[PATCH 2/2] NFSv4.2: condition READDIR's mask for security label based on LSM state

[Olga Kornievskaia]

On Thu, Nov 5, 2020 at 1:55 PM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> On Thu, Nov 5, 2020 at 6:33 PM Olga Kornievskaia
> <olga.kornievskaia at gmail.com> wrote:
> > From: Olga Kornievskaia <kolga at netapp.com>
> >
> > Currently, the client will always ask for security_labels if the server
> > returns that it supports that feature regardless of any LSM modules
> > (such as Selinux) enforcing security policy. This adds performance
> > penalty to the READDIR operation.
> >
> > Instead, query the LSM module to find if anything is enabled and
> > if not, then remove FATTR4_WORD2_SECURITY_LABEL from the bitmask.
>
> Having spent some time staring at some of the NFS code very recently,
> I can't help but suggest: Would it perhaps be enough to decide whether
> to ask for labels based on (NFS_SB(dentry->d_sb)->caps &
> NFS_CAP_SECURITY_LABEL)? It is set when mounting the FS iff some LSM
> confirms via the security_sb_*_mnt_opts() hook that it wants the
> filesystem to give it labels (or at least that's how I interpret the
> cryptic name) [1]. It's just a shot in the dark, but it seems to fit
> this use case.
>
> [1] https://elixir.bootlin.com/linux/v5.10-rc2/source/fs/nfs/getroot.c#L148

Very interesting. I was not aware of something like that nor was it
mentioned when I asked on the selinux mailing list. I wonder if this
is a supported feature that will always stay? In that case, NFS
wouldn't need the extra hook that was added for this series. I will
try this out and report back.

>
> >
> > Suggested-by: Scott Mayhew <smayhew at redhat.com>
> > Signed-off-by: Olga Kornievskaia <kolga at netapp.com>
> > ---
> >  fs/nfs/nfs4proc.c       | 5 +++++
> >  fs/nfs/nfs4xdr.c        | 3 ++-
> >  include/linux/nfs_xdr.h | 1 +
> >  3 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> > index 9e0ca9b2b210..774bc5e63ca7 100644
> > --- a/fs/nfs/nfs4proc.c
> > +++ b/fs/nfs/nfs4proc.c
> > @@ -55,6 +55,7 @@
> >  #include <linux/utsname.h>
> >  #include <linux/freezer.h>
> >  #include <linux/iversion.h>
> > +#include <linux/security.h>
> >
> >  #include "nfs4_fs.h"
> >  #include "delegation.h"
> > @@ -4968,6 +4969,7 @@ static int _nfs4_proc_readdir(struct dentry *dentry, const struct cred *cred,
> >                 .count = count,
> >                 .bitmask = NFS_SERVER(d_inode(dentry))->attr_bitmask,
> >                 .plus = plus,
> > +               .labels = true,
> >         };
> >         struct nfs4_readdir_res res;
> >         struct rpc_message msg = {
> > @@ -4977,10 +4979,13 @@ static int _nfs4_proc_readdir(struct dentry *dentry, const struct cred *cred,
> >                 .rpc_cred = cred,
> >         };
> >         int                     status;
> > +       int sec_flags = LSM_FQUERY_VFS_XATTRS;
> >
> >         dprintk("%s: dentry = %pd2, cookie = %Lu\n", __func__,
> >                         dentry,
> >                         (unsigned long long)cookie);
> > +       if (!security_func_query_vfs(sec_flags))
> > +               args.labels = false;
> >         nfs4_setup_readdir(cookie, NFS_I(dir)->cookieverf, dentry, &args);
> >         res.pgbase = args.pgbase;
> >         status = nfs4_call_sync(NFS_SERVER(dir)->client, NFS_SERVER(dir), &msg, &args.seq_args, &res.seq_res, 0);
> > diff --git a/fs/nfs/nfs4xdr.c b/fs/nfs/nfs4xdr.c
> > index c6dbfcae7517..585d5b5cc3dc 100644
> > --- a/fs/nfs/nfs4xdr.c
> > +++ b/fs/nfs/nfs4xdr.c
> > @@ -1605,7 +1605,8 @@ static void encode_readdir(struct xdr_stream *xdr, const struct nfs4_readdir_arg
> >                         FATTR4_WORD1_OWNER_GROUP|FATTR4_WORD1_RAWDEV|
> >                         FATTR4_WORD1_SPACE_USED|FATTR4_WORD1_TIME_ACCESS|
> >                         FATTR4_WORD1_TIME_METADATA|FATTR4_WORD1_TIME_MODIFY;
> > -               attrs[2] |= FATTR4_WORD2_SECURITY_LABEL;
> > +               if (readdir->labels)
> > +                       attrs[2] |= FATTR4_WORD2_SECURITY_LABEL;
> >                 dircount >>= 1;
> >         }
> >         /* Use mounted_on_fileid only if the server supports it */
> > diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
> > index d63cb862d58e..95f648b26525 100644
> > --- a/include/linux/nfs_xdr.h
> > +++ b/include/linux/nfs_xdr.h
> > @@ -1119,6 +1119,7 @@ struct nfs4_readdir_arg {
> >         unsigned int                    pgbase; /* zero-copy data */
> >         const u32 *                     bitmask;
> >         bool                            plus;
> > +       bool                            labels;
> >  };
> >
> >  struct nfs4_readdir_res {
> > --
> > 2.18.2
> >
>
>
> --
> Ondrej Mosnacek
> Software Engineer, Platform Security - SELinux kernel
> Red Hat, Inc.
>