Re: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

[AKASHI Takahiro]

On 08/27/2014 02:51 AM, Will Deacon wrote:
> On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote:
> > On 08/22/2014 02:08 AM, Kees Cook wrote:
> > > On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
> > > <takahiro.akashi@xxxxxxxxxx> wrote:
> > > > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> > > > index 8876049..c54dbcc 100644
> > > > --- a/arch/arm64/kernel/ptrace.c
> > > > +++ b/arch/arm64/kernel/ptrace.c
> > > > @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs,
> > > >
> > > >    asmlinkage int syscall_trace_enter(struct pt_regs *regs)
> > > >    {
> > > > +       unsigned int saved_syscallno = regs->syscallno;
> > > > +
> > > >           if (test_thread_flag(TIF_SYSCALL_TRACE))
> > > >                   tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
> > > >
> > > > +       if (IS_SKIP_SYSCALL(regs->syscallno)) {
> > > > +               /*
> > > > +                * RESTRICTION: we can't modify a return value of user
> > > > +                * issued syscall(-1) here. In order to ease this flavor,
> > > > +                * we need to treat whatever value in x0 as a return value,
> > > > +                * but this might result in a bogus value being returned.
> > > > +                */
> > > > +               /*
> > > > +                * NOTE: syscallno may also be set to -1 if fatal signal is
> > > > +                * detected in tracehook_report_syscall_entry(), but since
> > > > +                * a value set to x0 here is not used in this case, we may
> > > > +                * neglect the case.
> > > > +                */
> > > > +               if (!test_thread_flag(TIF_SYSCALL_TRACE) ||
> > > > +                               (IS_SKIP_SYSCALL(saved_syscallno)))
> > > > +                       regs->regs[0] = -ENOSYS;
> > > > +       }
> > > > +
> > >
> > > I don't have a runtime environment yet for arm64, so I can't test this
> > > directly myself, so I'm just trying to eyeball this. :)
> > >
> > > Once the seccomp logic is added here, I don't think using -2 as a
> > > special value will work. Doesn't this mean the Oops is possible by the
> > > user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and
> > > the user passed -2 as the syscall, audit will be called only on entry,
> > > and then skipped on exit?
> >
> > Oops, you're absolutely right. I didn't think of this case.
> > syscall_trace_enter() should not return a syscallno directly, but always
> > return -1 if syscallno < 0. (except when secure_computing() returns with -1)
> > This also implies that tracehook_report_syscall() should also have a return value.
> >
> > Will, is this fine with you?
>
> Well, the first thing that jumps out at me is why this is being done
> completely differently for arm64 and arm. I thought adding the new ptrace
> requests would reconcile the differences?

I'm not sure what portion of my code you mentioned as "completely different", but

1)
setting x0 to -ENOSYS is necessary because, otherwise, user-issued syscall(-1) will
return a bogus value when audit tracing is on.

Please note that, on arm,
                 not traced      traced
                 ------          ------
syscall(-1)      aborted         OOPs(BUG_ON)
syscall(-3000)   aborted         aborted
syscall(1000)    ENOSYS          ENOSYS

So, anyhow, its a bit difficult and meaningless to mimic these invalid cases.

2)
branching a new label, syscall_trace_return_skip (see entry.S), after syscall_trace_enter()
is necessary in order to avoid OOPS in audit_syscall_enter() as we discussed.

Did I make it clear?

-Takahiro AKASHI

> Will
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