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>

2 files changed, 24 insertions, 0 deletions

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index ad301f107dd2..bc8d4bbd82e2 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/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__ */
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 6391728c8f03..75f7a4e8541a 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/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