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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Oscar Salvador , Miaohe Lin , Naoya Horiguchi , Adam Litke , Mel Gorman , Rik van Riel , Peter Xu , Kiryl Shutsemau , Mike Rapoport , Sasha Levin Subject: [PATCH 6.12 0871/1536] mm/hugetlb: fix use-after-free of a new anon folio Date: Sat, 7 Nov 2026 08:48:07 -0500 Message-ID: <20261107135022.009325172@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261107134957.653319350@linuxfoundation.org> References: <20261107134957.653319350@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Josef Bacik [ Upstream commit 824cc56bf43c714319992bad3558803cac6e77c0 ] For a fault in a private mapping with nothing in the page cache, hugetlb_no_page() allocates a folio, locks it and maps it. The folio has one reference, which belongs to the page table entry from then on. After dropping the page table lock the function still sets the migratable flag of the folio and unlocks it. Whoever zaps the entry in between frees the folio. The fault holds the fault mutex and the lock of the vma. MADV_DONTNEED and the first unmap of a hole punch, hugetlb_vmdelete_list(), do not take the fault mutex, and a private vma only has a lock if it owns a reservation map: not with MAP_NORESERVE, and not in the child after fork(). Then the flag is set in, and folio_unlock() is called on, a folio that is free or has a new owner. A surplus page goes back to the page allocator while it is locked: BUG: Bad page state in process init pfn:108e00 flags: 0x200000000000059(locked|uptodate|dirty|head|node=0|zone=2) page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set A page of the pool can be given to the next user, of any file, who locks it and loses the lock to the late folio_unlock(): kernel BUG at mm/filemap.c:1532! hugetlb_no_page+0x111b/0x23e0 hugetlb_fault+0x3a9/0x14a0 An unprivileged process is enough: threads that write to a MAP_PRIVATE | MAP_NORESERVE hugetlb mapping, threads that call MADV_DONTNEED on it, and one that takes the freed pages for another file. Until commit 3a47c54f09c4 ("hugetlbfs: revert use i_mmap_rwsem for more pmd sharing synchronization") the fault held i_mmap_rwsem for reading, which everybody who zaps takes for writing. The lock that commit bf4916922c60 ("hugetlbfs: extend hugetlb_vma_lock to private VMAs") gave to private vmas closed the window again, but only for those with a reservation map. Be done with the folio before the page table lock is dropped, as hugetlb_wp() is with its copy. The function already unlocks page cache folios under that lock. Original fix generated by an LLM and sent in private by Josef. [david@kernel.org: simplify+move comment] Link: https://lore.kernel.org/20261015-urgent_fixes-v1-2-42796911043b@kernel.org Fixes: 3a47c54f09c4 ("hugetlbfs: revert use i_mmap_rwsem for more pmd sharing synchronization") Signed-off-by: Josef Bacik Signed-off-by: David Hildenbrand (Arm) Signed-off-by: Andrew Morton Reviewed-by: Lorenzo Stoakes (ARM) Acked-by: Muchun Song Reviewed-by: Lance Yang Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Jann Horn Cc: Pedro Falcato Cc: Oscar Salvador Cc: Miaohe Lin Cc: Naoya Horiguchi Cc: Adam Litke Cc: Mel Gorman Cc: Rik van Riel Cc: Peter Xu Cc: Kiryl Shutsemau Cc: Mike Rapoport Cc: Signed-off-by: Sasha Levin --- mm/hugetlb.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 5196c1a816e38..049726560c1aa 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -6295,8 +6295,6 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, ret = hugetlb_wp(vmf); } - spin_unlock(vmf->ptl); - /* * Only set hugetlb_migratable in newly allocated pages. Existing pages * found in the pagecache may not have hugetlb_migratable if they have @@ -6307,6 +6305,12 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, if (folio_locked) folio_unlock(folio); + + /* + * Hold the PTL until we're done with the folio: the page table owns the + * folio reference now. + */ + spin_unlock(vmf->ptl); out: hugetlb_vma_unlock_read(vma); -- 2.53.0