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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OQBViomQ; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 00CCA606A31C; Thu, 29 Oct 2026 10:46:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OQBViomQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1A31C1F01828; Thu, 29 Oct 2026 10:46:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793270763; bh=RWtxWJ8Mj0Rtl84/opbh6HrMgPdbznSuYDREGzIgtS8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=OQBViomQbwp7bZgr67nFX4bXyVvdihnfsX4qqLIJhw5cov8NwRlowbCfqCeB8B/TG vRZdctp8HDzZf5+MMfLQfGaLx8sLKTz+QD97JA9f/3ve2QDgYhC0hLH6dMJLQPhAiB mpAIGWRyRX/AroWWwl7GdtzWL5pw2DZ1FbKhQM1DCalokfCGrAWcRTTp67JDSuhVxS AQ4zKkwFJ5SSSBy1TDC7mw2jzppsjjBq202u+cwgkOE+I4z0mm1dat/+NS26OofQzA iIFxxdkBxEBRu6djTiz9092I9boXtgjVKklvBdRnPHpOicl4m8X4/d0eOZ3LVk21Ob sxvnPpw0oYzGA== Message-ID: Date: Thu, 29 Oct 2026 11:45:58 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] hugetlbfs: emit hugetlbfs_free_inode from ->destroy_inode() To: Joas Antonio dos Santos , Muchun Song , Oscar Salvador Cc: Andrew Morton , Hongbo Li , syzbot+91e1fc35f51774c479b2@syzkaller.appspotmail.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <179324176522.32673.7602596908234555806@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/29/26 03:42, Joas Antonio dos Santos wrote: > hugetlbfs_free_inode() calls trace_hugetlbfs_free_inode(), and the > hugetlbfs__inode event class records inode->i_sb->s_dev. ->free_inode() > runs from an RCU callback (destroy_inode() queues i_callback() with > call_rcu()), and Documentation/filesystems/porting.rst says that this > callback "is *NOT* ordered wrt superblock destruction; as the matter of > fact, the superblock and all associated structures might be already > gone". put_super() frees the superblock with call_rcu() followed by a > workqueue item (destroy_super_rcu() -> destroy_super_work()). syzbot > hit this with the event enabled: > > BUG: KASAN: slab-use-after-free in trace_event_raw_event_hugetlbfs__inode+0x45b/0x4c0 include/trace/events/hugetlbfs.h:36 > Read of size 4 at addr ffff888038850010 by task syz-executor952/6134 > hugetlbfs_free_inode+0x15e/0x1f0 fs/hugetlbfs/inode.c:1182 > i_callback+0x46/0x80 fs/inode.c:317 > rcu_do_batch kernel/rcu/tree.c:2650 [inline] > rcu_core+0x5f0/0x1220 kernel/rcu/tree.c:2919 > Allocated by task 6134: > alloc_super fs/super.c:326 [inline] > sget_fc+0x1ba/0x1f60 fs/super.c:874 > hugetlbfs_get_tree+0x1e7/0x2b0 fs/hugetlbfs/inode.c:1431 > Freed by task 855: > kfree+0x22b/0x6d0 mm/slub.c:6923 > process_one_work+0xac7/0x1b10 kernel/workqueue.c:3495 > > Emit the event from hugetlbfs_destroy_inode() instead. destroy_inode() > calls ->destroy_inode() synchronously, before it queues the RCU > callback, while the superblock is still alive; hugetlbfs_destroy_inode() > already dereferences inode->i_sb there. ->free_inode() is left with > only the kmem_cache_free(). > > Fixes: 014ad7c42a69 ("hugetlbfs: use tracepoints in hugetlbfs functions.") > Reported-by: syzbot+91e1fc35f51774c479b2@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/all/6ae27525.5451848c.f7707.0001.GAE@google.com/ > Cc: stable@vger.kernel.org > Signed-off-by: Joas Antonio dos Santos > Assisted-by: Yaga Agent HackerSec Claude:claude-opus-5-5 > --- > Tested on net/main 94cea342e891 (arm64, QEMU, generic KASAN, tracing > enabled) with the syzbot C reproducer, which enables > events/hugetlbfs/hugetlbfs_free_inode and races hugetlbfs mounts and > lazy unmounts with open + unlink. Without this patch KASAN reports the > slab-use-after-free in trace_event_raw_event_hugetlbfs__inode() from > hugetlbfs_free_inode() <- i_callback() during the first 10 second run, > on 3 of 3 boots. With it, 12 runs over 2 boots are clean, and the trace > buffer still holds the hugetlbfs_free_inode events (about 46k per boot) > with valid dev numbers. fs/hugetlbfs/inode.o builds cleanly with W=1. > > The event now fires when the inode is destroyed instead of when its > memory goes back to the slab cache, one RCU grace period earlier; the > recorded fields are the same. > > fs/hugetlbfs/inode.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c > index 6656807..27a3937 100644 > --- a/fs/hugetlbfs/inode.c > +++ b/fs/hugetlbfs/inode.c > @@ -1179,12 +1179,16 @@ static struct inode *hugetlbfs_alloc_inode(struct super_block *sb) > > static void hugetlbfs_free_inode(struct inode *inode) > { > - trace_hugetlbfs_free_inode(inode); > kmem_cache_free(hugetlbfs_inode_cachep, HUGETLBFS_I(inode)); > } > > static void hugetlbfs_destroy_inode(struct inode *inode) > { > + /* > + * Not in ->free_inode(): that runs from an RCU callback, when the > + * superblock the event reads may already be freed. > + */ > + trace_hugetlbfs_free_inode(inode); > hugetlbfs_inc_free_inodes(HUGETLBFS_SB(inode->i_sb)); > } > Should we rename the tracepoint then? -- Cheers, David