Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs

Leon Romanovsky wrote:

Hi Kirill,

On Mon, Jul 13, 2015 at 1:54 PM, Kirill A. Shutemov
[off-list ref] wrote:

quoted

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.

Build tested only.

Signed-off-by: Kirill A. Shutemov <redacted>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Thomas Gleixner <redacted>
---
 arch/x86/mm/mmap.c |  7 +++++++
 arch/x86/mm/mpx.c  | 20 +-------------------
 2 files changed, 8 insertions(+), 19 deletions(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 9d518d693b4b..844b06d67df4 100644
--- a/arch/x86/mm/mmap.c
+++ b/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;
+}

I sure that I'm missing something important. This function stores
"[mpx]" string on this function stack and returns the pointer to that
address. In current flow, this address is visible and accessible,
however in can be a different in general case.

The string is not on stack. String literals are in .rodata and caller is
not allowed to modify it since it's "const char *".

-- 
 Kirill

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