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 C336930F81F for ; Mon, 1 Sep 2025 12:25:30 +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=1756729530; cv=none; b=VEvJyArLvQreEO2E0C2/O4Dyx1yJRiiARQf2mKzydtHWCqh+kviALncnhOyJNHVpdMoVUsoIq7f96M9qLHS2urnf5y/tBtTJbsVTCnWZXrC5rV7cIOTfrHLehs90lg/xnmUeloPG/ZdUji/tjO5wxU93Ua9W4yrQxKC2Kr5yZwo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756729530; c=relaxed/simple; bh=ut6dLzV0s23NVocktYN8sYlOrmEc8JbGnLMFodGEF5k=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=NtQWqcVOaH4IhnddAAfoHBKvl3oe6gkggkAkvEVtqDL5ELKSdIIVC1XUPoB0uRYjo0220sE0HgHpXEBLHpuPQ5sC2TxnU573jwayc8tsBZd403oW3wXymiVNKMEiCKgD6aXQ8kqATiTmoiSrWLU2VjToyRLxTE34IVXW8ymPw4g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=k6jEwB3g; 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="k6jEwB3g" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3E599C4CEF0; Mon, 1 Sep 2025 12:25:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1756729530; bh=ut6dLzV0s23NVocktYN8sYlOrmEc8JbGnLMFodGEF5k=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=k6jEwB3gnPx8HDtYmLtA+IDKf+WmNTEMSJ/jOJmZrpR/QHqwyDCFphz+Y4twYRFwd SWIp5ut472cMpEmZsm8Jgd9Ut1Yr9Myi/zP+3Cf0iC9a/CCoSndCEk24mnaMOBv7/Q gne9M+0aIMVc10Xlnc2rWgiRdtHVrAYZxpwKAOff7SCAH12RvWyqR+6Px8SeNfnajJ HDh+ri2ba/v95V9Ii0Tt1Z3hEMXCAb6zpGZbx/0Mi8clur0zlpWTEcE9AACel+yyv3 hsshgfiQYaCP22TA/1wo2o+vYh8qlsn5K+eaBxhDLhqvdvL++g/ga06bXLpBVn5XnJ vmfeloN+KCyfg== Message-ID: <71fa0055f1ddd5a7f8606515579889e85390d8e9.camel@kernel.org> Subject: Re: Question about potential buffer issue in nfs_request_mount() - seeking feedback From: Jeff Layton To: SSH , "linux-nfs@vger.kernel.org" Cc: Kees Cook Date: Mon, 01 Sep 2025 08:25:29 -0400 In-Reply-To: References: Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn 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I think= I may have found something, although now the most up-to-date kernel HEAD i= s August, 2025 and most of all, I'm not a kernel developer so I wanted to a= sk for your feedback on whether my analysis makes sense. >=20 > ## Context > * This was on all NFS v3 TCP mounts > * The warning came from kernel's hardened memcpy detection > * The mount seemed to work despite the warning >=20 > ### Additional Context > I noticed this warning was originally reported around 6.1-rc1 timeframe (= ~2022), but checking the current kernel source, it would appear that the sa= me code pattern exists. > I'm not sure if this was previously reported to the NFS maintainers speci= fically, or if there's a reason it wasn't addressed. Either way, I thought = it was worth bringing up again in case it got missed or deprioritized. >=20 > Source: https://lkml.org/lkml/2022/10/16/461 >=20 > ## The Original Warning > I saw this warning during NFS v3 TCP mount: >=20 > ``` > [ =C2=A0 19.617475] memcpy: detected field-spanning write (size 28) of si= ngle field "request.sap" at fs/nfs/super.c:857 (size 18446744073709551615) > [ =C2=A0 19.617504] WARNING: CPU: 3 PID: 1300 at fs/nfs/super.c:857 nfs_r= equest_mount.constprop.0.isra.0+0x1c0/0x1f0 > ``` >=20 > ## Likely Source of Failure >=20 > Looking at `nfs_request_mount()` in `fs/nfs/super.c`, I see this code: >=20 > ```c > // Around line 850 > struct nfs_mount_request request =3D { > =C2=A0 =C2=A0 .sap =3D &ctx->mount_server._address, > =C2=A0 =C2=A0 // ... other fields > }; >=20 > // Later, around line 881 > if (ctx->mount_server.address.sa_family =3D=3D AF_UNSPEC) { > =C2=A0 =C2=A0 memcpy(request.sap, &ctx->nfs_server._address, ctx->nfs_ser= ver.addrlen); > =C2=A0 =C2=A0 ctx->mount_server.addrlen =3D ctx->nfs_server.addrlen; > } > ``` >=20 > My understanding is: > 1. `request.sap` points to `ctx->mount_server._address` > 2. We're copying from `ctx->nfs_server._address` (which could be 28 bytes= for IPv6) > 3. Into whatever `mount_server._address` points to (which might be smalle= r?) >=20 > The weird size value (18446744073709551615) in the warning makes me think= there might be memory corruption happening. >=20 > Does this seem like a real issue? If so, would adding a size check before= the memcpy make sense, something like: >=20 > ```c > if (ctx->mount_server.address.sa_family =3D=3D AF_UNSPEC) { > =C2=A0 =C2=A0 if (ctx->nfs_server.addrlen <=3D sizeof(ctx->mount_server._= address)) { > =C2=A0 =C2=A0 =C2=A0 =C2=A0 memcpy(request.sap, &ctx->nfs_server._address= , ctx->nfs_server.addrlen); > =C2=A0 =C2=A0 =C2=A0 =C2=A0 ctx->mount_server.addrlen =3D ctx->nfs_server= .addrlen; > =C2=A0 =C2=A0 } else { > =C2=A0 =C2=A0 =C2=A0 =C2=A0 // handle error case; maybe -EINVAL? > =C2=A0 =C2=A0 =C2=A0 =C2=A0 return -EINVAL; > =C2=A0 =C2=A0 } > } > ``` >=20 > I could easily be misunderstanding something fundamental here, so please = let me know if I'm off track. I just wanted to share this in case it's help= ful. >=20 > Thanks for your time and for maintaining NFS! >=20 (cc'ing Kees, our resident hardening expert) FYI, that large size field is 0xffffffffffffffff (a 64-bit integer with all bits set to 1). The doc header over __fortify_memcpy_chk() definition is a little helpful, but the commit it refers to (6f7630b1b5bc) has a bit more info. It looks like that means that the size detection was broken for this memcpy check? That commit mentions that this may be due to a GCC bug. Kees, any thoughts? --=20 Jeff Layton