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b=LhuMFxlJ+Sn+bK/n3A8CaZEya5rDRDXuzqmH4Rmb+ZzMFIU0N+638mkvPOxKux8T2 nenZffcs1R8sZ9OxOmcwysa+CI2hxBCdom0kuSUHZ7lqPAJE7RF408XYroYVuAg8PO 5D8mda/rzAPMZ5/+Ft3VnIW3BBFt+9BWUREovn/VKbu1AbvuIoqFRsqBrU4Hmqe/od OdqoUgjrcBhJpd4luZuZ8xTCT7n6yGmf8gQZRYsX9aB1s1bJNyU9xk1YujtizQYbze YLPPvISKdah6CX4SLnZpSVfE2kdhzXINurRtLkYBniUfRDpLegOuJdQCkIPC5XxOG9 CKB7mzLajrVdQ== Message-ID: <1fc81157-824c-4296-ac5f-e8965f64597e@kernel.org> Date: Tue, 14 Jul 2026 13:29:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: thp: unlock i_mmap before releasing split folios To: Kiryl Shutsemau , Zi Yan Cc: Hao Zhang , Andrew Morton , Lorenzo Stoakes , linux-mm@kvack.org References: <20260710071344.GA106129@zh-pc> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 6009D40004 X-Stat-Signature: 9zptpqnce7bndj11xakedwrdt4ty7nb8 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1784028599-350964 X-HE-Meta: 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 pwx57Y+O fMcwFAdz+rJXIs9AZB5K8FED44Cdr07O5PkD+fR29mcHHrfZCLzJGt6eDQAn/7tQcEjE7aukkHzyMTMmUyic6QXfdStEdjpdhdKBaTLjth4KG6yisE6rLde4TxZ3FGcZvQzw73bjCUMtBW6pF1XUQ+A2eqIOBfBVNzO2j/kcrw+BoOfOZkeeroYr9KIF8ADVh7lWYF2bzJYUEinHXvEJSWgpVk3heJLlBZ9D4GBJuGCg6zatoKMo3RISGXyqWUcgpj3+domy4vn+tUennfH0gObMKHX/lxOtCw5Ve+phfkqEholKnaVHiIurMXfZj7YW/bEweSkqdqOZzvBs= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/14/26 12:26, Kiryl Shutsemau wrote: > On Mon, Jul 13, 2026 at 10:15:40PM -0400, Zi Yan wrote: >>> This log line confirms my suspicion. >>> >>> The UAF fires at i_mmap_unlock_read() while try_to_split_thp_page() >>> still holds the poisoned page locked. For the inode to be freed at that >>> point, eviction must have completed truncation, which is impossible >>> while a locked folio remains in the page cache. So the after-split folio >>> containing lock_at was no longer in the page cache — and the only way to >>> remove a locked folio is the split itself: the new_folio->index >= end >>> drop in __folio_freeze_and_split_unmapped(). It requires lock_at to be a >>> tail page, which is what memory-failure passes. >>> >>> CPU0 CPU1 >>> i_mmap_lock_read(mapping) >>> __folio_freeze_and_split_unmapped() >>> __filemap_remove_folio(lock_at) <-- beyond EOF: leaves cache, stays LOCKED >>> >>> /* no locked folio pins the inode */ >>> evict() >>> truncate_inode_pages_final() /* nothing to block on */ >>> call_rcu() -> free inode >>> >>> i_mmap_unlock_read(mapping) <-- UAF: rwsem in freed inode >>> >>> Your fix moves i_mmap_unlock_read() out of the window, but I don't think >>> it is complete: shmem_uncharge(mapping->host) a few lines up also touches >>> the inode, and its nr_shmem_dropped guard is exactly the beyond-EOF drop >>> that triggers this. So it is in the same freed window; the patch just >>> relocates the one dereference KASAN caught. >> >> I was discussing this issue with Codex after I saw Kriyl's patch and the >> iput() issue raised by Sashiko[1]. Hao's patch might be still valid for >> shmem_uncharge(), since all after-split folios are locked at >> shmem_uncharge() and prevents inode from going away. So the rule is no >> mapping/inode dereferences after the after-split folios unlock loop >> begins. > > I agree that Hao's fix works, but I don't think "all after-split folios > are locked" is the right rule: the beyond-EOF folios are locked, yet no > longer in the page cache, so they don't block eviction. What actually > pins the inode is @folio itself: the drop loop in > __folio_freeze_and_split_unmapped() starts at folio_next(folio), so > @folio always stays in the page cache -- even if it is entirely beyond > EOF -- and stays locked until the unlock loop. > > But maybe we should sidestep the problem altogether and refuse the > split with -EBUSY if @lock_at is beyond @end. Then the caller's locked > folio is guaranteed to stay in the page cache and pins the inode for > the whole function. No ordering rules needed. > > It is normally only reachable as a race with truncate (or eviction), > where failing the split is harmless. The exception is a folio left > straddling EOF by a failed split in truncate_inode_partial_folio(): > memory_failure() on a beyond-EOF subpage of it would now fail to split > instead of dropping the poisoned page. That range is zeroed and > unreachable, so I think it is acceptable. > > Any opinions? I guess a deferred iput through some workqueue is not an option? I'd prefer if we could keep folio splitting working even when the passed page is EOF. -- Cheers, David