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

From: Peter Zijlstra
Date: Tue Nov 22 2016 - 04:57:25 EST



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

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/include/asm/uaccess.h | 7 +++++--
include/linux/preempt.h | 21 +++++++++++++--------
2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index faf3687f1035..b139c46ba122 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -88,8 +88,11 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, 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) \
+({ \
+ WARN_ON_ONCE(!in_task()); \
+ likely(!__range_not_ok(addr, size, user_addr_max())); \
+})

/*
* These are the main single-value transfer routines. They automatically
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 75e4e30677f1..7eeceac52dea 100644
--- 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();