Re: ipc/msg: zalloc struct msg_queue when creating a new msq

[Manfred Spraul]

Hello Dmitry,
On 07/04/2018 12:03 PM, Dmitry Vyukov wrote:
> On Wed, Jul 4, 2018 at 11:18 AM, Manfred Spraul
> <manfred@xxxxxxxxxxxxxxxx> 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?

--
ÂÂÂ Manfred