linux-kernel - Re: [PATCH 06/14] x86/ptrace: run seccomp after ptrace

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Date:	Thu, 9 Jun 2016 19:01:45 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Andy Lutomirski <luto@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Chris Metcalf <cmetcalf@...lanox.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Helge Deller <deller@....de>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	James Hogan <james.hogan@...tec.com>,
	Jeff Dike <jdike@...toit.com>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	linux-parisc <linux-parisc@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"Maciej W. Rozycki" <macro@...tec.com>,
	Mark Rutland <mark.rutland@....com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Paul Mackerras <paulus@...ba.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Richard Weinberger <richard@....at>,
	Russell King <linux@...linux.org.uk>,
	"user-mode-linux-devel@...ts.sourceforge.net" 
	<user-mode-linux-devel@...ts.sourceforge.net>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 06/14] x86/ptrace: run seccomp after ptrace

On Thu, Jun 9, 2016 at 3:52 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jun 9, 2016 at 2:01 PM, Kees Cook <keescook@...omium.org> wrote:
>> This moves seccomp after ptrace on x86 to that seccomp can catch changes
>> made by ptrace. Emulation should skip the rest of processing too.
>>
>> We can get rid of test_thread_flag because there's no longer any
>> opportunity for seccomp to mess with ptrace state before invoking
>> ptrace.
>>
>> Suggested-by: Andy Lutomirski <luto@...nel.org>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> Cc: x86@...nel.org
>> Cc: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/entry/common.c | 22 ++++++++++++----------
>>  1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>> index df56ca394877..81c0e12d831c 100644
>> --- a/arch/x86/entry/common.c
>> +++ b/arch/x86/entry/common.c
>> @@ -73,6 +73,7 @@ static long syscall_trace_enter(struct pt_regs *regs)
>>
>>         struct thread_info *ti = pt_regs_to_thread_info(regs);
>>         unsigned long ret = 0;
>> +       bool emulated = false;
>>         u32 work;
>>
>>         if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
>> @@ -80,11 +81,19 @@ static long syscall_trace_enter(struct pt_regs *regs)
>>
>>         work = ACCESS_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY;
>>
>> +       if (unlikely(work & _TIF_SYSCALL_EMU))
>> +               emulated = true;
>> +
>> +       if ((emulated || (work & _TIF_SYSCALL_TRACE)) &&
>> +           tracehook_report_syscall_entry(regs))
>> +               return -1L;
>> +
>> +       if (emulated)
>> +               return -1L;
>> +
>
> I think that this code will result in ptrace-induced skips calling the
> audit exit hook but not the audit entry hook.  I don't know whether
> this is a problem.  It's also worth making sure that ptracing a
> seccomp-skipped syscall calls the exit hook with the right regs.
>
> I suspect it's fine, but I want to think about it a little bit more.

I don't think this is true, since all architectures already needed to
handle an immediate return from seccomp, so audit shouldn't be touched
on the exit path either.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security