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

[Jeffrey Layton]

On Thu, 2024-05-02 at 13:58 -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.
> 

Nice catch. I think you're correct on file ACLs too.

We're probably saved in many cases by the fact that clients usually
send ACLs and seclabels alongside other attributes during a create.
Obviously, that's not _always_ the case though.

> 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>
> ---
>  fs/nfsd/vfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 2e41eb4c3cec..9b777ea7ef26 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1422,7 +1422,7 @@ nfsd_create_setattr(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	 * Callers expect new file metadata to be committed even
>  	 * if the attributes have not changed.
>  	 */
> -	if (iap->ia_valid)
> +	if (iap->ia_valid || (attrs->na_seclabel && attrs->na_seclabel->len))
>  		status = nfsd_setattr(rqstp, resfhp, attrs, NULL);
>  	else
>  		status = nfserrno(commit_metadata(resfhp));

This looks like the right approach to me, but can we instead add a
nfsd_attrs_valid() helper function that checks ia_valid and does the
test above?

Then we can add similar tests for ACLs to it later, once we do a bit
more investigation.

Thanks,
-- 
Jeffrey Layton <jlayton at kernel.org>