powerpc/rtas: mandate RTAS syscall filtering

CONFIG_PPC_RTAS_FILTER has been optional but default-enabled since its
introduction. It's been enabled in enterprise distro kernels for a
while without causing ABI breakage that wasn't easily fixed, and it
prevents harmful abuses of the rtas syscall.

Let's make it unconditional.
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221118150751.469393-10-nathanl@linux.ibm.com

arch/powerpc/Kconfig

@@ -1044,19 +1044,6 @@ config PPC_SECVAR_SYSFS
 	  read/write operations on these variables. Say Y if you have
 	  secure boot enabled and want to expose variables to userspace.
 
-config PPC_RTAS_FILTER
-	bool "Enable filtering of RTAS syscalls"
-	default y
-	depends on PPC_RTAS
-	help
-	  The RTAS syscall API has security issues that could be used to
-	  compromise system integrity. This option enforces restrictions on the
-	  RTAS calls and arguments passed by userspace programs to mitigate
-	  these issues.
-
-	  Say Y unless you know what you are doing and the filter is causing
-	  problems for you.
-
 endmenu
 
 config ISA_DMA_API

arch/powerpc/kernel/rtas.c

@@ -1050,8 +1050,6 @@ noinstr struct pseries_errorlog *get_pseries_errorlog(struct rtas_error_log *log
 	return NULL;
 }
 
-#ifdef CONFIG_PPC_RTAS_FILTER
-
 /*
  * The sys_rtas syscall, as originally designed, allows root to pass
  * arbitrary physical addresses to RTAS calls. A number of RTAS calls
@@ -1200,20 +1198,6 @@ static void __init rtas_syscall_filter_init(void)
 		rtas_filters[i].token = rtas_token(rtas_filters[i].name);
 }
 
-#else
-
-static bool block_rtas_call(int token, int nargs,
-			    struct rtas_args *args)
-{
-	return false;
-}
-
-static void __init rtas_syscall_filter_init(void)
-{
-}
-
-#endif /* CONFIG_PPC_RTAS_FILTER */
-
 /* We assume to be passed big endian arguments */
 SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
 {