Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 3 Jul 2014 09:13:50 -0700 | Subject | Re: [PATCH] arm: ptrace: fix syscall modification under PTRACE_O_TRACESECCOMP |
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On Thu, Jul 3, 2014 at 9:11 AM, Will Deacon <will.deacon@arm.com> 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@arm.com> 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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