parisc: Add lightweight spinlock checks

Add a lightweight spinlock check which uses only two instructions
per spinlock call. It detects if a spinlock has been trashed by
some memory corruption and then halts the kernel. It will not detect
uninitialized spinlocks, for which CONFIG_DEBUG_SPINLOCK needs to
be enabled.

This lightweight spinlock check shouldn't influence runtime, so it's
safe to enable it by default.

The __ARCH_SPIN_LOCK_UNLOCKED_VAL constant has been choosen small enough
to be able to be loaded by one LDI assembler statement.
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/Kconfig.debug

 # SPDX-License-Identifier: GPL-2.0
+#
+config LIGHTWEIGHT_SPINLOCK_CHECK
+	bool "Enable lightweight spinlock checks"
+	depends on SMP && !DEBUG_SPINLOCK
+	default y
+	help
+	  Add checks with low performance impact to the spinlock functions
+	  to catch memory overwrites at runtime. For more advanced
+	  spinlock debugging you should choose the DEBUG_SPINLOCK option
+	  which will detect unitialized spinlocks too.
+	  If unsure say Y here.

arch/parisc/include/asm/spinlock.h

@@ -7,10 +7,26 @@
 #include <asm/processor.h>
 #include <asm/spinlock_types.h>
 
+#define SPINLOCK_BREAK_INSN	0x0000c006	/* break 6,6 */
+
+static inline void arch_spin_val_check(int lock_val)
+{
+	if (IS_ENABLED(CONFIG_LIGHTWEIGHT_SPINLOCK_CHECK))
+		asm volatile(	"andcm,= %0,%1,%%r0\n"
+				".word %2\n"
+		: : "r" (lock_val), "r" (__ARCH_SPIN_LOCK_UNLOCKED_VAL),
+			"i" (SPINLOCK_BREAK_INSN));
+}
+
 static inline int arch_spin_is_locked(arch_spinlock_t *x)
 {
-	volatile unsigned int *a = __ldcw_align(x);
-	return READ_ONCE(*a) == 0;
+	volatile unsigned int *a;
+	int lock_val;
+
+	a = __ldcw_align(x);
+	lock_val = READ_ONCE(*a);
+	arch_spin_val_check(lock_val);
+	return (lock_val == 0);
 }
 
 static inline void arch_spin_lock(arch_spinlock_t *x)
@@ -18,9 +34,18 @@ static inline void arch_spin_lock(arch_spinlock_t *x)
 	volatile unsigned int *a;
 
 	a = __ldcw_align(x);
-	while (__ldcw(a) == 0)
+	do {
+		int lock_val_old;
+
+		lock_val_old = __ldcw(a);
+		arch_spin_val_check(lock_val_old);
+		if (lock_val_old)
+			return;	/* got lock */
+
+		/* wait until we should try to get lock again */
 		while (*a == 0)
 			continue;
+	} while (1);
 }
 
 static inline void arch_spin_unlock(arch_spinlock_t *x)
@@ -29,15 +54,19 @@ static inline void arch_spin_unlock(arch_spinlock_t *x)
 
 	a = __ldcw_align(x);
 	/* Release with ordered store. */
-	__asm__ __volatile__("stw,ma %0,0(%1)" : : "r"(1), "r"(a) : "memory");
+	__asm__ __volatile__("stw,ma %0,0(%1)"
+		: : "r"(__ARCH_SPIN_LOCK_UNLOCKED_VAL), "r"(a) : "memory");
 }
 
 static inline int arch_spin_trylock(arch_spinlock_t *x)
 {
 	volatile unsigned int *a;
+	int lock_val;
 
 	a = __ldcw_align(x);
-	return __ldcw(a) != 0;
+	lock_val = __ldcw(a);
+	arch_spin_val_check(lock_val);
+	return lock_val != 0;
 }
 
 /*

arch/parisc/include/asm/spinlock_types.h

@@ -2,13 +2,17 @@
 #ifndef __ASM_SPINLOCK_TYPES_H
 #define __ASM_SPINLOCK_TYPES_H
 
+#define __ARCH_SPIN_LOCK_UNLOCKED_VAL	0x1a46
+
 typedef struct {
 #ifdef CONFIG_PA20
 	volatile unsigned int slock;
-# define __ARCH_SPIN_LOCK_UNLOCKED { 1 }
+# define __ARCH_SPIN_LOCK_UNLOCKED { __ARCH_SPIN_LOCK_UNLOCKED_VAL }
 #else
 	volatile unsigned int lock[4];
-# define __ARCH_SPIN_LOCK_UNLOCKED	{ { 1, 1, 1, 1 } }
+# define __ARCH_SPIN_LOCK_UNLOCKED	\
+	{ { __ARCH_SPIN_LOCK_UNLOCKED_VAL, __ARCH_SPIN_LOCK_UNLOCKED_VAL, \
+	    __ARCH_SPIN_LOCK_UNLOCKED_VAL, __ARCH_SPIN_LOCK_UNLOCKED_VAL } }
 #endif
 } arch_spinlock_t;
 

arch/parisc/kernel/traps.c

@@ -47,6 +47,10 @@
 #include <linux/kgdb.h>
 #include <linux/kprobes.h>
 
+#if defined(CONFIG_LIGHTWEIGHT_SPINLOCK_CHECK)
+#include <asm/spinlock.h>
+#endif
+
 #include "../math-emu/math-emu.h"	/* for handle_fpe() */
 
 static void parisc_show_stack(struct task_struct *task,
@@ -309,6 +313,12 @@ static void handle_break(struct pt_regs *regs)
 	}
 #endif
 
+#ifdef CONFIG_LIGHTWEIGHT_SPINLOCK_CHECK
+        if ((iir == SPINLOCK_BREAK_INSN) && !user_mode(regs)) {
+		die_if_kernel("Spinlock was trashed", regs, 1);
+	}
+#endif
+
 	if (unlikely(iir != GDB_BREAK_INSN))
 		parisc_printk_ratelimited(0, regs,
 			KERN_DEBUG "break %d,%d: pid=%d command='%s'\n",