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Sat, 15 Aug 2026 18:16:40 +0000 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: baolin.wang@linux.alibaba.com Cc: vernon2gm@gmail.com, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, zokeefe@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Lance Yang Subject: Re: [PATCH v2 0/3] mm: khugepaged: fix tracepoint UAF Date: Sun, 16 Aug 2026 02:16:32 +0800 Message-Id: <20260815181632.21453-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: <20260815174444.4164-1-lance.yang@linux.dev> References: <20260815174444.4164-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 6A9A4A000B X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: dnnxa8dqdar9ccf6rh78wjqafst7rcn8 X-HE-Tag: 1786817802-922226 X-HE-Meta: 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 xd99OgQZ 4Qyl+hca4yKlX7cuZxjdWs9pAYCuM9GiFCTAw1rpVI+j4B6Np8WPvFlY/KLIRuvaQJSCUZu4Ra1n9EpbG7umhWgMmaRI49caQWYhCbOJuyr8/6MCeKb5719Ks6++6fO5/3FfZ3GiD9SLDz3XRWcfk18tqGr4AB+NfhvjNR3PvibovAapGuc7I/4Uf9lJbH7wCvmTI+07eS0OZ8xrbMhZlu1Nkd1rA5SlmNWt8TCBR6aRQXrQTyr3G7ljsA2bzShFbIRcLDk8FiNKJILOJEK/qFMkBxc9QXbeMllo1 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: +Cc Baolin On Sun, Aug 16, 2026 at 01:44:44AM +0800, Lance Yang wrote: > >On Sat, Aug 15, 2026 at 01:19:21PM +0800, Vernon Yang wrote: >>From: Vernon Yang >> >>The khugepaged tracepoints take a folio pointer and call folio_pfn(), >>but by then the folio may no longer be valid: freed after folio_put(), >>folio_unlock() or pte_unmap_unlock(), or not a folio at all but an >>xarray-encoded swap entry. On classic SPARSEMEM, dereferencing it oopses >>khugepaged as soon as the trace event is enabled; on other memory models >>it merely prints a bogus pfn. >> >>Pass the pfn to the tracepoints directly, captured while the folio is >>still pinned, closing the use-after-free windows in >>mm_khugepaged_scan_file(), mm_khugepaged_scan_pmd() and >>mm_khugepaged_collapse_file(). > >Well spotted! > >Gave the series a run on x86_64 (KVM), all good (only classic SPARSEMEM >untested) :) Hmm ... stumbled over something else while testing this ... With tmpfs mounted huge=advise, one MADV_HUGEPAGE isn't enough to get an unregistered mm onto khugepaged's list. Do it twice, and khugepaged starts scanning right away. The pending flags make it into khugepaged just fine: int hugepage_madvise(struct vm_area_struct *vma, vm_flags_t *vm_flags, int advice) { switch (advice) { case MADV_HUGEPAGE: *vm_flags &= ~VM_NOHUGEPAGE; *vm_flags |= VM_HUGEPAGE; ... khugepaged_enter_vma(vma, *vm_flags); break; ... } return 0; } and survive the common eligibility check: unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma, vm_flags_t vm_flags, enum tva_type type, unsigned long orders) { ... /* * Enabled via shmem mount options or sysfs settings. * Must be done before hugepage flags check since shmem has its * own flags. */ if (!in_pf && shmem_file(vma->vm_file)) return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file), vma, vma_start_pgoff(vma), 0, forced_collapse); ... } But then the shmem helper reads them back from the VMA: unsigned long shmem_allowable_huge_orders(struct inode *inode, struct vm_area_struct *vma, pgoff_t index, loff_t write_end, bool shmem_huge_force) { ... vm_flags_t vm_flags = vma ? vma->vm_flags : 0; ... } At that point vma->vm_flags still has the old value, so huge=advise quietly gives us no allowable order. First madvise still succeeds, of course. The second one works because it finds VM_HUGEPAGE already installed by the first call. Looked at history too ... we've been here before. 2cf1338454a8 ("mm: fix khugepaged with shmem_enabled=advise") fixed this exact ordering bug and tagged cd89fb065099 as the culprit. Then 6beeab870e70 was meant to be "No functional changes", but moving shmem_huge_global_enabled() into shmem_allowable_huge_orders() seems to have wired the stale vma->vm_flags read back in. So AFAICT, this regressed in v6.12 with 6beeab870e70. @Baolin, does that ring a bell? Any reason shmem_allowable_huge_orders() can't just take the pending vm_flags as well? Cheers, Lance