06 21:16 ` Eric W. Biederman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87614k73mo.fsf@x220.int.ebiederm.org \
--to=ebiederm@xmission.com \
--cc=akpm@linux-foundation.org \
--cc=dhowells@redhat.com \
--cc=jln@google.com \
--cc=keescook@chromium.org \
--cc=kirill.shutemov@linux.intel.com \
--cc=kirill@shutemov.name \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=oleg@redhat.com \
--cc=peterz@infradead.org \
--cc=rickyz@chromium.org \
--cc=riel@redhat.com \
--cc=vdavydov@parallels.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).