From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 5C97D492199 for ; Tue, 12 May 2026 11:25:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778585107; cv=none; b=QP/XDg/D9Bcfgws3qcBomZVya+RBiJr3FDEKnYQFidJZouiVhBhjhJtqsJGl7s4kb5nk8B+zEVwjol+zo6nYzl7QrIjasYaQLPUZRMxVey0j4oLMy/TOMJJfZr7qfFmrS0wl9kUrwWtIRYpDEkxln+qVlw+3lhFkDe3w2WNeBMc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778585107; c=relaxed/simple; bh=D497IKok0GrkFTKdOr6OX0NTExNJ+QVCTVOP1UgJ5Io=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=WlqfPfiqD1md2CmtXEZwZLzBc7ywhqa8B3iMzpmGFI0GgZpn18Gw2OecmhERIlV2SD6lIQrQTLiGQc3Mos4vKjF9GueMQ+CoQ6VlTC9Rlw95oXWzRT/VawSEwq96WeQY+GhuViXtCY5oEPO1BrqqL2jXdVh2Xwt+5nv9v4azThc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=p2eWRUBy; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="p2eWRUBy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AC831C2BCB0; Tue, 12 May 2026 11:25:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778585106; bh=D497IKok0GrkFTKdOr6OX0NTExNJ+QVCTVOP1UgJ5Io=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=p2eWRUBycUcaj3cpAEribbvwh6I2rM0FR66WGPaAvApyM2uQ+D54LIxc4JZsxvJa2 M8FYLaHgIANKj9g/1PGv4JRoVOT1zrGms6A+8jKE0gacV9Bzx8r4FDbZ+1azNOig7F XswW89kd5Cw8Wsq5G7AL+1aFoapU1aWlDp86+wJhUg9hrnTuMyZTDcIbFJlwKGxZXS pOKHqX5FbhPvXIUmnttsSJR6ahFJEPnNRQozUIkA/Dpsm7EBEqE7cu0H29w9+NQJfj MCpAkqv/N+7v1rWhIaz+wi5prChzFYDT82eq3Dfw0aMk1bIpJzvK7sVaDKgufNgfXV BwmILvB0Q/bmA== Message-ID: Subject: Re: [PATCH] Revert "NFSD: Defer sub-object cleanup in export put callbacks" From: Jeff Layton To: Yang Erkun , chuck.lever@oracle.com, misanjum@linux.ibm.com, neil@brown.name, okorniev@redhat.com, Dai.Ngo@oracle.com, tom@talpey.com Cc: linux-nfs@vger.kernel.org, yi.zhang@huawei.com, chengzhihao1@huawei.com, lilingfeng3@huawei.com, yangerkun@huaweicloud.com Date: Tue, 12 May 2026 07:25:02 -0400 In-Reply-To: <20260512023322.2828939-1-yangerkun@huawei.com> References: <20260512023322.2828939-1-yangerkun@huawei.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Tue, 2026-05-12 at 10:33 +0800, Yang Erkun wrote: > This reverts commit 48db892356d6cb80f6942885545de4a6dd8d2a29. >=20 > Commit 48db892356d6 ("NFSD: Defer sub-object cleanup in export put > callbacks") describes an issue where calling svc_export_put, path_put, > and auth_domain_put directly can cause use-after-free (UAF) errors when > accessing ex_path or ex_client->name. However, after discussion in [1], > it is clear that commit e7fcf179b82d ("NFSD: Hold net reference for the > lifetime of /proc/fs/nfs/exports fd") actually resolves this problem. >=20 > Additionally, commit 48db892356d6 ("NFSD: Defer sub-object cleanup in > export put callbacks") introduces a regression that was already fixed by > commit 69d803c40ede ("nfsd: Revert "nfsd: release svc_expkey/svc_export > with rcu_work""). Therefore, reverting commit 48db892356d6 ("NFSD: Defer > sub-object cleanup in export put callbacks") is necessary to fix this > regression. >=20 > Link: https://lore.kernel.org/all/10019b42-4589-4f9f-8d5b-d8197db1ce3c@hu= awei.com/ [1] > Fixes: 48db892356d6 ("NFSD: Defer sub-object cleanup in export put callba= cks") > Signed-off-by: Yang Erkun > --- > fs/nfsd/export.c | 63 +++++++----------------------------------------- > fs/nfsd/export.h | 7 ++---- > fs/nfsd/nfsctl.c | 8 +----- > 3 files changed, 12 insertions(+), 66 deletions(-) >=20 The LLMs don't seem to agree that this is safe: commit (not yet applied) Author: Yang Erkun Revert "NFSD: Defer sub-object cleanup in export put callbacks" This reverts commit 48db892356d6. The commit message states that e7fcf179b82d resolves the underlying UAF and that the reverted commit re-introduces an umount regression fixed by 69d803c40ede. Link: https://lore.kernel.org/all/10019b42-4589-4f9f-8d5b-d8197db1ce3c@huaw= ei.com/ > This reverts commit 48db892356d6cb80f6942885545de4a6dd8d2a29. > > Commit 48db892356d6 ("NFSD: Defer sub-object cleanup in export put > callbacks") describes an issue where calling svc_export_put, path_put, > and auth_domain_put directly can cause use-after-free (UAF) errors when > accessing ex_path or ex_client->name. However, after discussion in [1], > it is clear that commit e7fcf179b82d ("NFSD: Hold net reference for the > lifetime of /proc/fs/nfs/exports fd") actually resolves this problem. Is this accurate? Commit e7fcf179b82d holds a net reference for the lifetime of the /proc/fs/nfs/exports file descriptor. This prevents nfsd_net_exit() from calling nfsd_export_shutdown() while the exports fd is open, keeping the cache_detail alive. But the UAF described in 48db892356d6 is a different problem. cache_clean() periodically removes expired entries from the hash table via sunrpc_begin_cache_remove_entry() and drops their reference via cache_put(). If that was the last reference, svc_export_put() runs and frees sub-objects. Meanwhile, c_show() and e_show() iterate cache entries under rcu_read_lock() without holding a reference. Specifically, c_show() unconditionally calls cd->cache_show(): net/sunrpc/cache.c:c_show(): if (cache_check_rcu(cd, cp, NULL)) seq_puts(m, "# "); else if (cache_is_expired(cd, cp)) seq_puts(m, "# "); return cd->cache_show(m, cd, cp); svc_export_show() then accesses both exp->ex_path via seq_path() and exp->ex_client->name via seq_escape(). expkey_show() similarly accesses ek->ek_client->name and ek->ek_path. Commit e7fcf179b82d does not prevent cache_clean() from removing individual entries concurrently with these RCU readers. It seems like the claim in this commit message may be conflating two different problems. > diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c > index 665153f1720e..0baa58d1dbfc 100644 > --- a/fs/nfsd/export.c > +++ b/fs/nfsd/export.c [ ... ] > -static void expkey_release(struct work_struct *work) > +static void expkey_put(struct kref *ref) > { > - struct svc_expkey *key =3D container_of(to_rcu_work(work), > - struct svc_expkey, ek_rwork); > + struct svc_expkey *key =3D container_of(ref, struct svc_expkey, h.ref); > if (test_bit(CACHE_VALID, &key->h.flags) && > !test_bit(CACHE_NEGATIVE, &key->h.flags)) > path_put(&key->ek_path); > auth_domain_put(key->ek_client); > - kfree(key); > -} > - > -static void expkey_put(struct kref *ref) > -{ > - struct svc_expkey *key =3D container_of(ref, struct svc_expkey, h.ref); > - > - INIT_RCU_WORK(&key->ek_rwork, expkey_release); > - queue_rcu_work(nfsd_export_wq, &key->ek_rwork); > + kfree_rcu(key, ek_rcu); > } With this change, path_put() and auth_domain_put() run immediately when the last reference is dropped, before the RCU grace period. kfree_rcu() only defers the kfree of the svc_expkey struct itself. If cache_clean() drops the last reference concurrently with an RCU reader in c_show() -> expkey_show(), the reader can access ek->ek_client->name after auth_domain_put() has freed the auth_domain, and can call seq_path(&ek->ek_path) after path_put() has freed the underlying dentry/mnt. The rcu_read_lock() held by the reader prevents kfree_rcu() from freeing the struct, but does not prevent the direct path_put()/auth_domain_put() calls. Does this re-introduce the UAF that 48db892356d6 was fixing? > -static void svc_export_release(struct work_struct *work) > +static void svc_export_release(struct rcu_head *rcu_head) > { > - struct svc_export *exp =3D container_of(to_rcu_work(work), > - struct svc_export, ex_rwork); > + struct svc_export *exp =3D container_of(rcu_head, struct svc_export, > + ex_rcu); > > - path_put(&exp->ex_path); > - auth_domain_put(exp->ex_client); > nfsd4_fslocs_free(&exp->ex_fslocs); > export_stats_destroy(exp->ex_stats); > kfree(exp->ex_stats); > @@ -382,8 +369,9 @@ static void svc_export_put(struct kref *ref) > { > struct svc_export *exp =3D container_of(ref, struct svc_export, h.ref); > > - INIT_RCU_WORK(&exp->ex_rwork, svc_export_release); > - queue_rcu_work(nfsd_export_wq, &exp->ex_rwork); > + path_put(&exp->ex_path); > + auth_domain_put(exp->ex_client); > + call_rcu(&exp->ex_rcu, svc_export_release); > } Same concern here. With path_put() and auth_domain_put() called directly in svc_export_put() before call_rcu(), a concurrent RCU reader in e_show() -> svc_export_show() or c_show() -> svc_export_show() could dereference freed sub-objects through seq_path(&exp->ex_path) or exp->ex_client->name. --=20 Jeff Layton