Re: [PATCH v3 13/25] security: Introduce inode_post_removexattr hook

Stefan Berger
2023-09-05T18:00:59Z

On 9/4/23 09:34, Roberto Sassu wrote:

> From: Roberto Sassu <roberto.sassu@huawei.com>
>
> In preparation for moving IMA and EVM to the LSM infrastructure, introduce
> the inode_post_removexattr hook.
>
> It is useful for EVM to recalculate the HMAC on the remaining file xattrs,
> and other file metadata, after it verified the HMAC of current file
> metadata with the inode_removexattr hook.
>
> LSMs should use the new hook instead of inode_removexattr, when they need
> to know that the operation was done successfully (not known in
> inode_removexattr). The new hook cannot return an error and cannot cause
> the operation to be reverted.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
> fs/xattr.c | 9 +++++----
> include/linux/lsm_hook_defs.h | 2 ++
> include/linux/security.h | 5 +++++
> security/security.c | 14 ++++++++++++++
> 4 files changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index e7bbb7f57557..4a0280295686 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -552,11 +552,12 @@ __vfs_removexattr_locked(struct mnt_idmap *idmap,
> goto out;
>
> error = __vfs_removexattr(idmap, dentry, name);
> + if (error)
> + goto out;
>
> - if (!error) {
> - fsnotify_xattr(dentry);
> - evm_inode_post_removexattr(dentry, name);
> - }
> + fsnotify_xattr(dentry);
> + security_inode_post_removexattr(dentry, name);
> + evm_inode_post_removexattr(dentry, name);
>
> out:
> return error;
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 995d30336cfa..1153e7163b8b 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -148,6 +148,8 @@ LSM_HOOK(int, 0, inode_getxattr, struct dentry *dentry, const char *name)
> LSM_HOOK(int, 0, inode_listxattr, struct dentry *dentry)
> LSM_HOOK(int, 0, inode_removexattr, struct mnt_idmap *idmap,
> struct dentry *dentry, const char *name)
> +LSM_HOOK(void, LSM_RET_VOID, inode_post_removexattr, struct dentry *dentry,
> + const char *name)
> LSM_HOOK(int, 0, inode_set_acl, struct mnt_idmap *idmap,
> struct dentry *dentry, const char *acl_name, struct posix_acl *kacl)
> LSM_HOOK(int, 0, inode_get_acl, struct mnt_idmap *idmap,
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 820899db5276..665bba3e0081 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -374,6 +374,7 @@ int security_inode_getxattr(struct dentry *dentry, const char *name);
> int security_inode_listxattr(struct dentry *dentry);
> int security_inode_removexattr(struct mnt_idmap *idmap,
> struct dentry *dentry, const char *name);
> +void security_inode_post_removexattr(struct dentry *dentry, const char *name);
> int security_inode_need_killpriv(struct dentry *dentry);
> int security_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry);
> int security_inode_getsecurity(struct mnt_idmap *idmap,
> @@ -919,6 +920,10 @@ static inline int security_inode_removexattr(struct mnt_idmap *idmap,
> return cap_inode_removexattr(idmap, dentry, name);
> }
>
> +static inline void security_inode_post_removexattr(struct dentry *dentry,
> + const char *name)
> +{ }
> +
> static inline int security_inode_need_killpriv(struct dentry *dentry)
> {
> return cap_inode_need_killpriv(dentry);
> diff --git a/security/security.c b/security/security.c
> index 764a6f28b3b9..3947159ba5e9 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2354,6 +2354,20 @@ int security_inode_removexattr(struct mnt_idmap *idmap,
> return evm_inode_removexattr(idmap, dentry, name);
> }
>
> +/**
> + * security_inode_post_removexattr() - Update the inode after a removexattr op
> + * @dentry: file
> + * @name: xattr name
> + *
> + * Update the inode after a successful removexattr operation.
> + */
> +void security_inode_post_removexattr(struct dentry *dentry, const char *name)
> +{
> + if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> + return;
> + call_void_hook(inode_post_removexattr, dentry, name);
> +}
> +
> /**
> * security_inode_need_killpriv() - Check if security_inode_killpriv() required
> * @dentry: associated dentry

Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>