linux-kernel - Re: [PATCH 0/3] aio: ctx->dead cleanups

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Date:	Wed, 17 Jun 2015 03:14:14 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Jeff Moyer <jmoyer@...hat.com>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] aio: ctx->dead cleanups

On 06/17, Oleg Nesterov wrote:
>
> On 06/17, Al Viro wrote:
> >
> > On Wed, Jun 17, 2015 at 01:04:14AM +0200, Oleg Nesterov wrote:
> > > Al, please help. We are trying to backport some aio fixes and I am
> > > absolutely confused by your b2edffdd912b "fix mremap() vs. ioctx_kill()
> > > race".
> > >
> > >
> > > Firstly, I simply can't understand what exactly it tries to fix. OK,
> > > aio_free_ring() can race with kill and we can remap the soon-to-be-killed
> > > ctx. So what? kill_ioctx() will the the correct (already re-mapped)
> > > ctx->mmap_base after it drops mm->ioctx_lock.
> >
> > Huh?  kill_ioctx() picks ctx->mmap_base and passes it to vm_munmap().
> > Which tries to grab mmap_sem, blocks for mremap() from another thread
> > and waits for it to drop mmap_sem.  By that time ctx->mmap_base has
> > nothing whatsoever to the argument we'd passed to vm_munmap().
>
> Yes. But it seems that you missed another part of my email:
>
> 	So it seems to me we only need this change to ensure that move_vma() can
> 	not succeed if ctx was already removed from ->ioctx_table, or, if we race
> 	with ioctx_alloc(), it was not added to ->ioctx_table. IOW, we need to
> 	ensure that move_vma()->aio_ring_mmap() can not race with
> 	vm_munmap(ctx->mmap_base) in kill_ioctx() or ioctx_alloc(). And this race
> 	doesn't look really bad. The kernel can't crash, just the application can
> 	fool itself.
>
> So once again, could explain why do we really need to prevent this?
> Afaics, if the application is stupid, it can only fool itself.
>
> And please note that ctx->mmap_base or/and ctx->mmap_size can be wrong
> anyway. Say, an application can munmap() this vma, or munmap() the part
> of this vma.

And speaking of aio_ring_remap() it can "corrupt" ->mmap_base even with
this patch. Just you need to mremap() the tail of aio-mapped memory.

No?

Oleg.

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