x86/mpx: Do not set ->vm_ops on MPX VMAs

commit a896527 upstream.

MPX setups private anonymous mapping, but uses vma->vm_ops too.
This can confuse core VM, as it relies on vm->vm_ops to
distinguish file VMAs from anonymous.

As result we will get SIGBUS, because handle_pte_fault() thinks
it's file VMA without vm_ops->fault and it doesn't know how to
handle the situation properly.

Let's fix that by not setting ->vm_ops.

We don't really need ->vm_ops here: MPX VMA can be detected with
VM_MPX flag. And vma_merge() will not merge MPX VMA with non-MPX
VMA, because ->vm_flags won't match.

The only thing left is name of VMA. I'm not sure if it's part of
ABI, or we can just drop it. The patch keep it by providing
arch_vma_name() on x86.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@sr71.net
Link: http://lkml.kernel.org/r/20150720212958.305CC3E9@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/mm/mmap.c

@@ -126,3 +126,10 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 	}
 }
+
+const char *arch_vma_name(struct vm_area_struct *vma)
+{
+	if (vma->vm_flags & VM_MPX)
+		return "[mpx]";
+	return NULL;
+}

arch/x86/mm/mpx.c

@@ -18,26 +18,9 @@
 #include <asm/processor.h>
 #include <asm/fpu-internal.h>
 
-static const char *mpx_mapping_name(struct vm_area_struct *vma)
-{
-	return "[mpx]";
-}
-
-static struct vm_operations_struct mpx_vma_ops = {
-	.name = mpx_mapping_name,
-};
-
-static int is_mpx_vma(struct vm_area_struct *vma)
-{
-	return (vma->vm_ops == &mpx_vma_ops);
-}
-
 /*
  * This is really a simplified "vm_mmap". it only handles MPX
  * bounds tables (the bounds directory is user-allocated).
- *
- * Later on, we use the vma->vm_ops to uniquely identify these
- * VMAs.
  */
 static unsigned long mpx_mmap(unsigned long len)
 {
@@ -83,7 +66,6 @@ static unsigned long mpx_mmap(unsigned long len)
 		ret = -ENOMEM;
 		goto out;
 	}
-	vma->vm_ops = &mpx_vma_ops;
 
 	if (vm_flags & VM_LOCKED) {
 		up_write(&mm->mmap_sem);
@@ -661,7 +643,7 @@ static int zap_bt_entries(struct mm_struct *mm,
 		 * so stop immediately and return an error.  This
 		 * probably results in a SIGSEGV.
 		 */
-		if (!is_mpx_vma(vma))
+		if (!(vma->vm_flags & VM_MPX))
 			return -EINVAL;
 
 		len = min(vma->vm_end, end) - addr;