s390/syscalls: Add spectre boundary for syscall dispatch table

The s390 syscall number is directly controlled by userspace, but does
not have an array_index_nospec() boundary to prevent access past the
syscall function pointer tables.

Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Fixes: 56e62a7 ("s390: convert to generic entry")
Cc: stable@kernel.org
Assisted-by: gkh_clanker_2000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/2026032404-sterling-swoosh-43e6@gregkh
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: gregkh

arch/s390/kernel/syscall.c

@@ -13,6 +13,7 @@
  */
 
 #include <linux/cpufeature.h>
+#include <linux/nospec.h>
 #include <linux/errno.h>
 #include <linux/sched.h>
 #include <linux/mm.h>
@@ -131,8 +132,10 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 	if (unlikely(test_and_clear_pt_regs_flag(regs, PIF_SYSCALL_RET_SET)))
 		goto out;
 	regs->gprs[2] = -ENOSYS;
-	if (likely(nr < NR_syscalls))
+	if (likely(nr < NR_syscalls)) {
+		nr = array_index_nospec(nr, NR_syscalls);
 		regs->gprs[2] = sys_call_table[nr](regs);
+	}
 out:
 	syscall_exit_to_user_mode(regs);
 }