From: Stas Sergeev <stsp@list.ru>
To: Andy Lutomirski <luto@amacapital.net>
Cc: X86 ML <x86@kernel.org>, Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu
Date: Wed, 12 Aug 2015 22:55:58 +0300 [thread overview]
Message-ID: <55CBA4CE.1040108@list.ru> (raw)
In-Reply-To: <CALCETrXUW0cfvk3K4KsYBM3FiiLqGRGZkyDk8751UsXHu+MJSQ@mail.gmail.com>
12.08.2015 22:20, Andy Lutomirski пишет:
> On Wed, Aug 12, 2015 at 11:55 AM, Stas Sergeev <stsp@list.ru> wrote:
>> 12.08.2015 21:25, Andy Lutomirski пишет:
>>>>>>
>>>>>> https://github.com/stsp/dosemu2/commit/7898ac60d5e569964127d6cc48f592caecd20b81
>>>>> So the problem is that dosemu was actually hacking around the old
>>>>> buggy behavior and thus relying on it. Grr.
>>>> What else it could do? :(
>>> Going back in time? Ask the kernel to fix the issue.
>> Like this?
>> http://www.x86-64.org/pipermail/discuss/2007-May/009913.html
>> And this:
>> http://www.x86-64.org/pipermail/discuss/2007-May/009923.html
> I apologize on behalf of the upstream kernel in 2007. :-/ I wasn't
> really involved at that point.
:)
>>>>> Let me see if I can come up with a clean kernel fix.
>>>> The check for proper sigreturn would be good.
>>> I still don't see how sigreturn matters here. It's signal *delivery*
>>> that's the problem.
>> But the delivery can be easily checked with "if (ss & 4)".
>> What remains is just a sigreturn instead of iret.
>>
>>> I'm thinking of having signal delivery zap ss only if the old ss looks
>>> bogus instead of zapping it unconditionally. IOW, instead of setting
>>> regs->ss = __USER_DS unconditionally, we'd do larl on the old regs->ss
>>> and keep it if it's DPL 3 RW data (exp-down or otherwise) and present.
>> I am not sure how good is this.
>> Yes, may help for a backward-compatibility.
>> But OTOH the 32bit kernel saves _all_ registers, including
>> ss, which is IMHO the right thing to do in general.
> I agree. So does x32.
>
> Are you planning on merging your patches into upstream DOSEMU?
Unlikely.
My git tree counts ~3000 patches already, so it is an
entirely separate project, which just happen to share
some code with dosemu (and under different license).
But we can discuss a binary-compatible fix, so that the
original dosemu can work too.
>>> I'll have to check the precise rules in both the SDM and APM. The
>>> idea is that we don't want IRET to fail during signal delivery, which
>>> can happen due to a bad sigreturn or a race against modify_ldt.
>> Well, this is a "very basic" idea, so to say.
>> The fact that segregs are not restored, have much more
>> consequences, and since now you already broke things,
>> I wonder if something can be finally fixed for good...
>>
>> What alternatives do we have? Can we do something
>> really brave, introduce a new sigaction flag perhaps, that
>> will just restore all segregs for new apps, and none - for
>> old apps? I mean the above gcc bugzilla ticket in particular -
>> very annoying one...
> We might need to do that.
>
> Here's a nasty case:
>
> void sighandler(...) {
> switch_userspace_thread();
> }
>
> Suppose that switch_userspace_thread() changes fs. Now what? On
The crash - see the gcc ticket in the prev e-mail.
If fs on function entry differs from fs on exit, the gcc
stack protector will terminate the program.
So such code will not exist, except maybe in some
asm form...
> current kernels, it stays switched. If we change this, it won't stay
> switched. Even ignoring old ABI, it's not really clear to me what the
> right thing to do is.
There can be the following cases:
- switch_userspace_thread() switches fs to non-zero selector
- switch_userspace_thread() switches the fs base via syscall
- switch_userspace_thread() switches fs in sigcontext
- switch_userspace_thread() switches fs_base in sigcontext (???)
What exactly case do you have in mind?
I'd say, the way x86_32 is doing things - is good, but the
bases... perhaps, in ideal world, they should be a part of
the sigcontext as well?
next prev parent reply other threads:[~2015-08-12 19:56 UTC|newest]
Thread overview: 121+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-12 0:17 [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu Stas Sergeev
2015-08-12 0:38 ` Andy Lutomirski
2015-08-12 8:02 ` Stas Sergeev
2015-08-12 16:19 ` Andy Lutomirski
2015-08-12 17:00 ` Stas Sergeev
2015-08-12 18:25 ` Andy Lutomirski
2015-08-12 18:55 ` Stas Sergeev
2015-08-12 19:20 ` Andy Lutomirski
2015-08-12 19:55 ` Stas Sergeev [this message]
2015-08-12 20:01 ` Andy Lutomirski
2015-08-12 20:14 ` Stas Sergeev
2015-08-12 20:28 ` Andy Lutomirski
2015-08-12 20:45 ` Stas Sergeev
2015-08-12 20:47 ` Andy Lutomirski
2015-08-12 20:55 ` Stas Sergeev
2015-08-12 21:37 ` Andy Lutomirski
2015-08-12 21:50 ` Stas Sergeev
2015-08-12 22:00 ` Andy Lutomirski
2015-08-13 8:39 ` Ingo Molnar
2015-08-13 10:14 ` Stas Sergeev
2015-08-13 12:44 ` Stas Sergeev
2015-08-13 14:58 ` Andy Lutomirski
2015-08-13 15:22 ` Stas Sergeev
2015-08-13 15:38 ` Andy Lutomirski
2015-08-13 16:03 ` Stas Sergeev
2015-08-13 16:09 ` Andy Lutomirski
2015-08-13 16:20 ` Stas Sergeev
2015-08-13 16:24 ` Andy Lutomirski
2015-08-13 16:38 ` Stas Sergeev
2015-08-13 16:42 ` Andy Lutomirski
2015-08-13 16:48 ` Stas Sergeev
2015-08-13 16:59 ` Andy Lutomirski
2015-08-13 17:13 ` Stas Sergeev
2015-08-13 17:17 ` Andy Lutomirski
2015-08-13 18:00 ` Stas Sergeev
2015-08-13 18:05 ` Andy Lutomirski
2015-08-13 18:19 ` Stas Sergeev
2015-08-13 18:25 ` Andy Lutomirski
2015-08-13 18:35 ` Stas Sergeev
2015-08-22 12:38 ` Ingo Molnar
2015-08-22 14:19 ` Stas Sergeev
2015-08-23 6:25 ` Ingo Molnar
2015-08-13 11:08 ` Stas Sergeev
2015-08-13 15:37 ` Linus Torvalds
2015-08-13 15:43 ` Andy Lutomirski
2015-08-13 16:19 ` Linus Torvalds
2015-08-13 16:23 ` Andy Lutomirski
2015-08-13 16:34 ` Linus Torvalds
2015-08-13 16:43 ` Linus Torvalds
2015-08-13 16:44 ` Andy Lutomirski
2015-08-13 17:00 ` Brian Gerst
2015-08-18 6:29 ` Stas Sergeev
2015-08-18 22:42 ` Andy Lutomirski
2015-08-18 22:47 ` Andy Lutomirski
2015-08-19 9:35 ` Stas Sergeev
2015-08-19 15:46 ` Andy Lutomirski
2015-08-19 16:30 ` Stas Sergeev
2015-09-02 5:12 ` Andy Lutomirski
2015-09-02 9:17 ` Stas Sergeev
2015-09-02 14:21 ` Andy Lutomirski
2015-09-02 15:02 ` Andy Lutomirski
2015-09-02 17:46 ` Stas Sergeev
2015-09-02 18:17 ` Andy Lutomirski
2015-09-02 18:23 ` Stas Sergeev
2015-09-02 19:06 ` Andy Lutomirski
2015-09-02 21:01 ` Stas Sergeev
2015-09-02 21:39 ` Andy Lutomirski
2015-09-02 22:25 ` Stas Sergeev
2015-09-02 22:25 ` Andy Lutomirski
2015-09-02 23:01 ` Stas Sergeev
2015-08-19 10:10 ` Stas Sergeev
2015-08-19 15:35 ` Andy Lutomirski
2015-08-14 8:10 ` Cyrill Gorcunov
2015-08-13 17:51 ` Stas Sergeev
2015-08-13 18:35 ` Linus Torvalds
2015-08-13 18:41 ` Andy Lutomirski
2015-08-13 19:05 ` Stas Sergeev
2015-08-13 19:49 ` Andy Lutomirski
2015-08-13 20:09 ` Stas Sergeev
2015-08-13 19:53 ` Linus Torvalds
2015-08-13 20:08 ` Cyrill Gorcunov
2015-08-13 20:09 ` Linus Torvalds
2015-08-13 21:42 ` Raymond Jennings
2015-08-13 21:46 ` Linus Torvalds
2015-08-13 22:01 ` Raymond Jennings
2015-08-13 22:05 ` Stas Sergeev
2015-08-13 23:05 ` Linus Torvalds
2015-08-13 23:18 ` Linus Torvalds
2015-08-13 23:35 ` Raymond Jennings
2015-08-13 23:43 ` Stas Sergeev
2015-08-14 0:02 ` Linus Torvalds
2015-08-13 22:02 ` Stas Sergeev
2015-08-13 22:11 ` Andy Lutomirski
2015-08-13 22:25 ` Stas Sergeev
2015-08-13 22:29 ` Andy Lutomirski
2015-08-13 22:51 ` Stas Sergeev
2015-08-13 23:00 ` Andy Lutomirski
2015-08-13 23:17 ` Stas Sergeev
2015-08-14 0:00 ` Stas Sergeev
2015-08-14 0:05 ` Andy Lutomirski
2015-08-14 0:17 ` Stas Sergeev
2015-08-14 0:27 ` Linus Torvalds
2015-08-14 0:50 ` Stas Sergeev
2015-08-14 1:21 ` Andy Lutomirski
2015-08-14 1:32 ` Stas Sergeev
2015-08-14 1:37 ` Andy Lutomirski
2015-08-14 2:03 ` Stas Sergeev
2015-08-18 6:19 ` Stas Sergeev
2015-08-14 0:08 ` Linus Torvalds
2015-08-14 0:24 ` Andy Lutomirski
2015-08-14 0:40 ` Linus Torvalds
2015-08-14 7:22 ` Cyrill Gorcunov
2015-08-14 10:02 ` Pavel Emelyanov
2015-08-14 10:53 ` Cyrill Gorcunov
2015-08-13 18:57 ` Stas Sergeev
2015-08-13 19:01 ` Andy Lutomirski
2015-08-13 19:13 ` Stas Sergeev
2015-08-13 19:37 ` Linus Torvalds
2015-08-13 19:59 ` Stas Sergeev
2015-08-13 20:07 ` Linus Torvalds
2015-08-18 6:40 ` Stas Sergeev
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