From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 23CA42F1FDE; Sat, 23 May 2026 11:05:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779534342; cv=none; b=j9CXL7qbRB6vB+DlVCxbz7ALs7e6OJV4BsnKOE9chZnOGGgj01CF1+YD9ZoGaqEGnFN9kHF0iwVzLGkRyzIOAUEnrL0qPETkyOrucrg6UE8sdvvVOpVYB8ul9hOJi8xC851QNYuZQOKnUPW2scSe0LoJ90Wnhm/06sL/JUv3vfU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779534342; c=relaxed/simple; bh=5hIbtimAoeueDk68q+7boRHwpe0V0Mhhcqs71r4MUlI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=mt9gReerD4PdxIYJAsHHdVEsBb3zHMfjdmflBm/KV9Fc1GvCCRkqvmmwnwg3mnVGuSv1MrIjfi4D3X6QCUeHSQP3sLh8fe26Lz0VCQk3ncKwgAvbSPJgC98TzZ0nk5GBdixSGkgucShswEfGfbNhAWcduDrbu63Vl1LKpOrKkSE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Yj04OXDR; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Yj04OXDR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B68381F000E9; Sat, 23 May 2026 11:05:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779534340; bh=DHcPsLxl4tklqx/HrPQUMI4h8lhyyZSQ2vjQ3j8IezA=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=Yj04OXDRzVCOCz/aioL8499w0FQ0hhzf3uDwT/9SlQM10chPGw7u68ODmbbzLO7Tu soVDLtP3wzzExdNDNwNJ8F27LnKMf3cJzv2OoleMKQwuMZeZZ9r/ljgc3Ic06ZUQlf sFQTCvNtAd8Mqix7UQjBwQvN2zA+QrS5gQHAwdYJzpAvDYvdREURk45pNTsVDck+Si d89j5AipRYVtCzTVNVfq7UJYTfa9Zy6z3nWLuOU1CsQ4CxBFv8xULPMFI/i4z20uQH ff+FBBBXcTxGqiSJsHSlBd2kMTUwAXfnGgnyCUDv9HKASKpYAGJWn9Rw9h08nkvcdM XRbcxVtW5NwDw== Message-ID: Subject: Re: [PATCH] lockd: pin next file across nlm_inspect_file lock-drop From: Jeff Layton To: Michael Bommarito , Trond Myklebust , Anna Schumaker , Chuck Lever Cc: NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Date: Sat, 23 May 2026 07:05:37 -0400 In-Reply-To: <20260523014203.2462827-1-michael.bommarito@gmail.com> References: <20260523014203.2462827-1-michael.bommarito@gmail.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Fri, 2026-05-22 at 21:42 -0400, Michael Bommarito wrote: > nlm_traverse_files() walks each nlm_files[] hash bucket with > hlist_for_each_entry_safe(file, next, ...). For each matching file > it bumps f_count, drops nlm_file_mutex to run nlm_inspect_file() > (which may sleep walking blocks, shares, and the inode lock list), > then reacquires the mutex and decrements f_count before continuing > to the saved next. >=20 > The f_count bump pins the current file across the lock-drop, but > nothing pins next. Any nlmsvc thread that holds the last reference > on the file at next will, during that window, call > nlm_release_file() -> nlm_delete_file() under nlm_file_mutex, > hlist_del() it from the bucket, and kfree() it. When > nlm_traverse_files() reacquires the mutex and the macro reads the > next entry's f_list.next on the following iteration, the read lands > in the freed slab. >=20 > A naive restart-on-action variant would deadlock-spin against an > nlm_release_file holder: nlm_inspect_file() does not always drain > the file (it can return 1 with an RPC still holding f_count above > the cleanup threshold), and the outer predicate is_failover_file() > matches static attributes of the file, so a restart can keep > re-finding the same un-cleanable file until the external RPC ref > drops. >=20 > Pin the neighbour explicitly instead. Walk the bucket with two > locally-pinned cursors at a time: file (current, pinned by the > prior iteration's next bump) and next (one ahead). Drop file's pin > at the end of each iteration, then advance to next, which is still > alive because we hold its f_count above zero across the unlock. > This bounds the walk at O(N) per bucket and never observes a freed > neighbour. Factor the f_count/list/share/lock cleanup into a > helper so the no-match path also drops a stale empty file rather > than leaving it in the table. >=20 > Cc: stable@vger.kernel.org > Fixes: 01df9c5e918a ("LOCKD: Fix a deadlock in nlm_traverse_files()") > Assisted-by: Claude:claude-opus-4-7 > Signed-off-by: Michael Bommarito > --- > fs/lockd/svcsubs.c | 61 +++++++++++++++++++++++++++++++--------------- > 1 file changed, 42 insertions(+), 19 deletions(-) >=20 > Reproduced under UML + KASAN with a loopback NFSv3 mount, 768 > concurrent POSIX fcntl(F_SETLKW) holders, and parallel writes to > /proc/fs/nfsd/unlock_filesystem forcing nlmsvc_unlock_all_by_sb() > to walk the table while clients churn locks. >=20 > Stock kernel: >=20 > BUG: KASAN: slab-use-after-free in nlm_traverse_files+0x71d/0x9d0 > Read of size 8 at addr 0000000070314800 by task nlm-init-... >=20 > Allocated by: nlm_lookup_file via nlm4svc_proc_lock > Freed by: another nlm_traverse_files instance freeing a > file whose f_count dropped to zero during the > nlm_inspect_file() unlock window >=20 > Patched UML kernel ran the same harness silently. >=20 > Pin-next was chosen over restart-on-action because the latter can > livelock when nlm_inspect_file() returns 1 with an RPC reference > still holding the file above the cleanup threshold and the outer > is_failover_file() predicate matching static attributes. >=20 >=20 > diff --git a/fs/lockd/svcsubs.c b/fs/lockd/svcsubs.c > index dd0214dcb6950..2bfa32207f10c 100644 > --- a/fs/lockd/svcsubs.c > +++ b/fs/lockd/svcsubs.c > @@ -295,36 +295,59 @@ static void nlm_close_files(struct nlm_file *file) > /* > * Loop over all files in the file table. > */ > +static void nlm_file_release(struct nlm_file *file) > +{ > + if (list_empty(&file->f_blocks) && !file->f_locks > + && !file->f_shares && !file->f_count) { > + hlist_del(&file->f_list); > + nlm_close_files(file); > + kfree(file); > + } > +} > + > static int > nlm_traverse_files(void *data, nlm_host_match_fn_t match, > int (*is_failover_file)(void *data, struct nlm_file *file)) > { > - struct hlist_node *next; > - struct nlm_file *file; > + struct nlm_file *file, *next; > int i, ret =3D 0; > =20 > mutex_lock(&nlm_file_mutex); > for (i =3D 0; i < FILE_NRHASH; i++) { > - hlist_for_each_entry_safe(file, next, &nlm_files[i], f_list) { > - if (is_failover_file && !is_failover_file(data, file)) > - continue; > + file =3D hlist_entry_safe(nlm_files[i].first, > + struct nlm_file, f_list); > + if (file) > file->f_count++; > - mutex_unlock(&nlm_file_mutex); > - > - /* Traverse locks, blocks and shares of this file > - * and update file->f_locks count */ > - if (nlm_inspect_file(data, file, match)) > - ret =3D 1; > + while (file) { > + /* > + * Pin the next neighbour before we drop the mutex > + * for nlm_inspect_file(); a concurrent > + * nlm_release_file() under the same mutex would > + * otherwise be free to unlink and kfree it during > + * the unlock window, leaving us to dereference a > + * freed slab when we walked to next afterwards. > + */ > + next =3D hlist_entry_safe(file->f_list.next, > + struct nlm_file, f_list); > + if (next) > + next->f_count++; > + > + if (!is_failover_file || is_failover_file(data, file)) { > + mutex_unlock(&nlm_file_mutex); > + > + /* > + * Traverse locks, blocks and shares of this > + * file and update file->f_locks count. > + */ > + if (nlm_inspect_file(data, file, match)) > + ret =3D 1; > + > + mutex_lock(&nlm_file_mutex); > + } > =20 > - mutex_lock(&nlm_file_mutex); > file->f_count--; > - /* No more references to this file. Let go of it. */ > - if (list_empty(&file->f_blocks) && !file->f_locks > - && !file->f_shares && !file->f_count) { > - hlist_del(&file->f_list); > - nlm_close_files(file); > - kfree(file); > - } > + nlm_file_release(file); > + file =3D next; > } > } > mutex_unlock(&nlm_file_mutex); Sashiko seems to think there is a regression here. See: https://sashiko.dev/#/patchset/20260523014203.2462827-1-michael.bommarito@g= mail.com?part=3D1 --=20 Jeff Layton