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From:   =?UTF-8?B?5ZC05byC?= <wangzhibei1999@gmail.com>
Date:   Tue, 19 Jan 2021 00:29:28 +0800
Message-ID: <CAHxDmpQVxOPmA6o535yEC34fNrA2Of=_W-f49L6gDvxVC3FH6w@mail.gmail.com>
Subject: Re: nfsd vurlerability submit
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     "bfields@fieldses.org" <bfields@fieldses.org>,
        "security@kernel.org" <security@kernel.org>, "w@1wt.eu" <w@1wt.eu>,
        "greg@kroah.com" <greg@kroah.com>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "chuck.lever@oracle.com" <chuck.lever@oracle.com>
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hello,

I want to consult you on what is the original intention of designing
subtree_check and whether it is to solve the  'I want to export a
subtree of a filesystem' problem.

As far as I know, when opening subtree_check, the folder's  file
handle does not contain the inode information of its parent directory
and
'while (tdentry !=3D exp->ex_path.dentry && !IS_ROOT(tdentry))' in
nfsd_acceptable can work well to Intercept handles beyond the export
point.

This seems to delete code as follows in nfsfh.c could solve the  'I
want to export a subtree of a filesystem' problem and ensure safety:
if (exp->ex_flags & NFSEXP_NOSUBTREECHECK)
return 1;

Or replace by follow:
if (exp->ex_path.dentry =3D=3D exp->vfs_mount->mnt_root)
return 1;

When I was reading the nfsd code, I was confused about whether the
designer used the file system as a security boundary or an export
point.Since exporting a complete file system is the safest, why not
directly prohibit unsafe practices, but add code like subtree_check to
try to verify the file handle.

I may not understand your design ideas.

Yours sincerely,

Trond Myklebust <trondmy@hammerspace.com> =E4=BA=8E2021=E5=B9=B41=E6=9C=881=
3=E6=97=A5=E5=91=A8=E4=B8=89 =E4=B8=8A=E5=8D=8812:53=E5=86=99=E9=81=93=EF=
=BC=9A
>
> On Tue, 2021-01-12 at 10:32 -0500, J. Bruce Fields wrote:
> > On Tue, Jan 12, 2021 at 10:48:00PM +0800, =E5=90=B4=E5=BC=82 wrote:
> > > Telling users how to configure the exported file system in the most
> > > secure
> > > way does
> > > mitigate the problem to some extent, but this does not seem to
> > > address the
> > > security risks posed by no_ subtree_ check in the code. In my
> > > opinion,when
> > > the generated filehandle does not contain the inode information of
> > > the
> > > parent directory,the nfsd_acceptable function can also recursively
> > > determine whether the request file exceeds the export path
> > > dentry.Enabling
> > > subtree_check to add parent directory information only brings some
> > > troubles.
> >
> > Filesystems don't necessarily provide us with an efficient way to
> > find
> > parent directories from any given file.  (And note a single file may
> > have multiple parent directories.)
> >
> > (I do wonder if we could do better in the directory case, though.  We
> > already reconnect directories all the way back up to the root.)
> >
> > > I have a bold idea, why not directly remove the file handle
> > > modification in
> > > subtree_check, and then normalize the judgment of whether dentry
> > > exceeds
> > > the export point directory in nfsd_acceptable (line 38 to 54 in
> > > /fs/nfsd/nfsfh.c) .
> > >
> > > As far as I understand it, the reason why subtree_check is not
> > > turned on by
> > > default is that it will cause problems when reading and writing
> > > files,
> > > rather than it wastes more time when nfsd_acceptable.
> > >
> > > In short,I think it's open to question whether the security of the
> > > system
> > > depends on the user's complete correct configuration(the system
> > > does not
> > > prohibit the export of a subdirectory).
> >
> > > Enabling subtree_check to add parent directoryinformation only
> > > brings
> > > some troubles.
> > >
> > > In short,I think it's open to question whether the security of the
> > > system depends on the user's complete correct configuration(the
> > > system
> > > does not prohibit the export of a subdirectory).
> >
> > I'd love to replace the export interface by one that prohibited
> > subdirectory exports (or at least made it more obvious where they're
> > being used.)
> >
> > But given the interface we already have, that would be a disruptive
> > and
> > time-consuming change.
> >
> > Another approach is to add more entropy to filehandles so they're
> > harder
> > to guess; see e.g.:
> >
> >         https://www.fsl.cs.stonybrook.edu/docs/nfscrack-tr/index.html
> >
> > In the end none of these change the fact that a filehandle has an
> > infinite lifetime, so once it's leaked, there's nothing you can do.
> > The
> > authors suggest NFSv4 volatile filehandles as a solution to that
> > problem, but I don't think they've thought through the obstacles to
> > making volatile filehandles work.
> >
> > --b.
>
> The point is that there is no good solution to the 'I want to export a
> subtree of a filesystem' problem, and so it is plainly wrong to try to
> make a default of those solutions, which break the one sane case of
> exporting the whole filesystem.
>
> Just a reminder that we kicked out subtree_check not only because a
> trivial rename of a file breaks the client's ability to perform I/O by
> invalidating the filehandle. In addition, that option causes filehandle
> aliasing (i.e. multiple filehandles pointing to the same file) which is
> a major PITA for clients to try to manage for more or less the same
> reason that it is a major PITA to try to manage these files using
> paths.
>
> The discussion on volatile filehandles in RFC5661 does try to address
> some of the above issues, but ends up concluding that you need to
> introduce POSIX-incompatible restrictions, such as trying to ban
> renames and deletions of open files in order to make it work.
>
> None of these compromises are necessary if you export a whole
> filesystem (or a hierarchy of whole filesystems). That's the sane case.
> That's the one that people should default to using.
>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>
>