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 6AA27463B6C; Fri, 28 Aug 2026 14:39:54 +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=1787927995; cv=none; b=GhB8Q6VqTiVCWGm2GNtQURzT7lY1Emw3ec9jDDJaCMmnGsxxGVNrpGjCEvrfLF5XoUiATGDEyY7F6VsfuCIzkqB6+rwFB1F8xmuHJeQBYu9sy3IuefMZgqr+Zj8m93QrDtLxL80uLiAYj6nl25Y0kDjore0+OfA4FsnbIRlgl+Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787927995; c=relaxed/simple; bh=XZJkabDsz1o/isv8GnlIwvHXCQapvDug2lqh0wzM0uA=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=bwCw9S9g25UOxHnvmDicGLNQcjsBKkzWw9N7+VdwiV7kEPCtYrtF9+YwYTYRx3G9r/MNTtk1eGSJ/EHSkO1/DH6i9QiouN80QfPc3St9PITJuv+kpmqTa1R5Pnaoojz0MtyOZNlsiqaGDrpPnf/IOtesbHNV7OGXt634Pzp8txA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZOvgczhR; 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="ZOvgczhR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3D7D81F000E9; Fri, 28 Aug 2026 14:39:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787927994; bh=WbdV6eyXnQMzQohIWg66E1PllhjkKmNk21VhpQgF4DQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=ZOvgczhRpjakBqo71x01gh2pyyrydUU9UID+2PNXbAeHvJpuuYNdpDGv3gj9a3IeE M9IRbxEK/Uw4ZD9G46Y+GgX+x/tYk/ujap8MmZIunYM3nGmDvlhLJdeFX/nLXPCC85 5mMAnwD6yXkOGq+qslOaSSkgYPBm9I+YDGLab3drSOYlJZwvd1gkggpgRk/tXxQyR0 Bot0MC+mtwBCTW0mNJm/1p9X39w75wU/3GuEZZqZYBP5i3ybAia9O8bJGyMZ3nC+S+ 6uGn27WyqoUs2J2ii/pkHkSe9Jf+DnWPLmDirUfGAYDmvGKdTOEdeODgNs2cj8tJMV HWZL2pGkD4knA== Message-ID: <343cc59a0bf305ea5de8691257930aec4f44cc29.camel@kernel.org> Subject: Re: [PATCH] nfsd: don't modify a session slot when replaying its cached reply From: Jeff Layton To: Ameer Hamza , cel@kernel.org, neil@brown.name, okorniev@redhat.com, Dai.Ngo@oracle.com, tom@talpey.com Cc: linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, alexander.motin@truenas.com, caleb.stjohn@truenas.com Date: Fri, 28 Aug 2026 10:39:51 -0400 In-Reply-To: <20260824173641.3274260-1-ameer.hamza@truenas.com> References: <20260824173641.3274260-1-ameer.hamza@truenas.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2026-08-24 at 22:36 +0500, Ameer Hamza wrote: > nfsd4_sequence() claims a session slot by setting NFSD4_SLOT_INUSE > under nn->client_lock. A reply served from the slot's reply cache does > not claim it, and that distinction lived only in cstate->status, which > nfsd4_sequence() set to nfserr_replay_cache. Commit cc028a10a48c > ("NFSD: Hoist status code encoding into XDR encoder functions") moved > nfsd4_proc_compound()'s cstate->status assignment below the out: > label, and the replay path's goto out was the one path that relied on > skipping it. The test in nfsd4_sequence_done() therefore no longer > identifies a replay, and every replay now stores its reply and clears > NFSD4_SLOT_INUSE as though it owned the slot. >=20 > NFSD4_SLOT_INUSE is the interlock: check_slot_seqid() rejects every > sequence id for a slot that is in use, before replay_matches_cache() > runs. A replay never sets it, so two replays can be in flight on the > same slot at once. One can read slot->sl_cred under nn->client_lock > while the other's completion frees and rebuilds it from the XDR > encoder without that lock. Two completions can also collide with each > other and release the same group_info twice. free_svc_cred() leaves > cr_uid, cr_gid and cr_flavor intact, so the reader passes every > earlier test in same_creds() and dereferences a NULL cr_group_info. > From a 6.12.91 production server: >=20 > BUG: kernel NULL pointer dereference, address: 0000000000000004 > CPU: 63 UID: 0 PID: 39015 Comm: nfsd > RIP: 0010:same_creds+0x38/0xa0 [nfsd] > RDX: 0000000000000000 > Call Trace: > nfsd4_sequence+0x6a8/0x910 [nfsd] > nfsd4_proc_compound+0x345/0x670 [nfsd] > nfsd_dispatch+0x100/0x220 [nfsd] > svc_process_common+0x311/0x700 [sunrpc] > svc_process+0x131/0x1c0 [sunrpc] > svc_recv+0x7ef/0x9c0 [sunrpc] > nfsd+0xa3/0x100 [nfsd] > Kernel panic - not syncing: Fatal exception >=20 > Since v6.14 a live session's slot table can shrink, and the unowned > store becomes a use-after-free write. nfsd4_sequence() defers the > shrink while a slot is in use, but it tests NFSD4_SLOT_INUSE, which a > replay does not set, so free_session_slots() can kfree() the slot the > replay still holds in cstate->slot. >=20 > Record the claim in cstate->slot_owned when nfsd4_sequence() accepts a > request and test that in nfsd4_sequence_done(), so only the request > that claimed the slot updates its cached reply and clears > NFSD4_SLOT_INUSE. svc_generic_init_request() zeroes the compound > response before each request, so the flag starts clear. >=20 > Fixes: cc028a10a48c ("NFSD: Hoist status code encoding into XDR encoder f= unctions") > Assisted-by: Claude:claude-opus-5 > Signed-off-by: Ameer Hamza > --- > Reproduced on an unmodified tree with a client that pipelines two > copies of a cached request before reading either reply. >=20 > fs/nfsd/nfs4state.c | 7 ++++++- > fs/nfsd/xdr4.h | 1 + > 2 files changed, 7 insertions(+), 1 deletion(-) >=20 > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c > index 2c1b8b2cbbb5b..a4a75a512e9f0 100644 > --- a/fs/nfsd/nfs4state.c > +++ b/fs/nfsd/nfs4state.c > @@ -5170,6 +5170,7 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4= _compound_state *cstate, > slot->sl_flags &=3D ~NFSD4_SLOT_CACHETHIS; > =20 > cstate->slot =3D slot; > + cstate->slot_owned =3D true; > cstate->session =3D session; > cstate->clp =3D clp; > =20 > @@ -5244,7 +5245,11 @@ nfsd4_sequence_done(struct nfsd4_compoundres *resp= ) > struct nfsd4_compound_state *cs =3D &resp->cstate; > =20 > if (nfsd4_has_session(cs)) { > - if (cs->status !=3D nfserr_replay_cache) { > + /* > + * Only the request that claimed the slot may update its > + * cached reply and clear NFSD4_SLOT_INUSE. > + */ > + if (cs->slot_owned) { > nfsd4_store_cache_entry(resp); > cs->slot->sl_flags &=3D ~NFSD4_SLOT_INUSE; > } > diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h > index b841bc462dac8..5b7edf1893e0a 100644 > --- a/fs/nfsd/xdr4.h > +++ b/fs/nfsd/xdr4.h > @@ -62,6 +62,7 @@ struct nfsd4_compound_state { > struct nfsd4_slot *slot; > int data_offset; > bool spo_must_allowed; > + bool slot_owned; > size_t iovlen; > u32 minorversion; > __be32 status; >=20 > base-commit: 46ff234c7129c7c10ed493413d4f60d0cf4dd1f1 Reviewed-by: Jeff Layton