Re: [PATCH] arm: ptrace: fix syscall modification under PTRACE_O_TRACESECCOMP

[Andy Lutomirski]

On Thu, Jul 3, 2014 at 9:11 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Thu, Jul 03, 2014 at 04:39:21PM +0100, Andy Lutomirski wrote:
>> On Thu, Jul 3, 2014 at 3:24 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
>> > On Thu, Jul 03, 2014 at 08:43:07AM +0100, AKASHI Takahiro wrote:
>> >> On 06/24/2014 05:54 PM, Will Deacon wrote:
>> >> > On Mon, Jun 23, 2014 at 08:46:52PM +0100, Kees Cook wrote:
>> >> >> What's the state of seccomp on arm64? I saw a series back in March,
>> >> >> but nothing since then? It looked complete, but I haven't set up a
>> >> >> test environment yet to verify.
>> >> >
>> >> > I think Akashi was going to repost `real soon now' so we can include them
>> >> > for 3.17. He missed the merge window last time around.
>> >>
>> >> I took a quick look at the current implementation of ptrace.
>> >> ptrace(PTRACE_GETREGSET/SETREGSET), eventually gpr_get/set(), handles only
>> >> 'struct user_pt_regs', and we have no way to modify orig_x0 nor syscallno
>> >> in 'struct pt_regs' directly.
>> >> So it seems to me that we can't change a system call by ptrace().
>> >> Do I misunderstand anything?
>> >
>> > No, it looks like you have a point here. I don't think userspace has any
>> > business with orig_x0, but changing syscallno is certainly useful. I can
>> > think of two ways to fix this:
>> >
>> >   (1) Updating syscallno based on w8, but this ties us to the current ABI
>> >       and could get messy if this register changes in the future.
>> >
>> >   (2) Adding a PTRACE_SET_SYSCALL request, like we have for arch/arm/,
>> >       but that means adding arch-specific stuff to arch_ptrace (which
>> >       currently goes straight to ptrace_request on arm64).
>> >
>> > It looks like x86 uses orig_ax, which I *think* means we would go with
>> > (1) above if we followed their lead.
>>
>> w8 is a real register, right?  On x86, at least orig_ax isn't a real
>> register, so it's quite unlikely to conflict with hardware stuff.
>
> Yeah, w8 is the hardware register which the Linux ABI uses for the system
> call number. I was thinking We could allow the debugger/tracer to update
> the syscall number by updating that register, or do you see an issue with
> that? (other than tying us to the current ABI).

Not immediately, but I'm not super-familiar with ptrace.

Is w8 clobbered or otherwise changed by syscalls?  Using w8 for this
has the odd effect that tracers can't force a return with a specific
value of w8 without executing the corresponding syscall.  If that's a
meaningful limitation, then presumably some other channel should be
used.

>
>> On x86, the "user_struct" thing has nothing to do with any real kernel
>> data structure, so it's extensible.  Can you just add syscallno to it?
>
> I'm really not keen on changing user-facing structures like that. For
> example, KVM embeds user_pt_regs into kvm_regs.

Fair enough.

>
> We can add a new ptrace request if we have to.
>
> Will



-- 
Andy Lutomirski
AMA Capital Management, LLC
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