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Subject: Re: ipc/msg: zalloc struct msg_queue when creating a new msq
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        syzbot <syzbot+2827ef6b3385deb07eaf@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>, linux@dominikbrodowski.net,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Eric Dumazet <edumazet@google.com>
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From:   Manfred Spraul <manfred@colorfullife.com>
Message-ID: <b9ffe11e-c63f-c5fb-f5e8-fc4eb8c4f0fa@colorfullife.com>
Date:   Wed, 4 Jul 2018 16:08:33 +0200
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Hello Dmitry,
On 07/04/2018 12:03 PM, Dmitry Vyukov wrote:
> On Wed, Jul 4, 2018 at 11:18 AM, Manfred Spraul
> <manfred@colorfullife.com> wrote:
>>
>> There are 2 relevant values: kern_ipc_perm.id and kern_ipc_perm.seq.
>>
>> For kern_ipc_perm.id, it is possible to move the access to the codepath that
>> hold the lock.
>>
>> For kern_ipc_perm.seq, there are two options:
>> 1) set it before publication.
>> 2) initialize to an invalid value, and correct that at the end.
>>
>> I'm in favor of option 2, it avoids that we must think about reducing the
>> next sequence number or not:
>>
>> The purpose of the sequence counter is to minimize the risk that e.g. a
>> semop() will write into a newly created array.
>> I intentially write "minimize the risk", as it is by design impossible to
>> guarantee that this cannot happen, e.g. if semop() sleeps at the instruction
>> before the syscall.
>>
>> Therefore, we can set seq to ULONG_MAX, then ipc_checkid() will always fail
>> and the corruption is avoided.
>>
>> What do you think?
>>
>> And, obviously:
>> Just set seq to 0 is dangerous, as the first allocated sequence number is 0,
>> and if that object is destroyed, then the newly created object has
>> temporarily sequence number 0 as well.
> Hi Manfred,
>
> It still looks fishy to me. This code published uninitialized uid's
> for years (which lead not only to accidentally accessing wrong
> objects, but also to privilege escalation). Now it publishes uninit
> id/seq. The first proposed fix still did not make it correct. I can't
> say that I see a but in your patch, but initializing id/seq in a racy
> manner rings bells for me. Say, if we write/read seq ahead of id, can
> reader still get access to a wrong object?
> It all suggests some design flaw to me. Could ipc_idr_alloc() do full
> initialization, i.e. also do what ipc_buildid() does? This would
> ensure that we publish a fully constructed object in the first place.
> We already have cleanup for ipc_idr_alloc(), which is idr_remove(), so
> if we care about seq space conservation even in error conditions
> (ENOMEM?), idr_remove() could accept an additional flag saying "this
> object should not have been used by sane users yet, so retake its
> seq". Did I get your concern about seq properly?
You have convinced me, I'll rewrite the patch:

1) kern_ipc_perm.seq should be accessible under rcu_read_lock(), this 
means replacing ipc_build_id() with two functions:
One that initializes kern_ipc_perm.seq, and one that would set 
kern_ipc_perm.id.
2) the accesses to kern_ipc_perm.id must be moved to the position where 
the lock is held. This is trivial.
3) we need a clear table that describes which variables can be accessed 
under rcu_read_lock() and which need ipc_lock_object().
   e.g.: kern_ipc_perm.id would end up under ipc_lock_object, 
kern_ipc_perm.seq or the xuid fields can be read under rcu_read_lock().
   Everything that can be accessed without ipc_lock_object must be 
initialized before publication of a new object.

Or, as all access to kern_ipc_perm.id are in rare codepaths:
I'll remove kern_ipc_perm.id entirely, and build the id on demand.

Ok?

--
     Manfred