powerpc/44x: Implement Kernel Userspace Exec Protection (KUEP)

Powerpc 44x has two bits for exec protection in TLBs: one
for user (UX) and one for superviser (SX).

Clear SX on user pages in TLB miss handlers to provide KUEP.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/169310e08152aa1d96c979770291d165ec6896ae.1622616032.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/nohash/32/mmu-44x.h

@@ -113,6 +113,7 @@ typedef struct {
 
 /* patch sites */
 extern s32 patch__tlb_44x_hwater_D, patch__tlb_44x_hwater_I;
+extern s32 patch__tlb_44x_kuep, patch__tlb_47x_kuep;
 
 #endif /* !__ASSEMBLY__ */
 

arch/powerpc/kernel/head_44x.S

@@ -532,6 +532,10 @@ finish_tlb_load_44x:
 	andi.	r10,r12,_PAGE_USER		/* User page ? */
 	beq	1f				/* nope, leave U bits empty */
 	rlwimi	r11,r11,3,26,28			/* yes, copy S bits to U */
+#ifdef CONFIG_PPC_KUEP
+0:	rlwinm	r11,r11,0,~PPC44x_TLB_SX	/* Clear SX if User page */
+	patch_site 0b, patch__tlb_44x_kuep
+#endif
 1:	tlbwe	r11,r13,PPC44x_TLB_ATTRIB	/* Write ATTRIB */
 
 	/* Done...restore registers and get out of here.
@@ -743,6 +747,10 @@ finish_tlb_load_47x:
 	andi.	r10,r12,_PAGE_USER		/* User page ? */
 	beq	1f				/* nope, leave U bits empty */
 	rlwimi	r11,r11,3,26,28			/* yes, copy S bits to U */
+#ifdef CONFIG_PPC_KUEP
+0:	rlwinm	r11,r11,0,~PPC47x_TLB2_SX	/* Clear SX if User page */
+	patch_site 0b, patch__tlb_47x_kuep
+#endif
 1:	tlbwe	r11,r13,2
 
 	/* Done...restore registers and get out of here.

arch/powerpc/mm/nohash/44x.c

@@ -239,3 +239,16 @@ void __init mmu_init_secondary(int cpu)
 	}
 }
 #endif /* CONFIG_SMP */
+
+#ifdef CONFIG_PPC_KUEP
+void __init setup_kuep(bool disabled)
+{
+	if (disabled)
+		patch_instruction_site(&patch__tlb_44x_kuep, ppc_inst(PPC_RAW_NOP()));
+	else
+		pr_info("Activating Kernel Userspace Execution Prevention\n");
+
+	if (IS_ENABLED(CONFIG_PPC_47x) && disabled)
+		patch_instruction_site(&patch__tlb_47x_kuep, ppc_inst(PPC_RAW_NOP()));
+}
+#endif

arch/powerpc/platforms/Kconfig.cputype

@@ -61,6 +61,7 @@ config 44x
 	select 4xx_SOC
 	select HAVE_PCI
 	select PHYS_64BIT
+	select PPC_HAVE_KUEP
 
 endchoice