[PATCH v2] nfsd: set security label during create operations

[Stephen Smalley]

On Thu, May 2, 2024 at 6:34 PM Jeffrey Layton <jlayton at kernel.org> wrote:
>
> On Thu, 2024-05-02 at 15:58 -0400, Stephen Smalley wrote:
> > When security labeling is enabled, the client can pass a file security
> > label as part of a create operation for the new file, similar to mode
> > and other attributes. At present, the security label is received by nfsd
> > and passed down to nfsd_create_setattr(), but nfsd_setattr() is never
> > called and therefore the label is never set on the new file. I couldn't
> > tell if this has always been broken or broke at some point in time. Looking
> > at nfsd_setattr() I am uncertain as to whether the same issue presents for
> > file ACLs and therefore requires a similar fix for those. I am not overly
> > confident that this is the right solution.
> >
> > An alternative approach would be to introduce a new LSM hook to set the
> > "create SID" of the current task prior to the actual file creation, which
> > would atomically label the new inode at creation time. This would be better
> > for SELinux and a similar approach has been used previously
> > (see security_dentry_create_files_as) but perhaps not usable by other LSMs.
> >
> > Reproducer:
> > 1. Install a Linux distro with SELinux - Fedora is easiest
> > 2. git clone https://github.com/SELinuxProject/selinux-testsuite
> > 3. Install the requisite dependencies per selinux-testsuite/README.md
> > 4. Run something like the following script:
> > MOUNT=$HOME/selinux-testsuite
> > sudo systemctl start nfs-server
> > sudo exportfs -o rw,no_root_squash,security_label localhost:$MOUNT
> > sudo mkdir -p /mnt/selinux-testsuite
> > sudo mount -t nfs -o vers=4.2 localhost:$MOUNT /mnt/selinux-testsuite
> > pushd /mnt/selinux-testsuite/
> > sudo make -C policy load
> > pushd tests/filesystem
> > sudo runcon -t test_filesystem_t ./create_file -f trans_test_file \
> >       -e test_filesystem_filetranscon_t -v
> > sudo rm -f trans_test_file
> > popd
> > sudo make -C policy unload
> > popd
> > sudo umount /mnt/selinux-testsuite
> > sudo exportfs -u localhost:$MOUNT
> > sudo rmdir /mnt/selinux-testsuite
> > sudo systemctl stop nfs-server
> >
> > Expected output:
> > <eliding noise from commands run prior to or after the test itself>
> > Process context:
> >       unconfined_u:unconfined_r:test_filesystem_t:s0-s0:c0.c1023
> > Created file: trans_test_file
> > File context: unconfined_u:object_r:test_filesystem_filetranscon_t:s0
> > File context is correct
> >
> > Actual output:
> > <eliding noise from commands run prior to or after the test itself>
> > Process context:
> >       unconfined_u:unconfined_r:test_filesystem_t:s0-s0:c0.c1023
> > Created file: trans_test_file
> > File context: system_u:object_r:test_file_t:s0
> > File context error, expected:
> >       test_filesystem_filetranscon_t
> > got:
> >       test_file_t
> >
> > Signed-off-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> > ---
> > v2 introduces a nfsd_attrs_valid() helper and uses it as suggested by
> > Jeffrey Layton <jlayton at kernel.org>.
> >
> >  fs/nfsd/nfsproc.c | 2 +-
> >  fs/nfsd/vfs.c     | 2 +-
> >  fs/nfsd/vfs.h     | 8 ++++++++
> >  3 files changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
> > index 36370b957b63..3e438159f561 100644
> > --- a/fs/nfsd/nfsproc.c
> > +++ b/fs/nfsd/nfsproc.c
> > @@ -389,7 +389,7 @@ nfsd_proc_create(struct svc_rqst *rqstp)
> >                * open(..., O_CREAT|O_TRUNC|O_WRONLY).
> >                */
> >               attr->ia_valid &= ATTR_SIZE;
> > -             if (attr->ia_valid)
> > +             if (nfsd_attrs_valid(attr))
> >                       resp->status = nfsd_setattr(rqstp, newfhp, &attrs,
> >                                                   NULL);
> >       }
>
> This function is for NFSv2, which doesn't support any inode attributes
> that aren't represented in ia_valid. We could leave this as-is, but
> this is fine too.

Sorry, I got over-eager with trying to fix all ia_valid checks. It's
actually wrong so I'll send a 3rd version without it.