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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110614154052.GA22082@redhat.com>
Date:	Tue, 14 Jun 2011 17:40:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 06/14, Peter Zijlstra wrote:
>
> 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().

Currently it can't, write_opcode() does another cow. But that cow can,
and this is the same, yes.

> 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).

Yes, or write_opcode() could take mmap_sem for writing as Srikar suggests.

But do we really care? Whatever we do we can race with the other updates
to this memory. Say, someone can write to vma->vm_file.

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/