On Thu, Nov 10, 2022 at 6:09 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> When NFS superblocks are created by automounting, their LSM parameters
> aren't set in the fs_context struct prior to sget_fc() being called,
> leading to failure to match existing superblocks.
>
> Fix this by adding a new LSM hook to load fc->security for submount
> creation when alloc_fs_context() is creating the fs_context for it.
>
> However, this uncovers a further bug: nfs_get_root() initialises the
> superblock security manually by calling security_sb_set_mnt_opts() or
> security_sb_clone_mnt_opts() - but then vfs_get_tree() calls
> security_sb_set_mnt_opts(), which can lead to SELinux, at least,
> complaining.
>
> Fix that by adding a flag to the fs_context that suppresses the
> security_sb_set_mnt_opts() call in vfs_get_tree(). This can be set by NFS
> when it sets the LSM context on the new superblock.
>
> The first bug leads to messages like the following appearing in dmesg:
>
> NFS: Cache volume key already in use (nfs,4.2,2,108,106a8c0,1,,,,100000,100000,2ee,3a98,1d4c,3a98,1)
>
> Changes
> =======
> ver #5)
> - Removed unused variable.
> - Only allocate smack_mnt_opts if we're dealing with a submount.
>
> ver #4)
> - When doing a FOR_SUBMOUNT mount, don't set the root label in SELinux or
> Smack.
>
> ver #3)
> - Made LSM parameter extraction dependent on fc->purpose ==
> FS_CONTEXT_FOR_SUBMOUNT. Shouldn't happen on FOR_RECONFIGURE.
>
> ver #2)
> - Added Smack support
> - Made LSM parameter extraction dependent on reference != NULL.
>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Fixes: 9bc61ab18b1d ("vfs: Introduce fs_context, switch vfs_kern_mount() to it.")
> Fixes: 779df6a5480f ("NFS: Ensure security label is set for root inode)
> Tested-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
> Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> Acked-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
> cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
> cc: Anna Schumaker <anna@xxxxxxxxxx>
> cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> cc: Scott Mayhew <smayhew@xxxxxxxxxx>
> cc: Jeff Layton <jlayton@xxxxxxxxxx>
> cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> cc: linux-nfs@xxxxxxxxxxxxxxx
> cc: selinux@xxxxxxxxxxxxxxx
> cc: linux-security-module@xxxxxxxxxxxxxxx
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/r/165962680944.3334508.6610023900349142034.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1
> Link: https://lore.kernel.org/r/165962729225.3357250.14350728846471527137.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2
> Link: https://lore.kernel.org/r/165970659095.2812394.6868894171102318796.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v3
> Link: https://lore.kernel.org/r/166133579016.3678898.6283195019480567275.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v4
> Link: https://lore.kernel.org/r/217595.1662033775@xxxxxxxxxxxxxxxxxxxxxx/ # v5
> ---
>
> fs/fs_context.c | 4 +++
> fs/nfs/getroot.c | 1 +
> fs/super.c | 10 +++++---
> include/linux/fs_context.h | 1 +
> include/linux/lsm_hook_defs.h | 1 +
> include/linux/lsm_hooks.h | 6 ++++-
> include/linux/security.h | 6 +++++
> security/security.c | 5 ++++
> security/selinux/hooks.c | 25 +++++++++++++++++++
> security/smack/smack_lsm.c | 54 +++++++++++++++++++++++++++++++++++++++++
> 10 files changed, 108 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 24ce12f0db32..22248b8a88a8 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -282,6 +282,10 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
> break;
> }
>
> + ret = security_fs_context_init(fc, reference);
> + if (ret < 0)
> + goto err_fc;
> +
> /* TODO: Make all filesystems support this unconditionally */
> init_fs_context = fc->fs_type->init_fs_context;
> if (!init_fs_context)
> diff --git a/fs/nfs/getroot.c b/fs/nfs/getroot.c
> index 11ff2b2e060f..651bffb0067e 100644
> --- a/fs/nfs/getroot.c
> +++ b/fs/nfs/getroot.c
> @@ -144,6 +144,7 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
> }
> if (error)
> goto error_splat_root;
> + fc->lsm_set = true;
> if (server->caps & NFS_CAP_SECURITY_LABEL &&
> !(kflags_out & SECURITY_LSM_NATIVE_LABELS))
> server->caps &= ~NFS_CAP_SECURITY_LABEL;
> diff --git a/fs/super.c b/fs/super.c
> index 8d39e4f11cfa..f200ae0549ca 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1553,10 +1553,12 @@ int vfs_get_tree(struct fs_context *fc)
> smp_wmb();
> sb->s_flags |= SB_BORN;
>
> - error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
> - if (unlikely(error)) {
> - fc_drop_locked(fc);
> - return error;
> + if (!(fc->lsm_set)) {
> + error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
> + if (unlikely(error)) {
> + fc_drop_locked(fc);
> + return error;
> + }
> }
Thinking about all the different things that an LSM could do, would it
ever be possible that a LSM would want the security_sb_set_mnt_opts()
call to happen here? I'm wondering if we are better off leaving it up
to the LSM by passing the fs_context in the security_sb_set_mnt_opts()
hook; those that want to effectively skip this call due to a submount
setup already done in security_fs_context_init() can check the
fs_context::purpose value in the security_sb_set_mnt_opts() hook.
Thoughts?
--
paul-moore.com
