powerpc/32s: Fix DSI and ISI exceptions for CONFIG_VMAP_STACK

hash_page() needs to read page tables from kernel memory. When entire
kernel memory is mapped by BATs, which is normally the case when
CONFIG_STRICT_KERNEL_RWX is not set, it works even if the page hosting
the page table is not referenced in the MMU hash table.

However, if the page where the page table resides is not covered by
a BAT, a DSI fault can be encountered from hash_page(), and it loops
forever. This can happen when CONFIG_STRICT_KERNEL_RWX is selected
and the alignment of the different regions is too small to allow
covering the entire memory with BATs. This also happens when
CONFIG_DEBUG_PAGEALLOC is selected or when booting with 'nobats'
flag.

Also, if the page containing the kernel stack is not present in the
MMU hash table, registers cannot be saved and a recursive DSI fault
is encountered.

To allow hash_page() to properly do its job at all time and load the
MMU hash table whenever needed, it must run with data MMU disabled.
This means it must be called before re-enabling data MMU. To allow
this, registers clobbered by hash_page() and create_hpte() have to
be saved in the thread struct together with SRR0, SSR1, DAR and DSISR.
It is also necessary to ensure that DSI prolog doesn't overwrite
regs saved by prolog of the current running exception. That means:
- DSI can only use SPRN_SPRG_SCRATCH0
- Exceptions must free SPRN_SPRG_SCRATCH0 before writing to the stack.

This also fixes the Oops reported by Erhard when create_hpte() is
called by add_hash_page().

Due to prolog size increase, a few more exceptions had to get split
in two parts.

Fixes: cd08f10 ("powerpc/32s: Enable CONFIG_VMAP_STACK")
Reported-by: Erhard F. <erhard_f@mailbox.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Tested-by: Erhard F. <erhard_f@mailbox.org>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206501
Link: https://lore.kernel.org/r/64a4aa44686e9fd4b01333401367029771d9b231.1581761633.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/processor.h

@@ -168,6 +168,10 @@ struct thread_struct {
 	unsigned long	srr1;
 	unsigned long	dar;
 	unsigned long	dsisr;
+#ifdef CONFIG_PPC_BOOK3S_32
+	unsigned long	r0, r3, r4, r5, r6, r8, r9, r11;
+	unsigned long	lr, ctr;
+#endif
 #endif
 	/* Debug Registers */
 	struct debug_reg debug;

arch/powerpc/kernel/asm-offsets.c

@@ -132,6 +132,18 @@ int main(void)
 	OFFSET(SRR1, thread_struct, srr1);
 	OFFSET(DAR, thread_struct, dar);
 	OFFSET(DSISR, thread_struct, dsisr);
+#ifdef CONFIG_PPC_BOOK3S_32
+	OFFSET(THR0, thread_struct, r0);
+	OFFSET(THR3, thread_struct, r3);
+	OFFSET(THR4, thread_struct, r4);
+	OFFSET(THR5, thread_struct, r5);
+	OFFSET(THR6, thread_struct, r6);
+	OFFSET(THR8, thread_struct, r8);
+	OFFSET(THR9, thread_struct, r9);
+	OFFSET(THR11, thread_struct, r11);
+	OFFSET(THLR, thread_struct, lr);
+	OFFSET(THCTR, thread_struct, ctr);
+#endif
 #endif
 #ifdef CONFIG_SPE
 	OFFSET(THREAD_EVR0, thread_struct, evr[0]);

arch/powerpc/kernel/head_32.S

@@ -290,17 +290,55 @@ MachineCheck:
 7:	EXCEPTION_PROLOG_2
 	addi	r3,r1,STACK_FRAME_OVERHEAD
 #ifdef CONFIG_PPC_CHRP
-	bne	cr1,1f
+#ifdef CONFIG_VMAP_STACK
+	mfspr	r4, SPRN_SPRG_THREAD
+	tovirt(r4, r4)
+	lwz	r4, RTAS_SP(r4)
+	cmpwi	cr1, r4, 0
 #endif
-	EXC_XFER_STD(0x200, machine_check_exception)
-#ifdef CONFIG_PPC_CHRP
-1:	b	machine_check_in_rtas
+	beq	cr1, machine_check_tramp
+	b	machine_check_in_rtas
+#else
+	b	machine_check_tramp
 #endif
 
 /* Data access exception. */
 	. = 0x300
 	DO_KVM  0x300
 DataAccess:
+#ifdef CONFIG_VMAP_STACK
+	mtspr	SPRN_SPRG_SCRATCH0,r10
+	mfspr	r10, SPRN_SPRG_THREAD
+BEGIN_MMU_FTR_SECTION
+	stw	r11, THR11(r10)
+	mfspr	r10, SPRN_DSISR
+	mfcr	r11
+#ifdef CONFIG_PPC_KUAP
+	andis.	r10, r10, (DSISR_BAD_FAULT_32S | DSISR_DABRMATCH | DSISR_PROTFAULT)@h
+#else
+	andis.	r10, r10, (DSISR_BAD_FAULT_32S | DSISR_DABRMATCH)@h
+#endif
+	mfspr	r10, SPRN_SPRG_THREAD
+	beq	hash_page_dsi
+.Lhash_page_dsi_cont:
+	mtcr	r11
+	lwz	r11, THR11(r10)
+END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
+	mtspr	SPRN_SPRG_SCRATCH1,r11
+	mfspr	r11, SPRN_DAR
+	stw	r11, DAR(r10)
+	mfspr	r11, SPRN_DSISR
+	stw	r11, DSISR(r10)
+	mfspr	r11, SPRN_SRR0
+	stw	r11, SRR0(r10)
+	mfspr	r11, SPRN_SRR1		/* check whether user or kernel */
+	stw	r11, SRR1(r10)
+	mfcr	r10
+	andi.	r11, r11, MSR_PR
+
+	EXCEPTION_PROLOG_1
+	b	handle_page_fault_tramp_1
+#else	/* CONFIG_VMAP_STACK */
 	EXCEPTION_PROLOG handle_dar_dsisr=1
 	get_and_save_dar_dsisr_on_stack	r4, r5, r11
 BEGIN_MMU_FTR_SECTION
@@ -316,11 +354,32 @@ BEGIN_MMU_FTR_SECTION
 FTR_SECTION_ELSE
 	b	handle_page_fault_tramp_2
 ALT_MMU_FTR_SECTION_END_IFSET(MMU_FTR_HPTE_TABLE)
+#endif	/* CONFIG_VMAP_STACK */
 
 /* Instruction access exception. */
 	. = 0x400
 	DO_KVM  0x400
 InstructionAccess:
+#ifdef CONFIG_VMAP_STACK
+	mtspr	SPRN_SPRG_SCRATCH0,r10
+	mtspr	SPRN_SPRG_SCRATCH1,r11
+	mfspr	r10, SPRN_SPRG_THREAD
+	mfspr	r11, SPRN_SRR0
+	stw	r11, SRR0(r10)
+	mfspr	r11, SPRN_SRR1		/* check whether user or kernel */
+	stw	r11, SRR1(r10)
+	mfcr	r10
+BEGIN_MMU_FTR_SECTION
+	andis.	r11, r11, SRR1_ISI_NOPT@h	/* no pte found? */
+	bne	hash_page_isi
+.Lhash_page_isi_cont:
+	mfspr	r11, SPRN_SRR1		/* check whether user or kernel */
+END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
+	andi.	r11, r11, MSR_PR
+
+	EXCEPTION_PROLOG_1
+	EXCEPTION_PROLOG_2
+#else	/* CONFIG_VMAP_STACK */
 	EXCEPTION_PROLOG
 	andis.	r0,r9,SRR1_ISI_NOPT@h	/* no pte found? */
 	beq	1f			/* if so, try to put a PTE */
@@ -329,6 +388,7 @@ InstructionAccess:
 BEGIN_MMU_FTR_SECTION
 	bl	hash_page
 END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
+#endif	/* CONFIG_VMAP_STACK */
 1:	mr	r4,r12
 	andis.	r5,r9,DSISR_SRR1_MATCH_32S@h /* Filter relevant SRR1 bits */
 	stw	r4, _DAR(r11)
@@ -344,7 +404,7 @@ Alignment:
 	EXCEPTION_PROLOG handle_dar_dsisr=1
 	save_dar_dsisr_on_stack r4, r5, r11
 	addi	r3,r1,STACK_FRAME_OVERHEAD
-	EXC_XFER_STD(0x600, alignment_exception)
+	b	alignment_exception_tramp
 
 /* Program check exception */
 	EXCEPTION(0x700, ProgramCheck, program_check_exception, EXC_XFER_STD)
@@ -645,15 +705,100 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_NEED_DTLB_SW_LRU)
 
 	. = 0x3000
 
+machine_check_tramp:
+	EXC_XFER_STD(0x200, machine_check_exception)
+
+alignment_exception_tramp:
+	EXC_XFER_STD(0x600, alignment_exception)
+
 handle_page_fault_tramp_1:
+#ifdef CONFIG_VMAP_STACK
+	EXCEPTION_PROLOG_2 handle_dar_dsisr=1
+#endif
 	lwz	r4, _DAR(r11)
 	lwz	r5, _DSISR(r11)
 	/* fall through */
 handle_page_fault_tramp_2:
 	EXC_XFER_LITE(0x300, handle_page_fault)
 
+#ifdef CONFIG_VMAP_STACK
+.macro save_regs_thread		thread
+	stw	r0, THR0(\thread)
+	stw	r3, THR3(\thread)
+	stw	r4, THR4(\thread)
+	stw	r5, THR5(\thread)
+	stw	r6, THR6(\thread)
+	stw	r8, THR8(\thread)
+	stw	r9, THR9(\thread)
+	mflr	r0
+	stw	r0, THLR(\thread)
+	mfctr	r0
+	stw	r0, THCTR(\thread)
+.endm
+
+.macro restore_regs_thread	thread
+	lwz	r0, THLR(\thread)
+	mtlr	r0
+	lwz	r0, THCTR(\thread)
+	mtctr	r0
+	lwz	r0, THR0(\thread)
+	lwz	r3, THR3(\thread)
+	lwz	r4, THR4(\thread)
+	lwz	r5, THR5(\thread)
+	lwz	r6, THR6(\thread)
+	lwz	r8, THR8(\thread)
+	lwz	r9, THR9(\thread)
+.endm
+
+hash_page_dsi:
+	save_regs_thread	r10
+	mfdsisr	r3
+	mfdar	r4
+	mfsrr0	r5
+	mfsrr1	r9
+	rlwinm	r3, r3, 32 - 15, _PAGE_RW	/* DSISR_STORE -> _PAGE_RW */
+	bl	hash_page
+	mfspr	r10, SPRN_SPRG_THREAD
+	restore_regs_thread r10
+	b	.Lhash_page_dsi_cont
+
+hash_page_isi:
+	mr	r11, r10
+	mfspr	r10, SPRN_SPRG_THREAD
+	save_regs_thread	r10
+	li	r3, 0
+	lwz	r4, SRR0(r10)
+	lwz	r9, SRR1(r10)
+	bl	hash_page
+	mfspr	r10, SPRN_SPRG_THREAD
+	restore_regs_thread r10
+	mr	r10, r11
+	b	.Lhash_page_isi_cont
+
+	.globl fast_hash_page_return
+fast_hash_page_return:
+	andis.	r10, r9, SRR1_ISI_NOPT@h	/* Set on ISI, cleared on DSI */
+	mfspr	r10, SPRN_SPRG_THREAD
+	restore_regs_thread r10
+	bne	1f
+
+	/* DSI */
+	mtcr	r11
+	lwz	r11, THR11(r10)
+	mfspr	r10, SPRN_SPRG_SCRATCH0
+	SYNC
+	RFI
+
+1:	/* ISI */
+	mtcr	r11
+	mfspr	r11, SPRN_SPRG_SCRATCH1
+	mfspr	r10, SPRN_SPRG_SCRATCH0
+	SYNC
+	RFI
+
 stack_overflow:
 	vmap_stack_overflow_exception
+#endif
 
 AltiVecUnavailable:
 	EXCEPTION_PROLOG

arch/powerpc/kernel/head_32.h

@@ -64,11 +64,25 @@
 .endm
 
 .macro EXCEPTION_PROLOG_2 handle_dar_dsisr=0
+#if defined(CONFIG_VMAP_STACK) && defined(CONFIG_PPC_BOOK3S)
+BEGIN_MMU_FTR_SECTION
+	mtcr	r10
+FTR_SECTION_ELSE
+	stw	r10, _CCR(r11)
+ALT_MMU_FTR_SECTION_END_IFSET(MMU_FTR_HPTE_TABLE)
+#else
 	stw	r10,_CCR(r11)		/* save registers */
+#endif
+	mfspr	r10, SPRN_SPRG_SCRATCH0
 	stw	r12,GPR12(r11)
 	stw	r9,GPR9(r11)
-	mfspr	r10,SPRN_SPRG_SCRATCH0
 	stw	r10,GPR10(r11)
+#if defined(CONFIG_VMAP_STACK) && defined(CONFIG_PPC_BOOK3S)
+BEGIN_MMU_FTR_SECTION
+	mfcr	r10
+	stw	r10, _CCR(r11)
+END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
+#endif
 	mfspr	r12,SPRN_SPRG_SCRATCH1
 	stw	r12,GPR11(r11)
 	mflr	r10
@@ -83,6 +97,11 @@
 	stw	r10, _DSISR(r11)
 	.endif
 	lwz	r9, SRR1(r12)
+#if defined(CONFIG_VMAP_STACK) && defined(CONFIG_PPC_BOOK3S)
+BEGIN_MMU_FTR_SECTION
+	andi.	r10, r9, MSR_PR
+END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
+#endif
 	lwz	r12, SRR0(r12)
 #else
 	mfspr	r12,SPRN_SRR0