linux-kernel - Re: [PATCH] x86/ptrace: Remove questionable TS

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Date:	Mon, 20 Jun 2016 10:25:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Pedro Alves <pedro@...ves.net>, Kees Cook <keescook@...omium.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/ptrace: Remove questionable TS_COMPAT usage in ptrace

On Mon, Jun 20, 2016 at 9:14 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/20, Andy Lutomirski wrote:
>>
>> On Mon, Jun 20, 2016 at 8:24 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >
>> > How about the simple change below for now? IIRC 32-bit task can't use
>> > "syscall" so if syscall_get_nr() >= 0 then even the wrong TS_COMPAT is
>> > not that bad, even if it "leaks" to user-mode.
>>
>> Hmm.  That should fix the minor security issue, but it will even
>> further break cross-arch tracing: now a 32-bit tracer tracing a 64-bit
>> task that does int $0x80 will malfunction even more than it would
>> have.
>
> This is broken in any case. I mean, a 32-bit debugger can't really
> debug a 64-bit task.
>
> I don't think this change makes the things really worse.
>
>> Also, it relies on bizarre arch details IMO.
>
> Heh,  it looks as if your patch do not ;)
>
>> I think I prefer my version, coming momentarily.
>
> I disagree... I don't really understand why do we need the additional
> complications for the minimal fix which doesn't look very nice anyway.
>
> But I won't argue, and your patch looks correct to me.

Part of the reason is that I actually want to fix this in two parts.
After my TS_REGS_POKED_I386 patch, I have a follow-up patch that I
want to polish and test a bit more that's intended to get the
64-bit-tracer/32-bit-tracee case right, but it probably needs extra
testing, is less likely to make 4.8, and is definitely not for
4.7/stab.e.  That patch deletes TS_REGS_POKED_I386 again.  So by
having the minimal fix be more mindless, I'm more comfortable with the
backport.  I'll send out the second patch soon.


--Andy


-- 
Andy Lutomirski
AMA Capital Management, LLC