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Date: Tue, 29 Jul 2014 14:26:59 +0100
From: Will Deacon <will.deacon@arm.com>
To: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Andy Lutomirski <luto@amacapital.net>, Deepak Saxena <dsaxena@linaro.org>,
        Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
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        Catalin Marinas <Catalin.Marinas@arm.com>,
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Subject: Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after
 ptrace operations
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On Tue, Jul 29, 2014 at 07:49:47AM +0100, AKASHI Takahiro wrote:
> On 07/25/2014 08:03 PM, Will Deacon wrote:
> > On Fri, Jul 25, 2014 at 11:36:49AM +0100, AKASHI Takahiro wrote:
> >> On 07/25/2014 12:01 AM, Andy Lutomirski wrote:
> >>>>> 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.
> >
> > Can you reproduce this on arch/arm/ too? If so, we should also fix the code
> > there.
> 
> As far as I tried on arm with syscall auditing enabled,
> 
> 1) Changing a syscall number to -1 under seccomp doesn't hit BUG_ON(in_syscall).
> 2) But, in fact, audit_syscall_entry() is NOT called in this case because
>     __secure_computing() returns -1 and then it causes the succeeding tracing
>     in syscall_trace_enter(), including audit_syscall_entry(), skipped.

What happens if CONFIG_SECCOMP=n?

> 3) On the other hand, calling syscall(-1) from userspace hits BUG_ON because
>     the return path, ret_slow_syscall, doesn't contain syscall_trace_exit().
> 4) When we re-write a syscall number to -1 without seccomp, we will also see
>     BUG_ON hit, although I didn't try yet.
> 
> Fixing case 3 is easy, but should we also fix case 2?

I think so.

Will