[PATCH v9] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Christian Brauner]

On Tue, Aug 08, 2023 at 07:34:20AM -0400, Jeff Layton wrote:
> From: David Howells <dhowells at redhat.com>
> 
> 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.
> 
> This bug leads to messages like the following appearing in dmesg when
> fscache is enabled:
> 
>     NFS: Cache volume key already in use (nfs,4.2,2,108,106a8c0,1,,,,100000,100000,2ee,3a98,1d4c,3a98,1)
> 
> Fix this by adding a new LSM hook to load fc->security for submount
> creation.
> 
> Signed-off-by: David Howells <dhowells at redhat.com>
> Signed-off-by: Jeff Layton <jlayton at kernel.org>
> 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 at kernel.org>
> Reviewed-by: Jeff Layton <jlayton at kernel.org>
> Acked-by: Casey Schaufler <casey at schaufler-ca.com>
> Acked-by: "Christian Brauner (Microsoft)" <brauner at kernel.org>
> Link: https://lore.kernel.org/r/165962680944.3334508.6610023900349142034.stgit@warthog.procyon.org.uk/ # v1
> Link: https://lore.kernel.org/r/165962729225.3357250.14350728846471527137.stgit@warthog.procyon.org.uk/ # v2
> Link: https://lore.kernel.org/r/165970659095.2812394.6868894171102318796.stgit@warthog.procyon.org.uk/ # v3
> Link: https://lore.kernel.org/r/166133579016.3678898.6283195019480567275.stgit@warthog.procyon.org.uk/ # v4
> Link: https://lore.kernel.org/r/217595.1662033775@warthog.procyon.org.uk/ # v5
> ---
> ver #2)
> - Added Smack support
> - Made LSM parameter extraction dependent on reference != NULL.
> 
> ver #3)
> - Made LSM parameter extraction dependent on fc->purpose ==
>    FS_CONTEXT_FOR_SUBMOUNT.  Shouldn't happen on FOR_RECONFIGURE.
> 
> ver #4)
> - When doing a FOR_SUBMOUNT mount, don't set the root label in SELinux or Smack.
> 
> ver #5)
> - Removed unused variable.
> - Only allocate smack_mnt_opts if we're dealing with a submount.
> 
> ver #6)
> - Rebase onto v6.5.0-rc4
> - Link to v6: https://lore.kernel.org/r/20230802-master-v6-1-45d48299168b@kernel.org
> 
> ver #7)
> - Drop lsm_set boolean
> - Link to v7: https://lore.kernel.org/r/20230804-master-v7-1-5d4e48407298@kernel.org
> 
> ver #8)
> - Remove spurious semicolon in smack_fs_context_init
> - Make fs_context_init take a superblock as reference instead of dentry
> - WARN_ON_ONCE's when fc->purpose != FS_CONTEXT_FOR_SUBMOUNT
> - Call the security hook from fs_context_for_submount instead of alloc_fs_context
> - Link to v8: https://lore.kernel.org/r/20230807-master-v8-1-54e249595f10@kernel.org
> 
> ver #9)
> - rename *_fs_context_init to *_fs_context_submount
> - remove checks for FS_CONTEXT_FOR_SUBMOUNT and NULL reference pointers
> - fix prototype on smack_fs_context_submount

Thanks, this looks good from my perspective. If it looks fine to LSM
folks as well I can put it with the rest of the super work for this
cycle or it can go through the LSM tree.