On Wed, Sep 27, 2023 at 1:04 PM Jann Horn jannh@google.com wrote:
On Wed, Sep 27, 2023 at 8:08 PM Suren Baghdasaryan surenb@google.com wrote:
On Wed, Sep 27, 2023 at 5:47 AM Jann Horn jannh@google.com wrote:
On Sat, Sep 23, 2023 at 3:31 AM Suren Baghdasaryan surenb@google.com wrote:
From: Andrea Arcangeli aarcange@redhat.com
This implements the uABI of UFFDIO_REMAP.
Notably one mode bitflag is also forwarded (and in turn known) by the lowlevel remap_pages method.
Signed-off-by: Andrea Arcangeli aarcange@redhat.com Signed-off-by: Suren Baghdasaryan surenb@google.com
[...]
/** folio_referenced walks the anon_vma chain* without the folio lock. Serialize against it with* the anon_vma lock, the folio lock is not enough.*/src_anon_vma = folio_get_anon_vma(src_folio);if (!src_anon_vma) {/* page was unmapped from under us */err = -EAGAIN;goto out;}if (!anon_vma_trylock_write(src_anon_vma)) {pte_unmap(&orig_src_pte);pte_unmap(&orig_dst_pte);src_pte = dst_pte = NULL;/* now we can block and wait */anon_vma_lock_write(src_anon_vma);goto retry;}}So at this point we have:
- the current src_pte
- some referenced+locked src_folio that used to be mapped exclusively
at src_addr
- (the anon_vma associated with the src_folio)
err = remap_anon_pte(dst_mm, src_mm, dst_vma, src_vma,dst_addr, src_addr, dst_pte, src_pte,orig_dst_pte, orig_src_pte,dst_ptl, src_ptl, src_folio);And then this will, without touching folio mapcounts/refcounts, delete the current PTE at src_addr, and create a PTE at dst_addr pointing to the old src_folio, leading to incorrect refcounts/mapcounts?
I assume this still points to the missing previous_src_pte check discussed in the previous comments. Is that correct or is there yet another issue?
This is still referring to the missing previous_src_pte check.
} else {[...]
}+out:
if (src_anon_vma) {anon_vma_unlock_write(src_anon_vma);put_anon_vma(src_anon_vma);}if (src_folio) {folio_unlock(src_folio);folio_put(src_folio);}if (dst_pte)pte_unmap(dst_pte);if (src_pte)pte_unmap(src_pte);mmu_notifier_invalidate_range_end(&range);return err;+}
[...]
+ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm,
unsigned long dst_start, unsigned long src_start,unsigned long len, __u64 mode)+{
struct vm_area_struct *src_vma, *dst_vma;unsigned long src_addr, dst_addr;pmd_t *src_pmd, *dst_pmd;long err = -EINVAL;ssize_t moved = 0;/** Sanitize the command parameters:*/BUG_ON(src_start & ~PAGE_MASK);BUG_ON(dst_start & ~PAGE_MASK);BUG_ON(len & ~PAGE_MASK);/* Does the address range wrap, or is the span zero-sized? */BUG_ON(src_start + len <= src_start);BUG_ON(dst_start + len <= dst_start);/** Because these are read sempahores there's no risk of lock* inversion.*/mmap_read_lock(dst_mm);if (dst_mm != src_mm)mmap_read_lock(src_mm);/** Make sure the vma is not shared, that the src and dst remap* ranges are both valid and fully within a single existing* vma.*/src_vma = find_vma(src_mm, src_start);if (!src_vma || (src_vma->vm_flags & VM_SHARED))goto out;if (src_start < src_vma->vm_start ||src_start + len > src_vma->vm_end)goto out;dst_vma = find_vma(dst_mm, dst_start);if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))goto out;if (dst_start < dst_vma->vm_start ||dst_start + len > dst_vma->vm_end)goto out;err = validate_remap_areas(src_vma, dst_vma);if (err)goto out;for (src_addr = src_start, dst_addr = dst_start;src_addr < src_start + len;) {spinlock_t *ptl;pmd_t dst_pmdval;unsigned long step_size;BUG_ON(dst_addr >= dst_start + len);/** Below works because anonymous area would not have a* transparent huge PUD. If file-backed support is added,* that case would need to be handled here.*/src_pmd = mm_find_pmd(src_mm, src_addr);if (unlikely(!src_pmd)) {if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) {err = -ENOENT;break;}src_pmd = mm_alloc_pmd(src_mm, src_addr);if (unlikely(!src_pmd)) {err = -ENOMEM;break;}}dst_pmd = mm_alloc_pmd(dst_mm, dst_addr);if (unlikely(!dst_pmd)) {err = -ENOMEM;break;}dst_pmdval = pmdp_get_lockless(dst_pmd);/** If the dst_pmd is mapped as THP don't override it and just* be strict. If dst_pmd changes into TPH after this check, the* remap_pages_huge_pmd() will detect the change and retry* while remap_pages_pte() will detect the change and fail.*/if (unlikely(pmd_trans_huge(dst_pmdval))) {err = -EEXIST;break;}ptl = pmd_trans_huge_lock(src_pmd, src_vma);if (ptl && !pmd_trans_huge(*src_pmd)) {spin_unlock(ptl);ptl = NULL;}This still looks wrong - we do still have to split_huge_pmd() somewhere so that remap_pages_pte() works.
Hmm, I guess this extra check is not even needed...
Hm, and instead we'd bail at the pte_offset_map_nolock() in remap_pages_pte()? I guess that's unusual but works...
Yes, that's what I was thinking but I agree, that seems fragile. Maybe just bail out early if (ptl && !pmd_trans_huge())?
(It would be a thing to look out for if anyone tried to backport this, since the checks in pte_offset_map_nolock() were only introduced in 6.5, but idk if anyone's doing that)