linux-kernel - Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

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Message-ID: <4E52EA50.6070105@zytor.com>
Date:	Mon, 22 Aug 2011 16:46:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andrew Lutomirski <luto@....edu>,
	Al Viro <viro@...iv.linux.org.uk>,
	Borislav Petkov <bp@...64.org>, Ingo Molnar <mingo@...nel.org>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>
Subject: Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re:
 [RFC] weird crap with vdso on uml/i386)

On 08/22/2011 04:27 PM, Linus Torvalds wrote:
> On Mon, Aug 22, 2011 at 3:04 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> However, we could just issue a SIGILL or SIGSEGV at this point; the same
>> way we would if we got an #UD or #GP fault; SIGILL/#UD would be
>> consistent with Intel CPUs here.
> 
> Considering that this is not a remotely new issue, and that it has
> been around for years without anybody even noticing, I'd really prefer
> to just fix things going forwards rather than add any code to actively
> break any possible unlucky legacy users.
> 
> So I think the "let's fix the vdso case for sysenter" + "let's remove
> the 32-bit syscall vdso" is the right solution. If somebody has
> hardcoded syscall instructions, or generates them dynamically with
> some JIT, that's their problem. We'll continue to support it as well
> as we ever have (read: "almost nobody will ever notice").
> 
> One thing we *could* do is to just say "we never restart a x86-32
> 'syscall' instruction at all", and just make such a case return EINTR.
> IOW, do something along the lines of the appended pseudo-patch.
> 
> Because returning -EINTR is always "almost correct".
> 

I have to say it worries me from a potential security hole point of
view, especially since it clearly isn't very well trod ground to begin
with.  An almost-never-used path with access to the full system call
suite is scarier than hell in that sense.

Keep in mind support for SYSCALL32 is already (vendor-)conditional.

(The obvious solution of just putting the proper register frame back in
its place would be okay except for totally breaking anything
trace-on-exit as already hashed to death...)

	-hpa
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