Re: nfsd vurlerability submit

Trond Myklebust
2021-01-12T16:54:50Z

On Tue, 2021-01-12 at 10:32 -0500, J. Bruce Fields wrote:
> On Tue, Jan 12, 2021 at 10:48:00PM +0800, 吴异 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