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

Niklas Schnelle
2021-09-10T08:23:06Z

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.

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?

In any case:

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>