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

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Message-ID: <CALCETrUTJkGHWZ=3yKa-myVhKrNuoJhtingKv4B+YbUu5ZX=2g@mail.gmail.com>
Date:	Mon, 20 Jun 2016 09:30:47 -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 8:24 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/19, Andy Lutomirski wrote:
>>
>> On Sat, Jun 18, 2016 at 10:02 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> Step 1: for 4.7 and for -stable, introduce TS_I386_REGS_POKED.  Set it
>> in putreg32.  Use it in syscall_get_error, get_nr_restart_syscall,
>> etc.  Clear it in do_signal.
>
> do_signal() won't be necessarily called...

True.  But I should have said "clear it in prepare_exit_to_usermode",
and the patch I'm just about to send does that.
>
>> I wonder if we could actually get away with doing syscall restart
>> processing before ptrace invocation.
>
> How? this doesn't look possible or I misunderstood.
>
> 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.  Also, it relies on bizarre arch details IMO.

I think I prefer my version, coming momentarily.

--Andy