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

Since commit f086f674 ("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.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Catalin Marinas<catalin.marinas@arm.com>
Cc: <stable@vger.kernel.org>
Fixes: f086f674 ("arm64: ptrace: add support for syscall emulation")
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Timothy E Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Link: https://lore.kernel.org/r/90edd33b-6353-1228-791f-0336d94d5f8c@majoroak.me.ukSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/ptrace.c

@@ -1797,7 +1797,7 @@ 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;
 	}