Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

[AKASHI Takahiro]

On 07/25/2014 12:01 AM, Andy Lutomirski wrote:
> On Jul 23, 2014 10:57 PM, "AKASHI Takahiro" <takahiro.akashi@xxxxxxxxxx> wrote:
> > On 07/24/2014 12:54 PM, Andy Lutomirski wrote:
> > > On 07/22/2014 02:14 AM, AKASHI Takahiro wrote:
> > > > Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
> > > > its value either to:
> > > >     * any valid syscall number to alter a system call, or
> > > >     * -1 to skip a system call
> > > >
> > > > This patch implements this behavior by reloading that value into syscallno
> > > > in struct pt_regs after tracehook_report_syscall_entry() or
> > > > secure_computing(). In case of '-1', a return value of system call can also
> > > > be changed by the tracer setting the value to x0 register, and so
> > > > sys_ni_nosyscall() should not be called.
> > > >
> > > > See also:
> > > >       42309ab4, ARM: 8087/1: ptrace: reload syscall number after
> > > >            secure_computing() check
> > > >
> > > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > > > ---
> > > >    arch/arm64/kernel/entry.S  |    2 ++
> > > >    arch/arm64/kernel/ptrace.c |   13 +++++++++++++
> > > >    2 files changed, 15 insertions(+)
> > > >
> > > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > > > index 5141e79..de8bdbc 100644
> > > > --- a/arch/arm64/kernel/entry.S
> > > > +++ b/arch/arm64/kernel/entry.S
> > > > @@ -628,6 +628,8 @@ ENDPROC(el0_svc)
> > > >    __sys_trace:
> > > >        mov    x0, sp
> > > >        bl    syscall_trace_enter
> > > > +    cmp    w0, #-1                // skip syscall?
> > > > +    b.eq    ret_to_user
> > >
> > >
> > > Does this mean that skipped syscalls will cause exit tracing to be skipped?
> >
> >
> > Yes. (and I guess yes on arm, too)
> >
> >
> > > If so, then you risk (at least) introducing
> > >
> > > a nice user-triggerable OOPS if audit is enabled.
> >
> >
> > Can you please elaborate this?
> > Since I didn't find any definition of audit's behavior when syscall is
> > rewritten to -1, I thought it is reasonable to skip "exit tracing" of
> > "skipped" syscall.
> > (otherwise, "fake" seems to be more appropriate :)
>
> The audit entry hook will oops if you call it twice in a row without
> calling the exit hook in between.

Thank you, I could reproduce this problem which hits BUG(in_syscall) in
audit_syscall_entry(). Really bad, and I fixed it in my next version and
now a "skipped" system call is also traced by audit.

I ran libseccomp test and Kees' test under auditd running with a rule,
	auditctl -a exit,always -S all
and all the tests seemed to pass.

  I can also imagine ptracers getting
> confused if ptrace entry and exit don't line up.

FYI, on arm64, we can distinguish syscall enter/exit with x12 register.

> What happens if user code directly issues syscall ~0?  Does the return
> value register get set?  Is the behavior different between traced and
> untraced syscalls?

Interesting cases. Let me think about it.

Thanks,
-Takahiro AKASHI

>  The current approach seems a bit scary.
>
> --Andy
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