linux-kernel - Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

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Message-ID: <CA+55aFxSmtNaUrXUJMzD4u3PUs35Vhm1PGp0610TeDXnj0XpKA@mail.gmail.com>
Date:	Thu, 13 Aug 2015 09:34:36 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Stas Sergeev <stsp@...t.ru>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

On Thu, Aug 13, 2015 at 9:23 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Aug 13, 2015 at 9:19 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> So how about this "alternate" minimal patch instead. The difference is:
>>
>>  - we actually leave the
>>
>>         regs->ss = __USER_DS;
>>
>>    in __setup_rt_frame, to guarantee that when we take a signal, we do
>> take it with a valid SS
>
> That by itself is enough to break DOSEMU.  I think we may be stuck
> with my hack to only replace regs->ss if the old one was invalid.

Are you sure? From the description by Stas, the problem is literally
the *restoring* action of the sigcontext, and trying to restore a SS
value that is no longer valid.

 "The crash happens when DOS program terminates.
  At that point dosemu subverts the execution flow by
  replacing segregs and cs/ip ss/sp in sigcontext with its own.
  But __pad0 still has DOS SS, which crash because (presumably)
  the DOS LDT have been just removed"

and my "truly-minimal" patch removes all of the sigcontext games.

> You mean that we always set ss to __USER_DS on sigreturn?

No. We never touch SS at sigreturn time at all. Only when entering the
signal *handler* do we reset things to a known state. The signal
handler can do anything it wants, and sigreturn won't touch it (which
will obviously _leave_ it as __USER_DS, but avoids the problem with
sigreturn trying to load an SS that is no longer valid)

> If this regression were new in 4.2-rc, then I'd say revert first and
> ask questions later, but the regression is in 4.1 as well :(

Big deal. That's why we have the "cc stable". Distributions that ship
with 4.1 are still fairly few (but it's a LTS release so it will grow)
but they all pick up stable kernels.

And even if they temporarily have a broken situation, it's still
better to make sure that broken situation gets fixed, rather than say
"oh well, too late to do anything about it now".

                 Linus
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