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

Niklas Schnelle
2021-09-10T12:49:16Z

On Fri, 2021-09-10 at 11:23 +0200, David Hildenbrand wrote:
> 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.
>
> > 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 ...).

Yes and even if you had more hardware this code path is only hit by
very specialized workloads doing MMIO access of PCI devices from
userspace. I did test with such a workload (ib_send_bw test utility)
and all looks good.

Applied and will be sent out by Heiko or Vasily as part of the s390
tree.

>
> > In any case:
> >
> > Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
> >
>
> Thanks!
>