[PATCH] arm64: ptrace: Fix seccomp of traced syscall -1 (NO_SYSCALL)

[Sudeep Holla]

On Mon, Jan 18, 2021 at 02:58:58AM +0000, Timothy Baldwin wrote:
> From c047f549699d31ed91d5ac0cadbcf76a02cd801e Mon Sep 17 00:00:00 2001
> From: Timothy E Baldwin<T.E.Baldwin99 at members.leeds.ac.uk>
> Date: Sat, 16 Jan 2021 15:18:54 +0000
> Subject: [PATCH] arm64: ptrace: Fix seccomp of traced syscall -1 (NO_SYSCALL)
> 
> Since commit f086f67485c5 ("arm64: ptrace: add support for syscall
> emulation"), if system call number -1 is called and the process is being
> traced with PTRACE_SYSCALL, for example by strace, the seccomp check is
> skipped and -ENOSYS is returned unconditionally (unless altered by the
> tracer) rather than carrying out action specified in the seccomp filter.
> 
> The consequence of this is that it is not possible to reliably strace
> a seccomp based implementation of a foreign system call interface in
> which r7/x8 is permitted to be -1 on entry to a system call.
> 
> Also trace_sys_enter and audit_syscall_entry are skipped if a system
> call is skipped.
> 
> Fix by removing the in_syscall(regs) check restoring the previous behaviour
> which is like AArch32, x86 (which uses generic code) and everything else.
> 

Ah, my fault. At the time of timing this I didn't test with seccomp and
also for some reason IIRC I had assumed the flags SYSCALL_{EMU,TRACE}
and seccomp calls are mutually exclusive and can't happen together.

FWIW,
Reviewed-by: Sudeep Holla <sudeep.holla at arm.com>

Also I ran some minimal tests I have, so
Tested-by: Sudeep Holla <sudeep.holla at arm.com>

I have also asked Haibo Xu <Haibo.Xu at arm.com> who help me testing back then
to test again.


> Fixes: f086f67485c5 ("arm64: ptrace: add support for syscall emulation")
> Signed-off-by: Timothy E Baldwin<T.E.Baldwin99 at members.leeds.ac.uk>
> Cc: Sudeep Holla<sudeep.holla at arm.com>
> Cc: Oleg Nesterov <oleg at redhat.com>
> Cc: Catalin Marinas<catalin.marinas at arm.com>
> Cc: Will Deacon<will.deacon at arm.com>
> Cc: Kees Cook<keescook at chromium.org>
> Cc:stable at vger.kernel.org
> ---
>  arch/arm64/kernel/ptrace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 8ac487c84e37..1d75471979cb 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1796,7 +1796,7 @@ int syscall_trace_enter(struct pt_regs *regs)
>  	if (flags & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE)) {
>  		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
> -		if (!in_syscall(regs) || (flags & _TIF_SYSCALL_EMU))
> +		if (flags & _TIF_SYSCALL_EMU)
>  			return NO_SYSCALL;
>  	}
> -- 
> 2.27.0
> 
> The specific implementation of a seccomp based foreign system call interface
> is my port of RISC OS to Linux, in the spirit User Mode Linux:
> https://github.com/TimothyEBaldwin/RISC_OS_Linux_Binary
> 
> 
> 
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-- 
Regards,
Sudeep