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

From: [email-masked] (Andy Lutomirski)
Date: Wed, 23 Jul 2014 20:54:00 -0700

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 <[email-masked]>
> ---
>   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?  If so, then you risk (at least) introducing a nice 
user-triggerable OOPS if audit is enabled.  This bug existed for *years* 
on x86_32, and it amazes me that no one ever triggered it by accident. 
(Grr, audit.)

--Andy