powerpc/32: Warn and return ENOSYS on syscalls from kernel

Since commit b86fb888 ("powerpc/32: implement fast entry for
syscalls on non BOOKE") and commit 1a4b739b ("powerpc/32:
implement fast entry for syscalls on BOOKE"), syscalls from
kernel are unexpected and can have catastrophic consequences
as it will destroy the kernel stack.

Test MSR_PR on syscall entry. In case syscall is from kernel,
emit a warning and return ENOSYS error.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8ee3bdbbdfdfc64ca7001e90c43b2aee6f333578.1580470482.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/entry_32.S

@@ -575,6 +575,33 @@ syscall_exit_work:
 	bl	do_syscall_trace_leave
 	b	ret_from_except_full
 
+	/*
+	 * System call was called from kernel. We get here with SRR1 in r9.
+	 * Mark the exception as recoverable once we have retrieved SRR0,
+	 * trap a warning and return ENOSYS with CR[SO] set.
+	 */
+	.globl	ret_from_kernel_syscall
+ret_from_kernel_syscall:
+	mfspr	r9, SPRN_SRR0
+	mfspr	r10, SPRN_SRR1
+#if !defined(CONFIG_4xx) && !defined(CONFIG_BOOKE)
+	LOAD_REG_IMMEDIATE(r11, MSR_KERNEL & ~(MSR_IR|MSR_DR))
+	mtmsr	r11
+#endif
+
+0:	trap
+	EMIT_BUG_ENTRY 0b,__FILE__,__LINE__, BUGFLAG_WARNING
+
+	li	r3, ENOSYS
+	crset	so
+#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PERF_EVENTS)
+	mtspr	SPRN_NRI, r0
+#endif
+	mtspr	SPRN_SRR0, r9
+	mtspr	SPRN_SRR1, r10
+	SYNC
+	RFI
+
 /*
  * The fork/clone functions need to copy the full register set into
  * the child process. Therefore we need to save all the nonvolatile

arch/powerpc/kernel/head_32.h

@@ -111,14 +111,16 @@
 
 .macro SYSCALL_ENTRY trapno
 	mfspr	r12,SPRN_SPRG_THREAD
+	mfspr	r9, SPRN_SRR1
 #ifdef CONFIG_VMAP_STACK
-	mfspr	r9, SPRN_SRR0
-	mfspr	r11, SPRN_SRR1
-	stw	r9, SRR0(r12)
-	stw	r11, SRR1(r12)
+	mfspr	r11, SPRN_SRR0
+	stw	r11, SRR0(r12)
+	stw	r9, SRR1(r12)
 #endif
 	mfcr	r10
+	andi.	r11, r9, MSR_PR
 	lwz	r11,TASK_STACK-THREAD(r12)
+	beq-	99f
 	rlwinm	r10,r10,0,4,2	/* Clear SO bit in CR */
 	addi	r11, r11, THREAD_SIZE - INT_FRAME_SIZE
 #ifdef CONFIG_VMAP_STACK
@@ -128,15 +130,14 @@
 #endif
 	tovirt_vmstack r12, r12
 	tophys_novmstack r11, r11
-	mflr	r9
 	stw	r10,_CCR(r11)		/* save registers */
-	stw	r9, _LINK(r11)
+	mflr	r10
+	stw	r10, _LINK(r11)
 #ifdef CONFIG_VMAP_STACK
 	lwz	r10, SRR0(r12)
 	lwz	r9, SRR1(r12)
 #else
 	mfspr	r10,SPRN_SRR0
-	mfspr	r9,SPRN_SRR1
 #endif
 	stw	r1,GPR1(r11)
 	stw	r1,0(r11)
@@ -209,6 +210,7 @@
 	mtspr	SPRN_SRR0,r11
 	SYNC
 	RFI				/* jump to handler, enable MMU */
+99:	b	ret_from_kernel_syscall
 .endm
 
 .macro save_dar_dsisr_on_stack reg1, reg2, sp

arch/powerpc/kernel/head_booke.h

@@ -104,16 +104,18 @@ FTR_SECTION_ELSE
 #ifdef CONFIG_KVM_BOOKE_HV
 ALT_FTR_SECTION_END_IFSET(CPU_FTR_EMB_HV)
 #endif
+	mfspr	r9, SPRN_SRR1
 	BOOKE_CLEAR_BTB(r11)
+	andi.	r11, r9, MSR_PR
 	lwz	r11, TASK_STACK - THREAD(r10)
 	rlwinm	r12,r12,0,4,2	/* Clear SO bit in CR */
+	beq-	99f
 	ALLOC_STACK_FRAME(r11, THREAD_SIZE - INT_FRAME_SIZE)
 	stw	r12, _CCR(r11)		/* save various registers */
 	mflr	r12
 	stw	r12,_LINK(r11)
 	mfspr	r12,SPRN_SRR0
 	stw	r1, GPR1(r11)
-	mfspr	r9,SPRN_SRR1
 	stw	r1, 0(r11)
 	mr	r1, r11
 	stw	r12,_NIP(r11)
@@ -176,6 +178,7 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_EMB_HV)
 	mtspr	SPRN_SRR0,r11
 	SYNC
 	RFI				/* jump to handler, enable MMU */
+99:	b	ret_from_kernel_syscall
 .endm
 
 /* To handle the additional exception priority levels on 40x and Book-E