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

David Hildenbrand
2021-09-10T09:23:56Z

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 ...).

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

Thanks!

--
Thanks,

David / dhildenb