linux-kernel - Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.

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Date:	Tue, 14 Jun 2011 17:07:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 2/22]  2: uprobes: Breakground page
 replacement.

On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote:
> On 06/14, Peter Zijlstra wrote:
> >
> > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote:
> > >
> > > Also. This is called under down_read(mmap_sem), can't we race with
> > > access_process_vm() modifying the same memory?
> >
> > Shouldn't matter COW and similar things are serialized using the pte
> > lock.
> 
> Yes, but afaics this doesn't matter. Suppose that write_opcode() is
> called when access_process_vm() does copy_to_user_page(). We can cow
> the page before memcpy() completes.

access_process_vm() will end up doing a FOLL_WRITE itself when
copy_to_user_page() is called since write=1 in that case.

At that point we have a COW-race, someone wins, but the other will then
return the same page.

At this point further PTRACE pokes can indeed race with the memcpy in
write_opcode(). A possible fix would be to lock_page() around
copy_to_user_page() (its already done in set_page_dirty_lock(), so
pulling it out shouldn't matter much).


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