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

[AKASHI Takahiro]

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 :)

-Takahiro AKASHI

> This bug existed for *years* on x86_32, and it amazes me that no one
> ever triggered it by accident. (Grr, audit.)
>
> --Andy
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