On Sun, Dec 20, 2020 at 12:06:38AM -0800, Nadav Amit wrote: > > On Dec 19, 2020, at 10:05 PM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > > > On Sat, Dec 19, 2020 at 01:34:29PM -0800, Nadav Amit wrote: > >> [ cc’ing some more people who have experience with similar problems ] > >> > >>> On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > >>> > >>> Hello, > >>> > >>> On Fri, Dec 18, 2020 at 08:30:06PM -0800, Nadav Amit wrote: > >>>> Analyzing this problem indicates that there is a real bug since > >>>> mmap_lock is only taken for read in mwriteprotect_range(). This might > >>> > >>> Never having to take the mmap_sem for writing, and in turn never > >>> blocking, in order to modify the pagetables is quite an important > >>> feature in uffd that justifies uffd instead of mprotect. It's not the > >>> most important reason to use uffd, but it'd be nice if that guarantee > >>> would remain also for the UFFDIO_WRITEPROTECT API, not only for the > >>> other pgtable manipulations. > >>> > >>>> Consider the following scenario with 3 CPUs (cpu2 is not shown): > >>>> > >>>> cpu0 cpu1 > >>>> ---- ---- > >>>> userfaultfd_writeprotect() > >>>> [ write-protecting ] > >>>> mwriteprotect_range() > >>>> mmap_read_lock() > >>>> change_protection() > >>>> change_protection_range() > >>>> ... > >>>> change_pte_range() > >>>> [ defer TLB flushes] > >>>> userfaultfd_writeprotect() > >>>> mmap_read_lock() > >>>> change_protection() > >>>> [ write-unprotect ] > >>>> ... > >>>> [ unprotect PTE logically ] > >>>> ... > >>>> [ page-fault] > >>>> ... > >>>> wp_page_copy() > >>>> [ set new writable page in PTE] > > > > I don't see any problem in this example -- wp_page_copy() calls > > ptep_clear_flush_notify(), which should take care of the stale entry > > left by cpu0. > > > > That being said, I suspect the memory corruption you observed is > > related this example, with cpu1 running something else that flushes > > conditionally depending on pte_write(). > > > > Do you know which type of pages were corrupted? file, anon, etc. > > First, Yu, you are correct. My analysis is incorrect, but let me have > another try (below). To answer your (and Andrea’s) question - this happens > with upstream without any changes, excluding a small fix to the selftest, > since it failed (got stuck) due to missing wake events. [1] > > We are talking about anon memory. > > So to correct myself, I think that what I really encountered was actually > during MM_CP_UFFD_WP_RESOLVE (i.e., when the protection is removed). The > problem was that in this case the “write”-bit was removed during unprotect. Thanks. You are right about when the problem happens: UFD write- UNprotecting. But it's not UFD write-UNprotecting that removes the writable bit -- the bit can only be removed during COW or UFD write-protecting. So your original example was almost correct, except the last line describing cpu1. The problem is how do_wp_page() handles non-COW pages. (For COW pages, do_wp_page() works correctly by either reusing an existing page or make a new copy out of it.) In UFD case, the existing page may not have been properly write-protected. As you pointed out, the tlb flush may not be done yet. Making a copy can potentially race with the writer on cpu2. Should we fix the problem by ensuring integrity of the copy? IMO, no, because do_wp_page() shouldn't copy at all in this case. It seems it was recently broken by be068f29034f mm: fix misplaced unlock_page in do_wp_page() 09854ba94c6a mm: do_wp_page() simplification I haven't study them carefully. But if you could just revert them and run the test again, we'd know where exactly to look at next. > Sorry for the strange formatting to fit within 80 columns: > > > [ Start: PTE is writable ] > > cpu0 cpu1 cpu2 > ---- ---- ---- > [ Writable PTE > cached in TLB ] > userfaultfd_writeprotect() > [ write-*unprotect* ] > mwriteprotect_range() > mmap_read_lock() > change_protection() > > change_protection_range() > ... > change_pte_range() > [ *clear* “write”-bit ] > [ defer TLB flushes] > [ page-fault ] > … > wp_page_copy() > cow_user_page() > [ copy page ] > [ write to old > page ] > … > set_pte_at_notify() > > [ End: cpu2 write not copied form old to new page. ] > > > So this was actually resolved by the second part of the patch - changing > preserve_write in change_pte_range(). I removed the acquisition of mmap_lock > for write, left the change in change_pte_range() and the test passes. > > Let me give some more thought on whether a mmap_lock is needed > for write. I need to rehash this TLB flushing algorithm. > > Thanks, > Nadav > > [1] https://lore.kernel.org/patchwork/patch/1346386
