linux-kernel - Re: Sealed memfd & no-fault mmap

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Date:   Sat, 29 May 2021 00:31:39 -0700
From:   "Lin, Ming" <minggr@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>
Cc:     Simon Ser <contact@...rsion.fr>, Peter Xu <peterx@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Herrmann <dh.herrmann@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <greg@...ah.com>,
        "tytso@....edu" <tytso@....edu>
Subject: Re: Sealed memfd & no-fault mmap

On 5/28/2021 6:03 PM, Linus Torvalds wrote:
> On Fri, May 28, 2021 at 7:07 AM Lin, Ming <minggr@...il.com> 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)