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Date: Tue, 14 Nov 2017 16:18:23 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH 4.4 07/40] arm64: ensure __dump_instr() checks addr_limit
Message-ID: <20171114161756.eumtpoiid6j74o7f@salmiak>
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On Tue, Nov 14, 2017 at 01:52:14PM +0000, Catalin Marinas wrote:
> On Mon, Nov 13, 2017 at 07:05:12PM +0000, Ben Hutchings wrote:
> > On Mon, 2017-11-06 at 10:44 +0100, Greg Kroah-Hartman wrote:
> > > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Mark Rutland <mark.rutland@arm.com>
> > > 
> > > commit 7a7003b1da010d2b0d1dc8bf21c10f5c73b389f1 upstream.
> > > 
> > > It's possible for a user to deliberately trigger __dump_instr with a
> > > chosen kernel address.
> > > 
> > > Let's avoid problems resulting from this by using get_user() rather than
> > > __get_user(), ensuring that we don't erroneously access kernel memory.
> > > 
> > > Where we use __dump_instr() on kernel text, we already switch to
> > > KERNEL_DS, so this shouldn't adversely affect those cases.
> > > 
> > > Fixes: 60ffc30d5652810d ("arm64: Exception handling")
> > [...]
> > 
> > This seems harmless, but I don't think it will fix the bug in 4.4
> > unless you also cherry-pick:
> > 
> > commit c5cea06be060f38e5400d796e61cfc8c36e52924
> > Author: Mark Rutland <mark.rutland@arm.com>
> > Date:   Mon Jun 13 11:15:14 2016 +0100
> > 
> >     arm64: fix dump_instr when PAN and UAO are in use
> 
> I agree. In 4.4 dump_instr() doesn't do any checks, just
> set_fs(KERNEL_DS) and __get_user(). While commit c5cea06b was added to
> fix 57f4959bad0a154a ("arm64: kernel: Add support for User Access
> Override"; merged in 4.6), it also makes sense on its own as a security
> improvement for 4.4.
> 
> Mark is currently on holiday but he'll follow up next week if any
> patches need back-porting.

I accidentally glanced at my gmail filter this morning; so I can reply today.
;)

I agree that we need both patches. On its own (in the absence of PAN/UAO),
57f4959bad0a154a wouldn't have any effect, but it is critical to ensure that we
can dump kernel instructions when we intend to, with 7a7003b1da010d2b
backported.

Backporting 57f4959bad0a154a as a prerequisite makes sense to me.

I hadn't realised you could list prerequisites in a stable Cc -- I'll try to do
that in future.

Thanks,
Mark.