Re: [PATCH RFC 6/9] s390/pci_mmio: fully validate the VMA before calling follow_pte()

Liam Howlett
2021-09-10T14:12:40Z

* David Hildenbrand <david@redhat.com> [210910 05:23]:
> On 10.09.21 10:22, Niklas Schnelle wrote:
> > On Thu, 2021-09-09 at 16:59 +0200, David Hildenbrand wrote:
> > > We should not walk/touch page tables outside of VMA boundaries when
> > > holding only the mmap sem in read mode. Evil user space can modify the
> > > VMA layout just before this function runs and e.g., trigger races with
> > > page table removal code since commit dd2283f2605e ("mm: mmap: zap pages
> > > with read mmap_sem in munmap").
> > >
> > > find_vma() does not check if the address is >= the VMA start address;
> > > use vma_lookup() instead.
> > >
> > > Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > > ---
> > > arch/s390/pci/pci_mmio.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
> > > index ae683aa623ac..c5b35ea129cf 100644
> > > --- a/arch/s390/pci/pci_mmio.c
> > > +++ b/arch/s390/pci/pci_mmio.c
> > > @@ -159,7 +159,7 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
> > > mmap_read_lock(current->mm);
> > > ret = -EINVAL;
> > > - vma = find_vma(current->mm, mmio_addr);
> > > + vma = vma_lookup(current->mm, mmio_addr);
> > > if (!vma)
> > > goto out_unlock_mmap;
> > > if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > > @@ -298,7 +298,7 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
> > > mmap_read_lock(current->mm);
> > > ret = -EINVAL;
> > > - vma = find_vma(current->mm, mmio_addr);
> > > + vma = vma_lookup(current->mm, mmio_addr);
> > > if (!vma)
> > > goto out_unlock_mmap;
> > > if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> >
> > Oh wow great find thanks! If I may say so these are not great function
> > names. Looking at the code vma_lookup() is inded find_vma() plus the
> > check that the looked up address is indeed inside the vma.
> >
>
> IIRC, vma_lookup() was introduced fairly recently. Before that, this
> additional check was open coded (and still are in some instances). It's
> confusing, I agree.

This confusion is why I introduced vma_lookup(). My hope is to reduce
the users of find_vma() to only those that actually need the added
functionality, which are mostly in the mm code.

>
> > I think this is pretty independent of the rest of the patches, so do
> > you want me to apply this patch independently or do you want to wait
> > for the others?
>
> Sure, please go ahead and apply independently. It'd be great if you could
> give it a quick sanity test, although I don't expect surprises --
> unfortunately, the environment I have easily at hand is not very well suited
> (#cpu, #mem, #disk ...) for anything that exceeds basic compile tests (and
> even cross-compiling is significantly faster ...).
>
> >
> > In any case:
> >
> > Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
> >
>
> Thanks!
>
> --
> Thanks,
>
> David / dhildenb
>
>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>