ARM: BUG if jumping to usermode address in kernel mode

Detect if we are returning to usermode via the normal kernel exit paths
but the saved PSR value indicates that we are in kernel mode.  This
could occur due to corrupted stack state, which has been observed with
"ftracetest".

This ensures that we catch the problem case before we get to user code.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/include/asm/assembler.h

@@ -518,4 +518,22 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 #endif
 	.endm
 
+	.macro	bug, msg, line
+#ifdef CONFIG_THUMB2_KERNEL
+1:	.inst	0xde02
+#else
+1:	.inst	0xe7f001f2
+#endif
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+	.pushsection .rodata.str, "aMS", %progbits, 1
+2:	.asciz	"\msg"
+	.popsection
+	.pushsection __bug_table, "aw"
+	.align	2
+	.word	1b, 2b
+	.hword	\line
+	.popsection
+#endif
+	.endm
+
 #endif /* __ASM_ASSEMBLER_H__ */

arch/arm/kernel/entry-header.S

@@ -299,6 +299,8 @@
 	mov	r2, sp
 	ldr	r1, [r2, #\offset + S_PSR]	@ get calling cpsr
 	ldr	lr, [r2, #\offset + S_PC]!	@ get pc
+	tst	r1, #0xcf
+	bne	1f
 	msr	spsr_cxsf, r1			@ save in spsr_svc
 #if defined(CONFIG_CPU_V6) || defined(CONFIG_CPU_32v6K)
 	@ We must avoid clrex due to Cortex-A15 erratum #830321
@@ -313,6 +315,7 @@
 						@ after ldm {}^
 	add	sp, sp, #\offset + PT_REGS_SIZE
 	movs	pc, lr				@ return & move spsr_svc into cpsr
+1:	bug	"Returning to usermode but unexpected PSR bits set?", \@
 #elif defined(CONFIG_CPU_V7M)
 	@ V7M restore.
 	@ Note that we don't need to do clrex here as clearing the local
@@ -328,6 +331,8 @@
 	ldr	r1, [sp, #\offset + S_PSR]	@ get calling cpsr
 	ldr	lr, [sp, #\offset + S_PC]	@ get pc
 	add	sp, sp, #\offset + S_SP
+	tst	r1, #0xcf
+	bne	1f
 	msr	spsr_cxsf, r1			@ save in spsr_svc
 
 	@ We must avoid clrex due to Cortex-A15 erratum #830321
@@ -340,6 +345,7 @@
 	.endif
 	add	sp, sp, #PT_REGS_SIZE - S_SP
 	movs	pc, lr				@ return & move spsr_svc into cpsr
+1:	bug	"Returning to usermode but unexpected PSR bits set?", \@
 #endif	/* !CONFIG_THUMB2_KERNEL */
 	.endm