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

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Message-ID: <CALCETrWUmL5bPyxR0gvchtn433hdnSqOuRdBjFsqJqa_xbtBnw@mail.gmail.com>
Date:	Fri, 30 Oct 2015 22:43:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>, 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?

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
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