x86/entry: Treat out of range and gap system calls the same

The current 64-bit system call entry code treats out-of-range system
calls differently than system calls that map to a hole in the system
call table.

This is visible to the user if system calls are intercepted via ptrace or
seccomp and the return value (regs->ax) is modified: in the former case,
the return value is preserved, and in the latter case, sys_ni_syscall() is
called and the return value is forced to -ENOSYS.

The API spec in <asm-generic/syscalls.h> is very clear that only
(int)-1 is the non-system-call sentinel value, so make the system call
behavior consistent by calling sys_ni_syscall() for all invalid system
call numbers except for -1.

Although currently sys_ni_syscall() simply returns -ENOSYS, calling it
explicitly is friendly for tracing and future possible extensions, and
as this is an error path there is no reason to optimize it.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210518191303.4135296-6-hpa@zytor.com

Author: H. Peter Anvin (Intel)

arch/x86/entry/common.c

@@ -52,6 +52,8 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, unsigned long nr)
 					X32_NR_syscalls);
 		regs->ax = x32_sys_call_table[nr](regs);
 #endif
+	} else if (unlikely((int)nr != -1)) {
+		regs->ax = __x64_sys_ni_syscall(regs);
 	}
 	instrumentation_end();
 	syscall_exit_to_user_mode(regs);
@@ -76,6 +78,8 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs,
 	if (likely(nr < IA32_NR_syscalls)) {
 		nr = array_index_nospec(nr, IA32_NR_syscalls);
 		regs->ax = ia32_sys_call_table[nr](regs);
+	} else if (unlikely((int)nr != -1)) {
+		regs->ax = __ia32_sys_ni_syscall(regs);
 	}
 }