powerpc/64: allow alternate return locations for soft-masked interrupts

The exception table fixup adjusts a failed page fault's interrupt return
location if it was taken at an address specified in the exception table,
to a corresponding fixup handler address.

Introduce a variation of that idea which adds a fixup table for NMIs and
soft-masked asynchronous interrupts. This will be used to protect
certain critical sections that are sensitive to being clobbered by
interrupts coming in (due to using the same SPRs and/or irq soft-mask
state).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-10-npiggin@gmail.com

arch/powerpc/include/asm/interrupt.h

@@ -73,6 +73,11 @@
 #include <asm/kprobes.h>
 #include <asm/runlatch.h>
 
+#ifdef CONFIG_PPC64
+extern char __end_soft_masked[];
+unsigned long search_kernel_restart_table(unsigned long addr);
+#endif
+
 #ifdef CONFIG_PPC_BOOK3S_64
 static inline void srr_regs_clobbered(void)
 {
@@ -269,6 +274,14 @@ static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct inter
 	 * new work to do (must use irq_work for that).
 	 */
 
+#ifdef CONFIG_PPC64
+	if (arch_irq_disabled_regs(regs)) {
+		unsigned long rst = search_kernel_restart_table(regs->nip);
+		if (rst)
+			regs_set_return_ip(regs, rst);
+	}
+#endif
+
 #ifdef CONFIG_PPC64
 	if (nmi_disables_ftrace(regs))
 		this_cpu_set_ftrace_enabled(state->ftrace_enabled);

arch/powerpc/include/asm/ppc_asm.h

@@ -762,6 +762,14 @@ END_FTR_SECTION_NESTED(CPU_FTR_CELL_TB_BUG, CPU_FTR_CELL_TB_BUG, 96)
 	stringify_in_c(.long (_target) - . ;)	\
 	stringify_in_c(.previous)
 
+#define RESTART_TABLE(_start, _end, _target)	\
+	stringify_in_c(.section __restart_table,"a";)\
+	stringify_in_c(.balign 8;)		\
+	stringify_in_c(.llong (_start);)	\
+	stringify_in_c(.llong (_end);)		\
+	stringify_in_c(.llong (_target);)	\
+	stringify_in_c(.previous)
+
 #ifdef CONFIG_PPC_FSL_BOOK3E
 #define BTB_FLUSH(reg)			\
 	lis reg,BUCSR_INIT@h;		\

arch/powerpc/kernel/exceptions-64e.S

@@ -901,6 +901,28 @@ kernel_dbg_exc:
 	bl	unknown_exception
 	b	interrupt_return
 
+.macro SEARCH_RESTART_TABLE
+	LOAD_REG_IMMEDIATE_SYM(r14, r11, __start___restart_table)
+	LOAD_REG_IMMEDIATE_SYM(r15, r11, __stop___restart_table)
+300:
+	cmpd	r14,r15
+	beq	302f
+	ld	r11,0(r14)
+	cmpld	r10,r11
+	blt	301f
+	ld	r11,8(r14)
+	cmpld	r10,r11
+	bge	301f
+	ld	r11,16(r14)
+	b	303f
+301:
+	addi	r14,r14,24
+	b	300b
+302:
+	li	r11,0
+303:
+.endm
+
 /*
  * An interrupt came in while soft-disabled; We mark paca->irq_happened
  * accordingly and if the interrupt is level sensitive, we hard disable
@@ -909,6 +931,9 @@ kernel_dbg_exc:
  */
 
 .macro masked_interrupt_book3e paca_irq full_mask
+	std	r14,PACA_EXGEN+EX_R14(r13)
+	std	r15,PACA_EXGEN+EX_R15(r13)
+
 	lbz	r10,PACAIRQHAPPENED(r13)
 	.if \full_mask == 1
 	ori	r10,r10,\paca_irq | PACA_IRQ_HARD_DIS
@@ -918,15 +943,23 @@ kernel_dbg_exc:
 	stb	r10,PACAIRQHAPPENED(r13)
 
 	.if \full_mask == 1
-	rldicl	r10,r11,48,1		/* clear MSR_EE */
-	rotldi	r11,r10,16
+	xori	r11,r11,MSR_EE		/* clear MSR_EE */
 	mtspr	SPRN_SRR1,r11
 	.endif
 
+	mfspr	r10,SPRN_SRR0
+	SEARCH_RESTART_TABLE
+	cmpdi	r11,0
+	beq	1f
+	mtspr	SPRN_SRR0,r11		/* return to restart address */
+1:
+
 	lwz	r11,PACA_EXGEN+EX_CR(r13)
 	mtcr	r11
 	ld	r10,PACA_EXGEN+EX_R10(r13)
 	ld	r11,PACA_EXGEN+EX_R11(r13)
+	ld	r14,PACA_EXGEN+EX_R14(r13)
+	ld	r15,PACA_EXGEN+EX_R15(r13)
 	mfspr	r13,SPRN_SPRG_GEN_SCRATCH
 	rfi
 	b	.

arch/powerpc/kernel/exceptions-64s.S

@@ -591,6 +591,36 @@ END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
 	__GEN_COMMON_BODY \name
 .endm
 
+.macro SEARCH_RESTART_TABLE
+#ifdef CONFIG_RELOCATABLE
+	mr	r12,r2
+	ld	r2,PACATOC(r13)
+	LOAD_REG_ADDR(r9, __start___restart_table)
+	LOAD_REG_ADDR(r10, __stop___restart_table)
+	mr	r2,r12
+#else
+	LOAD_REG_IMMEDIATE_SYM(r9, r12, __start___restart_table)
+	LOAD_REG_IMMEDIATE_SYM(r10, r12, __stop___restart_table)
+#endif
+300:
+	cmpd	r9,r10
+	beq	302f
+	ld	r12,0(r9)
+	cmpld	r11,r12
+	blt	301f
+	ld	r12,8(r9)
+	cmpld	r11,r12
+	bge	301f
+	ld	r12,16(r9)
+	b	303f
+301:
+	addi	r9,r9,24
+	b	300b
+302:
+	li	r12,0
+303:
+.endm
+
 /*
  * Restore all registers including H/SRR0/1 saved in a stack frame of a
  * standard exception.
@@ -2646,6 +2676,7 @@ EXC_COMMON_BEGIN(soft_nmi_common)
 	mtmsrd	r9,1
 
 	kuap_kernel_restore r9, r10
+
 	EXCEPTION_RESTORE_REGS hsrr=0
 	RFI_TO_KERNEL
 
@@ -2703,6 +2734,16 @@ masked_interrupt:
 	stb	r9,PACASRR_VALID(r13)
 	.endif
 
+	SEARCH_RESTART_TABLE
+	cmpdi	r12,0
+	beq	3f
+	.if \hsrr
+	mtspr	SPRN_HSRR0,r12
+	.else
+	mtspr	SPRN_SRR0,r12
+	.endif
+3:
+
 	ld	r9,PACA_EXGEN+EX_CTR(r13)
 	mtctr	r9
 	lwz	r9,PACA_EXGEN+EX_CCR(r13)

arch/powerpc/kernel/vmlinux.lds.S

@@ -9,6 +9,14 @@
 #define EMITS_PT_NOTE
 #define RO_EXCEPTION_TABLE_ALIGN	0
 
+#define RESTART_TABLE(align)						\
+	. = ALIGN(align);						\
+	__restart_table : AT(ADDR(__restart_table) - LOAD_OFFSET) {	\
+		__start___restart_table = .;				\
+		KEEP(*(__restart_table))				\
+		__stop___restart_table = .;				\
+	}
+
 #include <asm/page.h>
 #include <asm-generic/vmlinux.lds.h>
 #include <asm/cache.h>
@@ -124,6 +132,8 @@ SECTIONS
 	RO_DATA(PAGE_SIZE)
 
 #ifdef CONFIG_PPC64
+	RESTART_TABLE(8)
+
 	. = ALIGN(8);
 	__stf_entry_barrier_fixup : AT(ADDR(__stf_entry_barrier_fixup) - LOAD_OFFSET) {
 		__start___stf_entry_barrier_fixup = .;

arch/powerpc/lib/Makefile

@@ -42,7 +42,7 @@ obj-$(CONFIG_PPC_BOOK3S_64) += copyuser_power7.o copypage_power7.o \
 			       memcpy_power7.o
 
 obj64-y	+= copypage_64.o copyuser_64.o mem_64.o hweight_64.o \
-	   memcpy_64.o copy_mc_64.o
+	   memcpy_64.o copy_mc_64.o restart_table.o
 
 ifndef CONFIG_PPC_QUEUED_SPINLOCKS
 obj64-$(CONFIG_SMP)	+= locks.o

arch/powerpc/lib/restart_table.c

@@ -0,0 +1,30 @@
+#include <asm/interrupt.h>
+#include <asm/kprobes.h>
+
+struct restart_table_entry {
+	unsigned long start;
+	unsigned long end;
+	unsigned long fixup;
+};
+
+extern struct restart_table_entry __start___restart_table[];
+extern struct restart_table_entry __stop___restart_table[];
+
+/* Given an address, look for it in the kernel exception table */
+unsigned long search_kernel_restart_table(unsigned long addr)
+{
+	struct restart_table_entry *rte = __start___restart_table;
+
+	while (rte < __stop___restart_table) {
+		unsigned long start = rte->start;
+		unsigned long end = rte->end;
+		unsigned long fixup = rte->fixup;
+
+		if (addr >= start && addr < end)
+			return fixup;
+
+		rte++;
+	}
+	return 0;
+}
+NOKPROBE_SYMBOL(search_kernel_restart_table);