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b=FY9LHIQRUp6sudA89KRTEkiY5idffNFEywi7FthL/xHsXkvV4nt7orzbZrAHuUwSA T4bIgUZI27T2LiJIlgDSvz82rBf2NJaOOHdcKuXsNGwWxJCPDbkcRLP8Pd2/uWpbvU u75B91AzbY/KwPXrmnyiak2iru6wZ76cI2/WF0AJaTzpwyHa3Vta0qePyIsawmzEiL D5ujFipSkoB5uAK7joB7cieTrz0d9Yc5y/4TDGCQmqgmfRFopDhSyZ82baZ7P/cbVZ 6GgkQ2zEIeeIz8gSMxtSS9zaNQ72ZLsNL9ozjhQ47DGOk4jnJfTSUAjYgFcXvzAXmC 9sBRJe/94VINA== Message-ID: <0feba0fb-4d23-47b4-ba37-5c272bceafd6@kernel.org> Date: Thu, 10 Sep 2026 09:00:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/rmap: fix missing barrier between anon_vma init and vma->anon_vma publish To: Jinjiang Tu , akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, lance.yang@linux.dev, riel@surriel.com, liam@infradead.org, harry@kernel.org, jannh@google.com, minchan.kim@gmail.com, lwoodman@redhat.com, kamezawa.hiroyu@jp.fujitsu.com, linux-mm@kvack.org Cc: wangkefeng.wang@huawei.com, sunnanyong@huawei.com References: <20260908122924.554373-1-tujinjiang@huawei.com> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: xii9xmoodqmbj8yf6a669yggnw93y9id X-Rspamd-Queue-Id: 08114140008 X-HE-Tag: 1789023664-864258 X-HE-Meta: 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 P7qRRWlE GqtHGmeUYWtYftok9zbznyryxHvbDBAkPy5oyJ8am5LtXJ0WHVewizOiKiFGvvXZSN6UxENpppZWZUTehbKzNRUpsKC2PCSiKZDJTT9Lbt2cAFQ5nWhFejT3uq0sLVthqCGhn2gcpkIG/Ct/Uxoe5mewPmWNzxcXTNZwnTfYC99EuOV2AxYUjw00aC+hyvv3QMrn6sb72M4xJgTERgcoJ85CgMJkF47nglNgdvqDpFqmFly6nQZNar4i1y2JH8RWv/rS/k07M8KZxeLKTRa4vx/Unp9hfCao+J7L+LagXq2dvk94naLaKQP7RcQfHRyFMYM7LbaerJ8Nn2CWFmT0qBiihxOV7S72aTlO0luG07qApo41ac798VOkqbT0zh5NKmHRWuUlmdhRk8N+Kaf060sYLJbYQvxdr20qCbP4ht0CK32E= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/8/26 14:29, Jinjiang Tu wrote: > On arm64 server, we find that a task trying to grab the anon_vma lock > triggers hungtask. > > INFO: task main:2354726 blocked for more than 120 seconds. > Tainted: G E 5.10.0-0021.aarch64 #1 > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > task:main state:D stack: 0 pid:2354726 ppid:2350673 flags:0x00000a01 > Call trace: > __switch_to+0x7c/0xbc > __schedule+0x3b4/0x8a0 > schedule+0x50/0xe0 > rwsem_down_write_slowpath+0x3cc/0x6cc > down_write+0x60/0x260 > __anon_vma_prepare+0x6c/0x210 > do_anonymous_page+0x258/0x660 > handle_pte_fault+0x188/0x214 > __handle_mm_fault+0x1b0/0x380 > handle_mm_fault+0xf4/0x284 > do_page_fault+0x19c/0x494 > do_translation_fault+0xcc/0xf8 > do_mem_abort+0x48/0xac > el0_da+0x44/0x80 > el0_sync_handler+0x88/0xb4 > el0_sync+0x160/0x180 > > After analyzing the vmcore, we found the anon_vma->root->rwsem.count is -1. > There is another anon_vma whose anon_vma->root->rwsem.count is 1, the > anon_vma->root->rwsem.owner shows the lock is held, but the stack of the > task shows the task doesn't hold the anon_vma lock. > > After adding more debugging info, we found __anon_vma_prepare() reuses > anon_vma and triggers the UAF of anon_vma->root due to missing memory > barrier, leading to locking and unlocking two different anon_vma->root, > thus leading to an anon_vma will never be unlocked, and another anon_vma > couldn't be locked anymore. > > This race requires two adjacent VMAs that are not merged but are > anon_vma-compatible (e.g., they differ in VMA_ACCESS_FLAGS that can be > changed by mprotect()). Two threads fault on each VMA concurrently, both > calling __anon_vma_prepare() with only mmap_lock held for reading. > > THREAD A THREAD B > __anon_vma_prepare __anon_vma_prepare > find_mergeable_anon_vma() -> NULL > anon_vma = anon_vma_alloc(); > anon_vma->root = anon_vma; > // the two stores may be reordered > vma->anon_vma = anon_vma; > // finds A's anon_vma > anon_vma = find_mergeable_anon_vma(vma); > anon_vma_lock_write(anon_vma); > // may still see the old root > down_write(&anon_vma->root->rwsem); > anon_vma_unlock_write(anon_vma); > // see the new root, never unlock old > up_write(&anon_vma->root->rwsem); > > thread A triggers page fault and calls __anon_vma_prepare() to prepare > anon_vma for the faulting vma. __anon_vma_prepare() allocates and > initializes a new anon_vma, and then publishes it to the vma with a plain > store. anon_vma_prepare() only requires the mmap_lock to be held for > reading, so two threads can fault on adjacent VMAs at the same time. While > thread A publishes a new anon_vma, thread B could find the anon_vma via > find_mergeable_anon_vma() and then locks anon_vma->root->rwsem. > > The store to anon_vma->root in anon_vma_alloc() and the store to > vma->anon_vma can be reordered. The anon_vma_lock_write() and spin_lock() > only provide acquire semantics, which do not prevent prior stores from > being reordered after them. The release semantics of the corresponding > spin_unlock() and anon_vma_unlock_write() come too late, the store to > vma->anon_vma is already published before they take effect. As a result, > thread B can observe the following order: > > vma->anon_vma = anon_vma; > anon_vma->root = anon_vma; > > The anon_vma slab is SLAB_TYPESAFE_BY_RCU, so a newly allocated anon_vma > may reuse memory from a previously freed one. The constructor > (anon_vma_ctor) does not reset anon_vma->root, and __put_anon_vma() > doesn't clear it either, so the old root value persists until > anon_vma_alloc() overwrites it. If that store isn't visible, thread B > reads a root that points to the old anon_vma and locks it. > > As a result, thread B can call anon_vma_lock_write() with the old root, > and call anon_vma_unlock_write() with the new root, leading to an anon_vma > will never be unlocked, and another anon_vma couldn't be locked anymore > (its count is dropped from 0 to -1 due to wrong unlock). > > To fix it, change the plain store `vma->anon_vma = anon_vma` to store > release, so that the fields of anon_vma are visible before anon_vma is > published to vma->anon_vma. > > At read side, the load of anon_vma and anon_vma->root have address > dependency. According to Documentation/memory-barriers.txt and some > investigations, only Alpha needs address-dependency barriers and it has > been handled by READ_ONCE() in reusable_anon_vma(). > > We reproduced this issue in v5.10 with KSM enabled. The kernel doesn't > merge commit cf7e7a3503df ("mm: prevent KSM from breaking VMA merging for > new VMAs"), so there are many adjacent VMAs that aren't merged but are > compatible for anon_vma. > > Without this fix, our production environment could reproduce this issue > about 2-5 times each month. After adding a smp_mb() before > anon_vma_lock_write(anon_vma) in __anon_vma_prepare(), which is different > to this patch, this issue hasn't been reproduced for one month. > > Cc: stable@vger.kernel.org > Fixes: 5c341ee1dfc8 ("mm: track the root (oldest) anon_vma") > Reviewed-by: Lance Yang > Reviewed-by: Lorenzo Stoakes (ARM) > Signed-off-by: Jinjiang Tu Acked-by: Vlastimil Babka (SUSE)