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

[Timothy Baldwin]

 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.

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