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

[Andy Lutomirski]

On Thu, Jun 9, 2016 at 3:52 PM, Andy Lutomirski  wrote:
> On Thu, Jun 9, 2016 at 2:01 PM, Kees Cook  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 
>> Signed-off-by: Kees Cook 
>> Cc: x...@kernel.org
>> Cc: Andy Lutomirski 
>> ---
>>  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 poked at it a bit and this seems to work correctly.
selftests/x86/ptrace_syscall.c exercises PTRACE_SYSCALL_EMU pretty
well, and it still passes.

--Andy
___
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev

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

[Kees Cook]

On Thu, Jun 9, 2016 at 3:52 PM, Andy Lutomirski  wrote:
> On Thu, Jun 9, 2016 at 2:01 PM, Kees Cook  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 
>> Signed-off-by: Kees Cook 
>> Cc: x...@kernel.org
>> Cc: Andy Lutomirski 
>> ---
>>  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
___
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev

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

[Andy Lutomirski]

On Thu, Jun 9, 2016 at 2:01 PM, Kees Cook  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 
> Signed-off-by: Kees Cook 
> Cc: x...@kernel.org
> Cc: Andy Lutomirski 
> ---
>  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.

--Andy
___
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev