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

[Kirill A. Shutemov]

Leon Romanovsky wrote:
> Hi Kirill,
> 
> On Mon, Jul 13, 2015 at 1:54 PM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > 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 <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > ---
> >  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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