Re: [PATCH v7] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

[Christian Brauner]

On Fri, Aug 04, 2023 at 12:09:34PM -0400, Jeff Layton wrote:
> From: David Howells <dhowells@xxxxxxxxxx>
> 
> 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 when alloc_fs_context() is creating the fs_context for it.
> 
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@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>
> 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
> ---
> ver #7)
>  - Drop lsm_set boolean
>  - Link to v6: https://lore.kernel.org/r/20230802-master-v6-1-45d48299168b@xxxxxxxxxx
> 
> ver #6)
>  - Rebase onto v6.5.0-rc4
> 
> 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.
> ---
>  fs/fs_context.c               |  4 ++++
>  include/linux/lsm_hook_defs.h |  1 +
>  include/linux/security.h      |  6 +++++
>  security/security.c           | 14 +++++++++++
>  security/selinux/hooks.c      | 25 ++++++++++++++++++++
>  security/smack/smack_lsm.c    | 54 +++++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 104 insertions(+)
> 
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 851214d1d013..a523aea956c4 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/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 7308a1a7599b..7ce3550154b1 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -54,6 +54,7 @@ LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, struct file *f
>  LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
>  LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
>  LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
> +LSM_HOOK(int, 0, fs_context_init, struct fs_context *fc, struct dentry *reference)
>  LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
>  	 struct fs_context *src_sc)
>  LSM_HOOK(int, -ENOPARAM, fs_context_parse_param, struct fs_context *fc,
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 32828502f09e..61fda06fac9d 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -293,6 +293,7 @@ int security_bprm_creds_from_file(struct linux_binprm *bprm, struct file *file);
>  int security_bprm_check(struct linux_binprm *bprm);
>  void security_bprm_committing_creds(struct linux_binprm *bprm);
>  void security_bprm_committed_creds(struct linux_binprm *bprm);
> +int security_fs_context_init(struct fs_context *fc, struct dentry *reference);
>  int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc);
>  int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param);
>  int security_sb_alloc(struct super_block *sb);
> @@ -629,6 +630,11 @@ static inline void security_bprm_committed_creds(struct linux_binprm *bprm)
>  {
>  }
>  
> +static inline int security_fs_context_init(struct fs_context *fc,
> +					   struct dentry *reference)

I think that's the wrong way of doing this hook. The security hook
really doesn't belong into alloc_fs_context().

I think what we want is a dedicated helper similar to vfs_dup_context():

// Only pass the superblock. There's no need for the dentry. I would
// avoid even passing fs_context but if that's preferred then sure.
security_fs_context_submount(struct fs_context *fc, const struct super_block *sb)

vfs_submount_fs_context(struct file_system_type *fs_type, struct dentry *reference)
{
        fc = fs_context_for_submount(fs_type, reference);

        security_fs_context_for_submount(fc, reference->d_sb);
}

This automatically ensures it's only called for submounts, the LSM
doesn't need to care about fc->purpose and this isn't called
in a pure allocation function for all allocation calls.

The we should switch all callers over to that new helper and unexport
that fs_context_for_submount() thing completely. Yes, that's more work
but that's the correct thing to do. And we need to audit fuse, cifs,
afs, and nfs anyway that they work fine with the new security hook.*


[1]: If really needed, then any additional fs specific work that needs
     to be done during submount allocation should probably probably be
     done in a new callback.

     struct fs_context_operations {
            void (*free)(struct fs_context *fc);
            int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
    +       int (*submount)(struct fs_context *fc, const struct super_block *sb);