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 8F3693876A7; Sat, 23 May 2026 10:55:56 +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=1779533758; cv=none; b=Kkjco0p4ML1L9Zg/Tzh+l5lG5aL9okWRPQnrPOuNUcjdG1bWCYZaX+/hADCebKL9r25fBBVWMGZlngO/S0UlcQkg+jEl3h26hl2uehBXDsUlFOiBhR3RnpWohoffAWiTJq6oYHoAC99byCRYyFEIfaNG7mbPUn09eEa0ytANQqc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779533758; c=relaxed/simple; bh=FvMux4Q+qubljKjKdYexEAEpcWPS3rNDT4d2kcBZOHQ=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=tu5Fv0bWSjldnB6Mo8eFc6zZDUCOgpe8kl9yP6trm/FsSMuYh6zPtoC5z98d6ROlJMKRMGWOfelMR44ALQUA9SheQLwGHJLkKBPmG58oI9yAApJ8q+NQNqn/Gimev63KgmhaPibaZkVljE7Vv2UUIeWXx+kuk9zytpeZTTgxwGA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZP1Gb61b; 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="ZP1Gb61b" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 553011F000E9; Sat, 23 May 2026 10:55:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779533756; bh=e51kdHNXyKAK8/FhvdkIzGvyPGuX+jymiccCBa72duA=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=ZP1Gb61b9o+17HtN2vtgi1fwEgLT4++Xb2PERgoH3lR6oRQB04YrdensIgES6SsSX EzRZQP7JwItNo3Rkj//rIqA9Zze87aUaXsk02bRyfiftK34ySFG4kGfr4J/2ZnHY4S t8GAyPxlGZedBcIbfWXF+IQ/boxkVzbO6itF2v/LZ6z8qPekQ/29RE1Vov+b+f1JpN q42H4bNsR5Lpp8pCIndJayQKawqndEweaTo1UGY1jWmy5URacifOuFj9KEf56KUzia 4zQuHaarbO1dIvs66AHw+YDUHPzoJLHWdiDEwv5Wtbnt+h9TUtzRShLwL3noQk/rRu jbTvKv5q9LoIQ== Message-ID: <8793ba93d173b82bd210a223a91664ee245b66dd.camel@kernel.org> Subject: Re: [PATCH] NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock From: Jeff Layton To: Michael Bommarito , 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 06:55:53 -0400 In-Reply-To: <20260523014107.2460863-1-michael.bommarito@gmail.com> References: <20260523014107.2460863-1-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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2026-05-22 at 21:41 -0400, Michael Bommarito wrote: > nfsd4_ssc_expire_umount() walks nn->nfsd_ssc_mount_list with > list_for_each_entry_safe(ni, tmp, ...). For each expired entry it > sets nsui_busy =3D true, drops nfsd_ssc_lock to run mntput() on the > source vfsmount, then reacquires the lock to list_del + kfree the > entry and continue iterating via the macro's saved tmp pointer. >=20 > The nsui_busy flag protects the current ni from concurrent > nfsd4_ssc_setup_dul() finders during the lock-drop window, but it > does not pin tmp. Another nfsd RPC thread that fails its source- > server mount and reaches nfsd4_ssc_cancel_dul() will, during that > same window, take nfsd_ssc_lock, list_del + kfree its own ssc_umount > item, and release the lock. If that item is the saved tmp of the > expire walk, the next iteration dereferences a freed > nfsd4_ssc_umount_item. >=20 > Reachability: triggered by any authenticated NFSv4.2 client that > can issue OP_COPY with cna_src.nl4_type =3D NL4_SERVER to a destination > nfsd built with CONFIG_NFSD_V4_2_INTER_SSC=3Dy and started with > inter_copy_offload_enable=3DY. The client chooses the source-server > netaddr and can pick one that fails vfs_kern_mount() (unreachable, > RST after EXCHANGE_ID, etc.) to drive nfsd4_ssc_cancel_dul() into > the laundromat's lock-drop window. Default Linux nfsd ships with > inter_copy_offload_enable=3DN, so the bug is reachable only on servers > where the administrator has explicitly opted into inter-SSC offload. >=20 > Restart the walk from the head after the mntput() unlock window so > no saved next pointer survives the lock-drop. The list is bounded > by the number of active inter-server source mounts (typically small) > and the expire delayed-work runs periodically rather than per-IO, > so the restart is cheap. >=20 > Cc: stable@vger.kernel.org > Fixes: f4e44b393389 ("NFSD: delay unmount source's export after inter-ser= ver copy completed.") > Assisted-by: Claude:claude-opus-4-7 > Signed-off-by: Michael Bommarito > --- > fs/nfsd/nfs4state.c | 45 ++++++++++++++++++++++++++------------------- > 1 file changed, 26 insertions(+), 19 deletions(-) >=20 > Reproduced under QEMU/KVM with KASAN, three nfsd network namespaces > on a single host so the kernel client treats them as distinct > servers, and Linux fault injection forcing vfs_kern_mount allocations > inside the destination nfsd to fail. This drives nfsd4_ssc_cancel_dul > into a tight loop concurrent with the laundromat workqueue. >=20 > Stock kernel: >=20 > BUG: KASAN: slab-use-after-free in laundromat_main+0x1756/0x1be0 > Read of size 8 at addr ffff88800ce9b200 by task kworker/u16:3 > Workqueue: nfsd4 laundromat_main >=20 > Allocated by task 229: > nfsd4_interssc_connect+0x3f5/0xd90 (nfsd4_ssc_setup_dul, inlined) > nfsd4_copy+0x117d/0x1a30 > nfsd4_proc_compound+0xbe9/0x23f0 >=20 > Freed by task 229: > kfree+0x18f/0x520 > nfsd4_interssc_connect+0xaff/0xd90 (nfsd4_ssc_cancel_dul, inlined) > nfsd4_copy+0x117d/0x1a30 >=20 > The buggy address belongs to the cache kmalloc-128 of size 128. > Kernel panic - not syncing: Fatal exception >=20 > Patched kernel ran the equivalent workload to completion with the > inter-SSC code path exercised 21-22 times per run and no KASAN > report. >=20 > The fault-injection knobs are standard Linux testing infrastructure > (see Documentation/fault-injection/) exercising the existing failure > path in nfsd; no kernel source was modified. The same primitive > class was previously addressed by the OPEN-error path fix in > __nfs42_ssc_open(); this patch closes the corresponding hole in > the laundromat-driven delayed-unmount path. >=20 >=20 > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c > index 6b9c399b89dfb..03582f15e3e7e 100644 > --- a/fs/nfsd/nfs4state.c > +++ b/fs/nfsd/nfs4state.c > @@ -6728,30 +6728,37 @@ static void nfsd4_ssc_shutdown_umount(struct nfsd= _net *nn) > static void nfsd4_ssc_expire_umount(struct nfsd_net *nn) > { > bool do_wakeup =3D false; > - struct nfsd4_ssc_umount_item *ni =3D NULL; > - struct nfsd4_ssc_umount_item *tmp; > + struct nfsd4_ssc_umount_item *ni; > =20 > +restart: > spin_lock(&nn->nfsd_ssc_lock); > - list_for_each_entry_safe(ni, tmp, &nn->nfsd_ssc_mount_list, nsui_list) = { > - if (time_after(jiffies, ni->nsui_expire)) { > - if (refcount_read(&ni->nsui_refcnt) > 1) > - continue; > + list_for_each_entry(ni, &nn->nfsd_ssc_mount_list, nsui_list) { > + if (!time_after(jiffies, ni->nsui_expire)) > + break; > + if (refcount_read(&ni->nsui_refcnt) > 1) > + continue; > =20 > - /* mark being unmount */ > - ni->nsui_busy =3D true; > - spin_unlock(&nn->nfsd_ssc_lock); > - mntput(ni->nsui_vfsmount); > - spin_lock(&nn->nfsd_ssc_lock); > + /* mark being unmount */ > + ni->nsui_busy =3D true; > + spin_unlock(&nn->nfsd_ssc_lock); > + mntput(ni->nsui_vfsmount); > + spin_lock(&nn->nfsd_ssc_lock); > =20 > - /* waiters need to start from begin of list */ > - list_del(&ni->nsui_list); > - kfree(ni); > + /* waiters need to start from begin of list */ > + list_del(&ni->nsui_list); > + kfree(ni); > =20 > - /* wakeup ssc_connect waiters */ > - do_wakeup =3D true; > - continue; > - } > - break; > + /* wakeup ssc_connect waiters */ > + do_wakeup =3D true; > + /* > + * The list_for_each_entry_safe() saved-next pointer was Comment is a bit confusing, given that you replaced list_for_each_entry_safe() with list_for_each_entry(). > + * not pinned across the spin_unlock() above: a concurrent > + * nfsd4_ssc_cancel_dul() can free the next item under the > + * same spinlock while mntput() runs. Restart the walk > + * from the head so no stale next is dereferenced. > + */ > + spin_unlock(&nn->nfsd_ssc_lock); > + goto restart; > } > if (do_wakeup) > wake_up_all(&nn->nfsd_ssc_waitq); The bug and patch look correct to me though. Reviewed-by: Jeff Layton