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smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b="b6R/J3oD"; arc=none smtp.remote-ip=91.218.175.55 Received: from mta0.migadu.com (out-55.mta0.migadu.com [91.218.175.55]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B7259606A326 for ; Sun, 1 Nov 2026 03:13:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org B7259606A326 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=b6R/J3oD X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=9EHwrI8bkAWMARTdLxi0cB28WQ9c8LLtlAJzoaChO84=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793502820; v=1; x=1794107620; b=b6R/J3oD9Mt3LlXnLwajFd7+WOQcAce4m3j5KWvdNrmex1E9nerZkVONXfReFMeOrmqWIbQg A95yHdAEN0hc45oeIJiicazdWnGLlXXXymdhpsjdI0Ik3/Q48hH/mnCxYMG3Im21SCIrtyQ0CxO 4ic5vh+KRpfcmwO0mpL0fMX8= X-Envelope-To: linux-kernel@vger.kernel.org Received: by mta11.migadu.com with ESMTPS id a030bda49eede247; Sun, 01 Nov 2026 03:13:40 +0000 X-Mizu-Trace-ID: a030bda49eede247 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii 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 (Mac OS X Mail 16.0 \(3901.100.1.1.12\)) Subject: Re: [PATCH] hugetlbfs: emit hugetlbfs_free_inode from ->destroy_inode() From: Muchun Song In-Reply-To: <20261031-b4-hugetlbfs-trace-v1-1-4ec8d02a30cc@gmail.com> Date: Sun, 1 Nov 2026 11:13:18 +0800 Cc: Oscar Salvador , David Hildenbrand , linux-mm@kvack.org, Andrew Morton , Hongbo Li , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+91e1fc35f51774c479b2@syzkaller.appspotmail.com, stable@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20261031-b4-hugetlbfs-trace-v1-1-4ec8d02a30cc@gmail.com> To: joasantonio108@gmail.com X-Mailer: Apple Mail (2.3901.100.1.1.12) This is the V2 version. you should specify it in your subject line = prefix. > On Nov 1, 2026, at 00:06, Joas Antonio dos Santos via B4 Relay = wrote: >=20 > From: Joas Antonio dos Santos >=20 > 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: >=20 > 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 >=20 > 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(). >=20 > 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 > --- > Resend: my mail provider blocked the original submission to the lists > on 2026-10-29, so only the direct recipients got it. No change to the > patch, except dropping a deprecated Assisted-by trailer. >=20 > David asked whether the tracepoint should be renamed now that it no > longer fires from ->free_inode(). I'd keep this patch as is, since it > is tagged for stable and the event name is visible in tracefs > (events/hugetlbfs/hugetlbfs_free_inode); renaming it in stable kernels > would break existing tracing setups just to fix the use-after-free. I > can send a follow-up for mm-unstable that renames it to > hugetlbfs_destroy_inode, next to hugetlbfs_evict_inode, or fold the > rename into a v2 if preferred. At least for me, I think we should rename it, and it belongs to the fix itself. Otherwise, we will introduce another behavior change, someone thinks the tracepoint is fired through free_inode callback, but it's not, it's weird. Thanks, Muchun >=20 > Tested with the syzbot C reproducer on a KASAN kernel with the event > enabled: the use-after-free reproduced in 3 of 3 boots without the > patch and in none of 12 runs with it, and the event is still emitted. >=20 > Tool-use note: the analysis and testing were done with help from an > LLM-based assistant; I reviewed the change and ran the tests myself. > --- > fs/hugetlbfs/inode.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) >=20 > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c > index 01c4d6f44..5bd348288 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) >=20 > static void hugetlbfs_free_inode(struct inode *inode) > { > - trace_hugetlbfs_free_inode(inode); > kmem_cache_free(hugetlbfs_inode_cachep, HUGETLBFS_I(inode)); > } >=20 > 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)); > } >=20 >=20 > --- > base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8 > change-id: 20261031-b4-hugetlbfs-trace-341b338ab57b >=20 > Best regards, > --=20 > Joas Antonio dos Santos >=20 >=20