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Sun, 18 Oct 2026 06:16:37 +0000 X-Mizu-Trace-ID: 8a974899a536c980 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, pfalcato@suse.de, muchun.song@linux.dev, osalvador@suse.de, linmiaohe@huawei.com, nao.horiguchi@gmail.com, agl@us.ibm.com, mel@csn.ul.ie, riel@surriel.com, peterx@redhat.com, kas@kernel.org, rppt@kernel.org, josef@toxicpanda.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 02/10] mm/hugetlb: fix use-after-free of a new anon folio Date: Sun, 18 Oct 2026 14:16:32 +0800 Message-ID: <20261018061632.14622-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261015-urgent_fixes-v1-2-42796911043b@kernel.org> References: <20261015-urgent_fixes-v1-2-42796911043b@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 0082D120007 X-Rspam-User: X-Stat-Signature: nnhe18ssnw6berdkxdekebzga5tkzffw X-HE-Tag: 1792304198-887000 X-HE-Meta: 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 RBxr2HMI AunRioGCBLIkPfJRbdniMLcaffkVoxd4V2OSHpZE3AKHJ4KCdUGQV638MB37nfOewhW9AMNuaYhO9NWbcvfTwJoDpUgligiQ22L/mDupMXo0S/80L1Cy6IqQr39Defpyq/DzORPtL5U6cPidx/L4DC3UAgMsQjHhVWKLYcgZ+zbTAMgWx60IcxewcdAXSZhVCwPFJTk2svuz/rkDHeHtTd3OIIFZ4f092TaM0QCO5n6NufKFakkfNMGTYHQ83q9l567r6GE7hcngQ6DH5n0YbFlkk7gSF8TsxRZfXU13l1WuSbuW1FRlDHTq3AKknBnltcTGYnyDV33urCFioDG9wqX8KHQ1uWKZq/1dYM59IfUHNaCJHRCTT6yZUCi1cTovf53UwCbs8kt/9WIk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Oct 15, 2026 at 06:07:08PM +0200, David Hildenbrand (Arm) wrote: >From: Josef Bacik > >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. > >Fixes: 3a47c54f09c4 ("hugetlbfs: revert use i_mmap_rwsem for more pmd sharing synchronization") >Cc: stable@vger.kernel.org >Signed-off-by: Josef Bacik >Reviewed-by: Lorenzo Stoakes (ARM) >[ david: simplify+move comment ] >Signed-off-by: David Hildenbrand (Arm) >--- Nice work! Nothing jumped out at me, so: Reviewed-by: Lance Yang