[PATCH v5] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Christian Brauner]

On Thu, Sep 01, 2022 at 01:02:55PM +0100, David Howells 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 at redhat.com>
> 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>
> Acked-by: Casey Schaufler <casey at schaufler-ca.com>
> cc: Trond Myklebust <trond.myklebust at hammerspace.com>
> cc: Anna Schumaker <anna at kernel.org>
> cc: Alexander Viro <viro at zeniv.linux.org.uk>
> cc: Scott Mayhew <smayhew at redhat.com>
> cc: Jeff Layton <jlayton at kernel.org>
> cc: Paul Moore <paul at paul-moore.com>
> cc: linux-nfs at vger.kernel.org
> cc: selinux at vger.kernel.org
> cc: linux-security-module at vger.kernel.org
> cc: linux-fsdevel at vger.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
> ---

Acked-by: Christian Brauner (Microsoft) <brauner at kernel.org>