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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/31/26 19:22, Zhao Gongyi wrote: Hi, thanks for the report! CCing Lorenzo. > KASAN reports a use-after-free in page_address_in_vma() while ksmd > merges pages. ksmd keeps a folio reference across an mmap_lock drop, > during which mremap() can remove its last mapping and free its > anon_vma. page_address_in_vma() then dereferences the stale pointer > from folio->mapping. Do you have a link to the report or the actual KASAN report? > > KSM needs only a candidate address. write_protect_page() checks that > the PTE maps the folio, and replace_page() verifies the saved PTE > before replacing it. Compute the candidate from the folio index and > VMA instead of dereferencing the folio-side anon_vma. Reject KSM > folios because their page index is not meaningful. > > Fixes: 21d0d443cdc1 ("rmap: resurrect page_address_in_vma anon_vma check") > Signed-off-by: Zhao Gongyi > --- > Testing: > Unpatched mm-new: fixed-pool KSM/mremap reproduced the KASAN UAF. > Patched mm-new: the same workload ran for 12 minutes without KASAN, > BUG or WARNING; fork/COW KSM smoke passed. > > mm/ksm.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index d6f736a..f8af5ee 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -1307,7 +1307,7 @@ static int write_protect_page(struct vm_area_struct *vma, struct folio *folio, > if (WARN_ON_ONCE(folio_test_large(folio))) > return err; > > - pvmw.address = page_address_in_vma(folio, folio_page(folio, 0), vma); > + pvmw.address = vma_anon_address(vma, folio_pgoff(folio), 1); > if (pvmw.address == -EFAULT) > goto out; > > @@ -1408,7 +1408,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page, > int err = -EFAULT; > struct mmu_notifier_range range; > > - addr = page_address_in_vma(folio, page, vma); > + addr = vma_anon_address(vma, page_pgoff(folio, page), 1); > if (addr == -EFAULT) > goto out; > > @@ -1515,7 +1515,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma, > if (page == kpage) /* ksm page forked */ > return 0; > > - if (!folio_test_anon(folio)) > + if (!folio_test_anon(folio) || folio_test_ksm(folio)) > goto out; > > /* > base-commit: 243e8cdab64129d6207c398f8cf5fbb1962ff7e1 -- Cheers, David