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 B48C634F24A for ; Tue, 12 May 2026 12:30:37 +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=1778589037; cv=none; b=WAgCtKDujQNW/4VXHy19NjoXV6VAgGBqGbREEATFlAD2nHok6C/kIoHRE8e/8zU2mQOtvHJZOshfrztyQviSk5B39YMkmHCJvK//6yTbL9ypM/DfKX+TcAP6XWvMzJWN8/wTzKGInTu9y6PflRXj0XAT1iFCqNoqHHK4yuoHP60= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778589037; c=relaxed/simple; bh=02Nnu23Uu7eG96yKSKnQpac4o44Jx0WV9khru12eqhg=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=oe69QExg1ic5uXb3gq0U+hHaHB7EAXtVvqmRjJsu0C67dOlwILU3X9yO7O4WECAxFMMGdeet4S7aSd+fcZB+bTUmVXI37rJT8ufuztWt92Im+evRC4oovYX+3qTvHdEeO4xkay7Q0UUokPshbh0VG/EMFyWBRBZxrRfgwBl1pcU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gKcL2RGg; 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="gKcL2RGg" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EAFA3C2BCB0; Tue, 12 May 2026 12:30:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778589037; bh=02Nnu23Uu7eG96yKSKnQpac4o44Jx0WV9khru12eqhg=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=gKcL2RGgVkHC105A2GwPncEZfSBp+7RtWZRCFOEfA8WqTdb5MMYVpuJ+YFX3ZMgiB 0rP0U6xDgtldm41NvofNLI10gAwEQq26tcuBBnJQc2loYgtvQpI/uwqk6+AxLDYjRH kzwEcckTOByQkrq5oWnrRdrKDUMkOLbPEOgnjqfuYcgQU4aNWLC9KhJeD/j3N/c3+s 1crBmmLMGg7B0k/TK5yu4tFeQlYCLM9W/YXrjqxs01jN165wnpCB3frO6qJaHupaHs Jf2nVWZWocEGZiQ5brdq6sLP7IVlGmgtzE8VY0Co3nWBMuAmoa86/sUXo8nva23UwI 2jw2SffGGHclQ== Message-ID: Subject: Re: [PATCH] Revert "NFSD: Defer sub-object cleanup in export put callbacks" From: Jeff Layton To: yangerkun , 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 Date: Tue, 12 May 2026 08:30:33 -0400 In-Reply-To: <5701706a-6a54-4bcb-a2ce-83eac3d4b715@huaweicloud.com> References: <20260512023322.2828939-1-yangerkun@huawei.com> <5701706a-6a54-4bcb-a2ce-83eac3d4b715@huaweicloud.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-05-12 at 20:19 +0800, yangerkun wrote: > Hi Jeff, >=20 > =E5=9C=A8 2026/5/12 19:25, Jeff Layton =E5=86=99=E9=81=93: > > On 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 wh= en > > > accessing ex_path or ex_client->name. However, after discussion in [1= ], > > > it is clear that commit e7fcf179b82d ("NFSD: Hold net reference for t= he > > > 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_expo= rt > > > with rcu_work""). Therefore, reverting commit 48db892356d6 ("NFSD: De= fer > > > sub-object cleanup in export put callbacks") is necessary to fix this > > > regression. > > >=20 > > > Link: https://lore.kernel.org/all/10019b42-4589-4f9f-8d5b-d8197db1ce3= c@huawei.com/ [1] > > > Fixes: 48db892356d6 ("NFSD: Defer sub-object cleanup in export put ca= llbacks") > > > 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 > >=20 > > The LLMs don't seem to agree that this is safe: > >=20 > > commit (not yet applied) > > Author: Yang Erkun > >=20 > > Revert "NFSD: Defer sub-object cleanup in export put callbacks" > >=20 > > 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. > >=20 > > Link: https://lore.kernel.org/all/10019b42-4589-4f9f-8d5b-d8197db1ce3c@= huawei.com/ > >=20 > > > 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 wh= en > > > accessing ex_path or ex_client->name. However, after discussion in [1= ], > > > it is clear that commit e7fcf179b82d ("NFSD: Hold net reference for t= he > > > lifetime of /proc/fs/nfs/exports fd") actually resolves this problem. > >=20 > > 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. >=20 > Yes. >=20 > >=20 > > 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. >=20 > Yes. >=20 > >=20 > > 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(): > >=20 > > 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); > >=20 > > 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. >=20 > Yes. >=20 > >=20 > > Commit e7fcf179b82d does not prevent cache_clean() from removing > > individual entries concurrently with these RCU readers. It seems >=20 > Yes. >=20 > > like the claim in this commit message may be conflating two different > > problems. >=20 > True, the msg describe here seems confused. That should change to: >=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. But after discussion in [1], it=20 > seems cannot happen and either will introduce a gression 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 >=20 > >=20 > > > 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 > >=20 > > [ ... ] > >=20 > > > -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.r= ef); > > > 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.r= ef); > > > - > > > - INIT_RCU_WORK(&key->ek_rwork, expkey_release); > > > - queue_rcu_work(nfsd_export_wq, &key->ek_rwork); > > > + kfree_rcu(key, ek_rcu); > > > } > >=20 > > 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. > >=20 > > 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. >=20 > Yes. It won't prevent the direct path_put/auth_domain_put calls. >=20 > >=20 > > Does this re-introduce the UAF that 48db892356d6 was fixing? >=20 > No, something like release of dentry/mnt ek->ek_client->name all=20 > protected will call_rcu(call_rcu(&dentry->d_rcu, __d_free) in=20 > dentry_free, call_rcu(&mnt->mnt_rcu, delayed_free_vfsmnt) in=20 > cleanup_mnt, call_rcu(&dom->rcu_head, svcauth_gss_domain_release_rcu) in= =20 > svcauth_gss_domain_release, call_rcu(&dom->rcu_head,=20 > svcauth_unix_domain_release_rcu) in svcauth_unix_domain_release), so=20 > rcu_read_lock/rcu_read_unlock for c_show/e_show can protect this trigger= =20 > UAF. >=20 Got it. Thanks for the explanation. In that case, I'm fine with reverting this patch. Reviewed-by: Jeff Layton