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

+/* 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__
 
 /*

arch/powerpc/include/asm/kup.h

@@ -8,6 +8,9 @@
 #ifdef CONFIG_PPC_8xx
 #include <asm/nohash/32/kup-8xx.h>
 #endif
+#ifdef CONFIG_PPC_BOOK3S_32
+#include <asm/book3s/32/kup.h>
+#endif
 
 #ifdef __ASSEMBLY__
 #ifndef CONFIG_PPC_KUAP

arch/powerpc/kernel/entry_32.S

@@ -162,6 +162,9 @@ transfer_to_handler:
 	andis.	r12,r12,DBCR0_IDM@h
 #endif
 	ACCOUNT_CPU_USER_ENTRY(r2, r11, r12)
+#ifdef CONFIG_PPC_BOOK3S_32
+	kuep_lock r11, r12
+#endif
 #if defined(CONFIG_40x) || defined(CONFIG_BOOKE)
 	beq+	3f
 	/* From user and task is ptraced - load up global dbcr0 */
@@ -427,6 +430,9 @@ BEGIN_FTR_SECTION
 END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
 	stwcx.	r0,0,r1			/* to clear the reservation */
 	ACCOUNT_CPU_USER_EXIT(r2, r5, r7)
+#ifdef CONFIG_PPC_BOOK3S_32
+	kuep_unlock r5, r7
+#endif
 	kuap_check r2, r4
 	lwz	r4,_LINK(r1)
 	lwz	r5,_CCR(r1)
@@ -821,6 +827,9 @@ restore_user:
 	bnel-	load_dbcr0
 #endif
 	ACCOUNT_CPU_USER_EXIT(r2, r10, r11)
+#ifdef CONFIG_PPC_BOOK3S_32
+	kuep_unlock	r10, r11
+#endif
 
 	b	restore
 

arch/powerpc/kernel/head_32.S

@@ -896,14 +896,24 @@ load_up_mmu:
 	tophys(r6,r6)
 	lwz	r6,_SDR1@l(r6)
 	mtspr	SPRN_SDR1,r6
-	li	r0,16		/* load up segment register values */
+	li	r0, NUM_USER_SEGMENTS	/* load up segment register values */
 	mtctr	r0		/* for context 0 */
 	lis	r3,0x2000	/* Ku = 1, VSID = 0 */
+#ifdef CONFIG_PPC_KUEP
+	oris	r3, r3, SR_NX@h	/* Set Nx */
+#endif
 	li	r4,0
 3:	mtsrin	r3,r4
 	addi	r3,r3,0x111	/* increment VSID */
 	addis	r4,r4,0x1000	/* address of next segment */
 	bdnz	3b
+	li	r0, 16 - NUM_USER_SEGMENTS /* load up kernel segment registers */
+	mtctr	r0			/* for context 0 */
+	rlwinm	r3, r3, 0, ~SR_NX	/* Nx = 0 */
+3:	mtsrin	r3, r4
+	addi	r3, r3, 0x111	/* increment VSID */
+	addis	r4, r4, 0x1000	/* address of next segment */
+	bdnz	3b
 
 /* Load the BAT registers with the values set up by MMU_init.
    MMU_init takes care of whether we're on a 601 or not. */
@@ -1007,6 +1017,9 @@ _ENTRY(switch_mmu_context)
 	mulli	r3,r3,897	/* multiply context by skew factor */
 	rlwinm	r3,r3,4,8,27	/* VSID = (context & 0xfffff) << 4 */
 	addis	r3,r3,0x6000	/* Set Ks, Ku bits */
+#ifdef CONFIG_PPC_KUEP
+	oris	r3, r3, SR_NX@h	/* Set Nx */
+#endif
 	li	r0,NUM_USER_SEGMENTS
 	mtctr	r0
 

arch/powerpc/mm/ppc_mmu_32.c

@@ -394,3 +394,16 @@ void setup_initial_memory_limit(phys_addr_t first_memblock_base,
 	else /* Anything else has 256M mapped */
 		memblock_set_current_limit(min_t(u64, first_memblock_size, 0x10000000));
 }
+
+#ifdef CONFIG_PPC_KUEP
+void __init setup_kuep(bool disabled)
+{
+	pr_info("Activating Kernel Userspace Execution Prevention\n");
+
+	if (cpu_has_feature(CPU_FTR_601))
+		pr_warn("KUEP is not working on powerpc 601 (No NX bit in Seg Regs)\n");
+
+	if (disabled)
+		pr_warn("KUEP cannot be disabled yet on 6xx when compiled in\n");
+}
+#endif

arch/powerpc/platforms/Kconfig.cputype

@@ -25,6 +25,7 @@ config PPC_BOOK3S_32
 	bool "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx"
 	select PPC_FPU
 	select PPC_HAVE_PMU_SUPPORT
+	select PPC_HAVE_KUEP
 
 config PPC_85xx
 	bool "Freescale 85xx"