Re: [PATCH v5 10/23] security: Introduce inode_post_setattr hook

[Casey Schaufler]

On 11/7/2023 5:39 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> the inode_post_setattr hook.
>
> At inode_setattr hook, EVM verifies the file's existing HMAC value. At
> inode_post_setattr, EVM re-calculates the file's HMAC based on the modified
> file attributes and other file metadata.
>
> Other LSMs could similarly take some action after successful file attribute
> change.
>
> The new hook cannot return an error and cannot cause the operation to be
> reverted.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Reviewed-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>

Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>


> ---
>  fs/attr.c                     |  1 +
>  include/linux/lsm_hook_defs.h |  2 ++
>  include/linux/security.h      |  7 +++++++
>  security/security.c           | 16 ++++++++++++++++
>  4 files changed, 26 insertions(+)
>
> diff --git a/fs/attr.c b/fs/attr.c
> index 498e673bdf06..221d2bb0a906 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -502,6 +502,7 @@ int notify_change(struct mnt_idmap *idmap, struct dentry *dentry,
>  
>  	if (!error) {
>  		fsnotify_change(dentry, ia_valid);
> +		security_inode_post_setattr(idmap, dentry, ia_valid);
>  		ima_inode_post_setattr(idmap, dentry, ia_valid);
>  		evm_inode_post_setattr(idmap, dentry, ia_valid);
>  	}
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index f5db5e993cd8..67410e085205 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -137,6 +137,8 @@ LSM_HOOK(int, 0, inode_follow_link, struct dentry *dentry, struct inode *inode,
>  LSM_HOOK(int, 0, inode_permission, struct inode *inode, int mask)
>  LSM_HOOK(int, 0, inode_setattr, struct mnt_idmap *idmap, struct dentry *dentry,
>  	 struct iattr *attr)
> +LSM_HOOK(void, LSM_RET_VOID, inode_post_setattr, struct mnt_idmap *idmap,
> +	 struct dentry *dentry, int ia_valid)
>  LSM_HOOK(int, 0, inode_getattr, const struct path *path)
>  LSM_HOOK(int, 0, inode_setxattr, struct mnt_idmap *idmap,
>  	 struct dentry *dentry, const char *name, const void *value,
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 750130a7b9dd..664df46b22a9 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -361,6 +361,8 @@ int security_inode_follow_link(struct dentry *dentry, struct inode *inode,
>  int security_inode_permission(struct inode *inode, int mask);
>  int security_inode_setattr(struct mnt_idmap *idmap,
>  			   struct dentry *dentry, struct iattr *attr);
> +void security_inode_post_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> +				 int ia_valid);
>  int security_inode_getattr(const struct path *path);
>  int security_inode_setxattr(struct mnt_idmap *idmap,
>  			    struct dentry *dentry, const char *name,
> @@ -877,6 +879,11 @@ static inline int security_inode_setattr(struct mnt_idmap *idmap,
>  	return 0;
>  }
>  
> +static inline void
> +security_inode_post_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> +			    int ia_valid)
> +{ }
> +
>  static inline int security_inode_getattr(const struct path *path)
>  {
>  	return 0;
> diff --git a/security/security.c b/security/security.c
> index 7935d11d58b5..ce3bc7642e18 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2222,6 +2222,22 @@ int security_inode_setattr(struct mnt_idmap *idmap,
>  }
>  EXPORT_SYMBOL_GPL(security_inode_setattr);
>  
> +/**
> + * security_inode_post_setattr() - Update the inode after a setattr operation
> + * @idmap: idmap of the mount
> + * @dentry: file
> + * @ia_valid: file attributes set
> + *
> + * Update inode security field after successful setting file attributes.
> + */
> +void security_inode_post_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> +				 int ia_valid)
> +{
> +	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> +		return;
> +	call_void_hook(inode_post_setattr, idmap, dentry, ia_valid);
> +}
> +
>  /**
>   * security_inode_getattr() - Check if getting file attributes is allowed
>   * @path: file