linux-kernel - Re: 4.3 regression: task_work corruption in vm86 mode?

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Message-ID: <5634AB0E.3060707@list.ru>
Date:	Sat, 31 Oct 2015 14:50:38 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: 4.3 regression: task_work corruption in vm86 mode?

31.10.2015 08:43, Andy Lutomirski пишет:
> On Fri, Oct 30, 2015 at 6:44 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> Hi all-
>>
>> In 4.3-rc7, running dosemu2 (https://github.com/stsp/dosemu2/) oopses
>> the system very quickly, as long as CONFIG_VM86=y.  It blows up
>> because snd_seq_delete_port walks ports_list_head, finds two valid
>> ports, and then starts finding obviously invalid pointers in the list.
>>
>> git bisect blames:
>>
>> commit 5ed92a8ab71f8865ba07811429c988c72299b315
>> Author: Brian Gerst <brgerst@...il.com>
>> Date:   Wed Jul 29 01:41:19 2015 -0400
>>
>>      x86/vm86: Use the normal pt_regs area for vm86
>>
>> I haven't spotted the problem yet.  It seems to happen when
>> task_work_run fires in get_signal, which happens before
>> save_v86_state.  I'm not entirely sure what causes task work to be
>> scheduled at all while in v86 land.  Could we somehow be processing
>> task_work later than we should?
>>
> Nope, the bug has nothing to do with task_work.  Patches sent.
Andy, thanks for finally fixing this attack surface!
So after all, the comments you put into Kconfig, were justified.
Now I can seriously consider the dosemu2-specific vm86-light.
Having the machine to crash, was not a good starting point for
the clean-ups.
Also there is an interesting thread here:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1499089
I wonder if they are affected now by that bug or not...
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