linux-kernel - Re: [PATCH 3/3] uprobes: write_opcode()->__replace_page() can race with try_to

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Message-ID: <20120608100358.GA19131@redhat.com>
Date:	Fri, 8 Jun 2012 12:03:58 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Hugh Dickins <hughd@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] uprobes: write_opcode()->__replace_page() can race
	with try_to_unmap()

On 06/08, Peter Zijlstra wrote:
>
> On Thu, 2012-06-07 at 19:00 +0200, Oleg Nesterov wrote:
> > write_opcode() gets old_page via get_user_pages() and then calls
> > __replace_page() which assumes that this old_page is still mapped
> > after pte_offset_map_lock().
> >
> > This is not true if this old_page was already try_to_unmap()'ed,
> > and in this case everything __replace_page() does with old_page
> > is wrong. Just for example, put_page() is not balanced.
> >
> > I think it is possible to teach __replace_page() to handle this
> > unlikely case correctly, but this patch simply changes it to use
> > page_check_address() and return -EAGAIN if it fails. The caller
> > should notice this error code and retry.
>
> Note that replace_page() was nicked from ksm, does that suffer a similar
> problem?

Yes, I looked at replace_page() too. Afaics it looks fine, it does the
additional pte_same(pte, orig_pte) check.

Oleg.

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