Re: [RFC][PATCH] x86: Verify access_ok() context

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 22, 2016 at 11:42:19AM -0800, Linus Torvalds wrote:
> On Tue, Nov 22, 2016 at 11:37 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > CONFIG_DEBUG_VM seems somehow inappropriate.
> 
> The usual might_fault() logic? That uses
> 
>     defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)
> 
> (and "might_sleep()" uses just CONFIG_DEBUG_ATOMIC_SLEEP, maybe that's fine).
> 

Fair enough; something like so then?

---
Subject: x86: Verify access_ok() context
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 22 Nov 2016 10:57:15 +0100

I recently encountered wreckage because access_ok() was used where it
should not be, add an explicit WARN when access_ok() is used wrongly.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/uaccess.h |   13 +++++++++++--
 include/linux/preempt.h        |   21 +++++++++++++--------
 2 files changed, 24 insertions(+), 10 deletions(-)

--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -68,6 +68,12 @@ static inline bool __chk_range_not_ok(un
 	__chk_range_not_ok((unsigned long __force)(addr), size, limit); \
 })
 
+#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
+#define ACCESS_OK_WARN()	WARN_ON_ONCE(!in_task())
+#else
+#define ACCESS_OK_WARN()
+#endif
+
 /**
  * access_ok: - Checks if a user space pointer is valid
  * @type: Type of access: %VERIFY_READ or %VERIFY_WRITE.  Note that
@@ -88,8 +94,11 @@ static inline bool __chk_range_not_ok(un
  * checks that the pointer is in the user space range - after calling
  * this function, memory access functions may still return -EFAULT.
  */
-#define access_ok(type, addr, size) \
-	likely(!__range_not_ok(addr, size, user_addr_max()))
+#define access_ok(type, addr, size)					\
+({									\
+	ACCESS_OK_WARN();						\
+	likely(!__range_not_ok(addr, size, user_addr_max()));		\
+})
 
 /*
  * These are the main single-value transfer routines.  They automatically
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -65,19 +65,24 @@
 
 /*
  * Are we doing bottom half or hardware interrupt processing?
- * Are we in a softirq context? Interrupt context?
- * in_softirq - Are we currently processing softirq or have bh disabled?
- * in_serving_softirq - Are we currently processing softirq?
+ *
+ * in_irq()       - We're in (hard) IRQ context
+ * in_softirq()   - We have BH disabled, or are processing softirqs
+ * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
+ * in_serving_softirq() - We're in softirq context
+ * in_nmi()       - We're in NMI context
+ * in_task()	  - We're in task context
+ *
+ * Note: due to the BH disabled confusion: in_softirq(),in_interrupt() really
+ *       should not be used in new code.
  */
 #define in_irq()		(hardirq_count())
 #define in_softirq()		(softirq_count())
 #define in_interrupt()		(irq_count())
 #define in_serving_softirq()	(softirq_count() & SOFTIRQ_OFFSET)
-
-/*
- * Are we in NMI context?
- */
-#define in_nmi()	(preempt_count() & NMI_MASK)
+#define in_nmi()		(preempt_count() & NMI_MASK)
+#define in_task()		(!(preempt_count() & \
+				   (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
 
 /*
  * The preempt_count offset after preempt_disable();