[ cc’ing some more people who have experience with similar problems ]
On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli aarcange@redhat.com 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?