Re: Sealed memfd & no-fault mmap

[Lin, Ming]

On 5/28/2021 6:03 PM, Linus Torvalds wrote:
> On Fri, May 28, 2021 at 7:07 AM Lin, Ming <minggr@xxxxxxxxx> wrote:
> > Does something like following draft patch on the right track?
>
> No, I don't think this can work:
>
> > +               _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
> > +                                        vma->vm_page_prot));
>
> You can't just blindly insert the zero pfn - for a shared write
> mapping, that would actually allow writes to the zeropage. That would
> be horrible.

I should check the vma is not writable.

diff --git a/mm/shmem.c b/mm/shmem.c
index 856d2d8d4cdf..fa23e38bc692 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1820,7 +1820,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
                spinlock_t *ptl;
                int ret;
 
-               if (!IS_NOFAULT(inode))
+               if (!IS_NOFAULT(inode) || (vma->vm_flags & VM_WRITE))
                        return -EINVAL;
 
                _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr)