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 29CDB368D73; Fri, 29 May 2026 10:48:55 +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=1780051737; cv=none; b=dI6It/hAPdQJlJMYrHyvLrgZcLem3cQYc5LQAWDqR7MjmwbEnferlORBZHOH3IsoDWTCD9gvLyE6/ViA8Bjdqfv1T8RGgzkcONbQtqdufrhuSO8Lhpk/b3NLiKn9sT4BYctlYQEk7sTNub4O74kJ6BSZX0XHuJrNQuP8vg73tDw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780051737; c=relaxed/simple; bh=RmkBqZlu4zNY+YYMrfb1783b9PAuugPUjj0Fe6IQndg=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=bFJGwOzYS55QEjE2YayVzbUjjwtSc4ulhKj/1t3v+GixfqW0Slm2vyD/S+/amcyeyUjqMh4yP3BOZS8J0Om3fbAmBPjHi3F5Lq/FTvC80oB8SQhiK8Aihcme/taj5Ljc5sSbFj3Rd4DR9uBKKmeNVrwx7jXXHLOKWVIrNaJnnGU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bO8pLHmu; 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="bO8pLHmu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 63F3F1F00893; Fri, 29 May 2026 10:48:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780051735; bh=sjLcQVpA4qQvyyNA9Tp3iQXS/SqZ9LXAb9ArX5f6/MY=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=bO8pLHmusFzI+LKJ2EiyiNavEkaLXqTZLOkPE6Kc2aHOdY3aCr3EHKuCE8lnhQSxd 6xL+dKOgj4epOsGMI+sigsJSPleraPe+k8oqDTgRqlZrim8n3r+AMPHtVOy60XZHHq LniCy7MCa9TbdhW2IoQ6KueCwHs7Gm+ZTcUJyDY5Wu8kfGx2I/NJuU3riXo6sBA6Xs HNHWcUXsANSiIIzc/MCuLDeRbqlODxao5uNYxJIW01X5OzLhD7Y9MIfoka9ZZjQT1i 8WZitP64lqrxE0PsUYKp2GYN9jxwxxGlgCvP8nkrtoe1v+s9hDiONTjxs7o8igdil8 VkQx/aHpy16tg== Message-ID: <4a35675f5627c2e1e3464cb893ff6e619e41e74b.camel@kernel.org> Subject: Re: [PATCH 09/10] nfsd: cap decoded POSIX ACL count to bound sort cost From: Jeff Layton To: Chuck Lever , Rick Macklem Cc: Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , "J. Bruce Fields" , Scott Mayhew , Trond Myklebust , Andreas Gruenbacher , Mike Snitzer , Rick Macklem , Chris Mason , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 29 May 2026 06:48:52 -0400 In-Reply-To: <71e5e6b6-f2d6-48ca-bc66-32064a2e0798@app.fastmail.com> References: <20260528-nfsd-fixes-v1-0-e78708eff77d@kernel.org> <20260528-nfsd-fixes-v1-9-e78708eff77d@kernel.org> <71e5e6b6-f2d6-48ca-bc66-32064a2e0798@app.fastmail.com> 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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DfX/sn9XcfoVZIxMpCRE750zvJvcCUz9HojsrMQ1NFc7MFT1z3MOW2/RlzPcog7xvR5ENPH19ojRD CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-05-28 at 20:07 -0400, Chuck Lever wrote: >=20 > On Thu, May 28, 2026, at 7:11 PM, Chuck Lever wrote: > > On Thu, May 28, 2026, at 6:11 PM, Rick Macklem wrote: > > > On Thu, May 28, 2026 at 2:56=E2=80=AFPM Jeff Layton wrote: > > > >=20 > > > > CAUTION: This email originated from outside of the University of Gu= elph. Do not click links or open attachments unless you recognize the sende= r and know the content is safe. If you are unsure, forward the message to I= THelp@uoguelph.ca for review. > > > >=20 > > > >=20 > > > > From: Chris Mason > > > >=20 > > > > nfsd4_decode_posixacl() reads a u32 entry count off the wire and pa= sses > > > > it straight to posix_acl_alloc() and sort_pacl_range(). The latter = is > > > > an O(n^2) bubble sort, so a client-chosen count drives unbounded CP= U in > > > > the server's compound processing path. > > > >=20 > > > > nfsd4_decode_posixacl() > > > > xdr_stream_decode_u32(&count) /* uncapped u32 */ > > > > posix_acl_alloc(count, GFP_KERNEL) > > > > sort_pacl_range(*acl, 0, count - 1) /* O(n^2) bubble sort */ > > > >=20 > > > > The encoder side in the same file already rejects ACLs whose a_coun= t > > > > exceeds NFS_ACL_MAX_ENTRIES, but the decoder introduced in commit > > > > 5fc51dfc2eb1 ("NFSD: Add support for XDR decoding POSIX draft ACLs"= ) > > > > omitted the symmetric check. > > > My recollection is that Chuck didn't like this limit. He argued that = it was > > > specific to the NFSv3 ACL protocol and that the limit on the size of = a NFSv4 > > > RPC message was sufficient. I, personally, think that 1024 is a reas= onable > > > limit for # of ACEs, but Chuck can jump in here if he doesn't agree. > >=20 > > The NFSACL protocol limits the number of ACEs in an ACL to NFS_ACL_MAX_= ENTRIES > > (1024). It=E2=80=99s a limit that is defined in the protocol itself. > >=20 > > The NFSv4 protocol sets no similar limit. In particular, NFS_ACL_MAX_EN= TRIES > > is not a constant defined or used by the NFSv4.x family of protocols II= RC. > >=20 > > Using NFS_ACL_MAX_ENTRIES to cap the number of ACEs in NFSv4 ACLs is a > > convenience, but it adds technical debt (slight though it may be). > >=20 > > So=E2=80=A6 We can define an implementation limit for NFSv4 ACL support= in NFSD. > > But it shouldn=E2=80=99t be called NFS_ACL_MAX_ENTRIES, IMHO. For clari= ty of > > documentation, pick a number (could be 1024) and state in a comment tha= t > > it is an NFSD implementation constraint, not a protocol limit. Name the > > constant something like NFSD4_MAX_yada to make it clear it is an > > implementation limit, not a protocol limit. >=20 > A different take on this might be that we want to ensure that ACLs set > via the NFSv4 POSIX ACL extension can always be retrieved via NFSACL. > In that case, capping the ACE count at the same number makes sense. > As long as the reason for this is clearly mentioned. >=20 First, I'll note that the encoder already has this limit in place: static __be32 nfsd4_encode_posixacl(struct xdr_stream *xdr, struct svc_rqst *rqstp, struct posix_acl *acl) { [...] if (acl->a_count > NFS_ACL_MAX_ENTRIES) return nfserr_resource; [...] } ...so if you set an ACL that is longer than NFS_ACL_MAX_ENTRIES you already can't retrieve it with NFSv4. Given that, I went with the above suggestion, and added a comment to the patch. Seem ok? diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c index c6c50c376b23..eaf71c65d665 100644 --- a/fs/nfsd/nfs4xdr.c +++ b/fs/nfsd/nfs4xdr.c @@ -448,6 +448,14 @@ nfsd4_decode_posixacl(struct nfsd4_compoundargs *argp,= struct posix_acl **acl) =20 if (xdr_stream_decode_u32(argp->xdr, &count) < 0) return nfserr_bad_xdr; + /* + * RFC8881 doesn't define a max number of ACE's, but the NFSACL spe= c + * does. For NFSv4, cap the number of entries to the v3 limit, as w= e + * want to ensure that ACLs set via NFSv4 POSIX ACL extensions are + * retrievable via NFSv3. + */ + if (count > NFS_ACL_MAX_ENTRIES) + return nfserr_resource; =20 *acl =3D posix_acl_alloc(count, GFP_KERNEL); if (*acl =3D=3D NULL)