Re: [PATCH 4.4 07/40] arm64: ensure __dump_instr() checks addr_limit

[Catalin Marinas]

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@xxxxxxx>
> > 
> > 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@xxxxxxx>
> 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.

-- 
Catalin