Re: [bug report] mm/hugetlb: possible data leak with huge pmd sharing

[Mike Kravetz]

On 07/26/22 15:14, Miaohe Lin wrote:
> On 2022/7/26 2:35, Mike Kravetz wrote:
> > On 07/25/22 17:07, Miaohe Lin wrote:
> >> Hi all:
> >>     When I investigate the mm/hugetlb code, I found there's a possible data leak issue
> >> with huge pmd sharing. Thank about the below scene:
> >>
> >>     1. Process A and process B shares huge pmd page.(vm_flags: VM_MAYSHARE but !VM_SHARED)
> > 
> > Thanks,
> > 
> > I often get confused about the setting of VM_MAYSHARE and VM_SHARED.  When
> > you throw in the possibility of shared and anonymous, then I struggle a bit
> > more.  At one time did an audit to get the meaning clear in my mind, but still
> > struggle with the meanings.
> > 
> > Is it possible to have VM_MAYSHARE and !VM_SHARED on a hugetlb vma?  I only
> > took a quick look and could not find a way for this to happen.  But, I> could have easily missed something.
> 
> Thanks for your reply. It's possible to have VM_MAYSHARE and !VM_SHARED on a hugetlb vma
> with below code snippet:
> 
> ...
>     fd = open("/root/huge/hugepagefile", O_CREAT | O_RDONLY, 0755);
>     if (fd < 0) {
>             perror("Open failed");
>             exit(1);
>     }
> 
>     addr = mmap(0, 32UL*1024*1024, PROT_READ, MAP_SHARED, fd, 0);
> ...
> 
> cat /proc/<pid>/smaps:
> 
> 400000000000-400002000000 r--s 00000000 00:2f 153780886                  /root/huge/hugepagefile
> Size:              32768 kB
> KernelPageSize:     2048 kB
> MMUPageSize:        2048 kB
> ...
> VmFlags: rd mr me ms de ht
> 
> /* sh: VM_SHARED, mw: VM_MAYWRITE, ms:VM_MAYSHARE */
> 
> So vm_flags is VM_MAYSHARE but !VM_SHARED.
> 
> But in this case, it's readonly. So the above scene won't happen. Sorry for make noise.
> 

No worries!  And, thank you for looking at the pmd sharing code.  In concept
the functionality is simple.  However, details and edge cases make things
complicated.

If you are interested in the pmd sharing code, more eyes on this proposal
would be appreciated.

https://lore.kernel.org/linux-mm/20220706202347.95150-1-mike.kravetz@xxxxxxxxxx/

-- 
Mike Kravetz