> On Dec 19, 2020, at 1:34 PM, Nadav Amit <nadav.amit@xxxxxxxxx> 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] >> >> Can't we check mm_tlb_flush_pending(vma->vm_mm) if MM_CP_UFFD_WP_ALL >> is set and do an explicit (potentially spurious) tlb flush before >> write-unprotect? > > There is a concrete scenario that I actually encountered and then there is a > general problem. > > In general, the kernel code assumes that PTEs that are read from the > page-tables are coherent across all the TLBs, excluding permission promotion > (i.e., the PTE may have higher permissions in the page-tables than those > that are cached in the TLBs). > > We therefore need to both: (a) protect change_protection_range() from the > changes of others who might defer TLB flushes without taking mmap_sem for > write (e.g., try_to_unmap_one()); and (b) to protect others (e.g., > page-fault handlers) from concurrent changes of change_protection(). > > We have already encountered several similar bugs, and debugging such issues > s time consuming and these bugs impact is substantial (memory corruption, > security). So I think we should only stick to general solutions. > > So perhaps your the approach of your proposed solution is feasible, but it > would have to be applied all over the place: we will need to add a check for > mm_tlb_flush_pending() and conditionally flush the TLB in every case in > which PTEs are read and there might be an assumption that the > access-permission reflect what the TLBs hold. This includes page-fault > handlers, but also NUMA migration code in change_protection(), softdirty > cleanup in clear_refs_write() and maybe others. > > [ I have in mind another solution, such as keeping in each page-table a > “table-generation” which is the mm-generation at the time of the change, > and only flush if “table-generation”==“mm-generation”, but it requires > some thought on how to avoid adding new memory barriers. ] > > IOW: I think the change that you suggest is insufficient, and a proper > solution is too intrusive for “stable". > > As for performance, I can add another patch later to remove the TLB flush > that is unnecessarily performed during change_protection_range() that does > permission promotion. I know that your concern is about the “protect” case > but I cannot think of a good immediate solution that avoids taking mmap_lock > for write. > > Thoughts? On a second thought (i.e., I don’t know what I was thinking), doing so — checking mm_tlb_flush_pending() on every PTE read which is potentially dangerous and flushing if needed - can lead to huge amount of TLB flushes and shootodowns as the counter might be elevated for considerable amount of time. So this solution seems to me as a no-go.
