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

[Miaohe Lin]

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.

> 

Thanks for your comment again. :)