On 3/12/20 1:16 PM, Minchan Kim wrote: > On Thu, Mar 12, 2020 at 09:22:48AM +0100, Michal Hocko wrote: > I't likde to wait Jann's reply since Dave gave his opinion about the vulnerability. > https://lore.kernel.org/linux-mm/cf95db88-968d-fee5-1c15-10d024c09d8a@xxxxxxxxx/ > Jann, could you give your insigh about that practically it's possible? FWIW, just checking for mapcount>=1 seems like a pretty sane fix to me. I went looking at doing it another way, but Michal was quite correct. We'd probably end up having to special-case something underneath shrink_page_list(). > A real dumb question to understand vulnerability: > > The attacker would be able to trigger heavy memory consumption so that he > could make paging them out without MADV_PAGEOUT. I know MADV_PAGEOUT makes > it easier but he still could do without MADV_PAGEOUT. > What makes difference here? Causing memory pressure is quite a bit more disruptive than MADV_PAGEOUT. It's a much more blunt instrument and is likely to result in a lot of collateral damage and a lot of I/O. MADV_PAGEOUT is *surgical*. You can target one very specific page if, for instance, you think that your victim is reading it in a way that is vulnerable. You can also do it with zero I/O (after the initial pageout). > To clarify how MADV_PAGEWORK works: > If other process has accessed the page so that his page table has access > bit marked, MADV_PAGEOUT couldn't page it out. The attacker doesn't need to get the victim to get a major fault, it just needs to induce *a* fault. I actually did an experiment to see how this would work in practice. 1. Allocate some memory(), touch it 2. fork() 3. In the parent: Loop reading the memory 4. In the child: loop running MADV_PAGEOUT The pages stayed in the swap cache and the parent reading the memory saw a constant stream of faults.
