powerpc/32s: Implement Kernel Userspace Execution Prevention.

To implement Kernel Userspace Execution Prevention, this patch
sets NX bit on all user segments on kernel entry and clears NX bit
on all user segments on kernel exit.

Note that powerpc 601 doesn't have the NX bit, so KUEP will not
work on it. A warning is displayed at startup.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/32/kup.h

@@ -0,0 +1,42 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_POWERPC_BOOK3S_32_KUP_H
+#define _ASM_POWERPC_BOOK3S_32_KUP_H
+
+#include <asm/book3s/32/mmu-hash.h>
+
+#ifdef __ASSEMBLY__
+
+.macro kuep_update_sr	gpr1, gpr2		/* NEVER use r0 as gpr2 due to addis */
+101:	mtsrin	\gpr1, \gpr2
+	addi	\gpr1, \gpr1, 0x111		/* next VSID */
+	rlwinm	\gpr1, \gpr1, 0, 0xf0ffffff	/* clear VSID overflow */
+	addis	\gpr2, \gpr2, 0x1000		/* address of next segment */
+	bdnz	101b
+	isync
+.endm
+
+.macro kuep_lock	gpr1, gpr2
+#ifdef CONFIG_PPC_KUEP
+	li	\gpr1, NUM_USER_SEGMENTS
+	li	\gpr2, 0
+	mtctr	\gpr1
+	mfsrin	\gpr1, \gpr2
+	oris	\gpr1, \gpr1, SR_NX@h		/* set Nx */
+	kuep_update_sr \gpr1, \gpr2
+#endif
+.endm
+
+.macro kuep_unlock	gpr1, gpr2
+#ifdef CONFIG_PPC_KUEP
+	li	\gpr1, NUM_USER_SEGMENTS
+	li	\gpr2, 0
+	mtctr	\gpr1
+	mfsrin	\gpr1, \gpr2
+	rlwinm	\gpr1, \gpr1, 0, ~SR_NX		/* Clear Nx */
+	kuep_update_sr \gpr1, \gpr2
+#endif
+.endm
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_POWERPC_BOOK3S_32_KUP_H */

arch/powerpc/include/asm/book3s/32/mmu-hash.h

@@ -63,6 +63,9 @@ typedef pte_t *pgtable_t;
 #define PP_RWRW 2	/* Supervisor read/write, User read/write */
 #define PP_RXRX 3	/* Supervisor read,       User read */
 
+/* Values for Segment Registers */
+#define SR_NX	0x10000000	/* No Execute */
+
 #ifndef __ASSEMBLY__
 
 /*