mm/gup: Factor out VMA fault permission checking

This code matches a fault condition up with the VMA and ensures
that the VMA allows the fault to be handled instead of just
erroring out.

We will be extending this in a moment to comprehend protection
keys.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dominik Dingel <dingel@linux.vnet.ibm.com>
Cc: Eric B Munson <emunson@akamai.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20160212210216.C3824032@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

mm/gup.c

@@ -610,6 +610,18 @@ next_page:
 }
 EXPORT_SYMBOL(__get_user_pages);
 
+bool vma_permits_fault(struct vm_area_struct *vma, unsigned int fault_flags)
+{
+	vm_flags_t vm_flags;
+
+	vm_flags = (fault_flags & FAULT_FLAG_WRITE) ? VM_WRITE : VM_READ;
+
+	if (!(vm_flags & vma->vm_flags))
+		return false;
+
+	return true;
+}
+
 /*
  * fixup_user_fault() - manually resolve a user page fault
  * @tsk:	the task_struct to use for page fault accounting, or
@@ -645,7 +657,6 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 		     bool *unlocked)
 {
 	struct vm_area_struct *vma;
-	vm_flags_t vm_flags;
 	int ret, major = 0;
 
 	if (unlocked)
@@ -656,8 +667,7 @@ retry:
 	if (!vma || address < vma->vm_start)
 		return -EFAULT;
 
-	vm_flags = (fault_flags & FAULT_FLAG_WRITE) ? VM_WRITE : VM_READ;
-	if (!(vm_flags & vma->vm_flags))
+	if (!vma_permits_fault(vma, fault_flags))
 		return -EFAULT;
 
 	ret = handle_mm_fault(mm, vma, address, fault_flags);