Re: [PATCH v5 11/23] security: Introduce inode_post_removexattr hook

[Casey Schaufler]

On 11/20/2023 9:31 AM, Roberto Sassu wrote:
> On Tue, 2023-11-07 at 09:33 -0800, Casey Schaufler wrote:
>> On 11/7/2023 5:40 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_removexattr hook.
>>>
>>> At inode_removexattr hook, EVM verifies the file's existing HMAC value. At
>>> inode_post_removexattr, EVM re-calculates the file's HMAC with the passed
>>> xattr removed and other file metadata.
>>>
>>> Other LSMs could similarly take some action after successful xattr removal.
>>>
>>> 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>
>>> ---
>>>  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 09d927603433..84a4aa566c02 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;
>> Shouldn't this be simply "return error" rather than a goto to nothing
>> but "return error"?
> I got a review from Andrew Morton. His argument seems convincing, that
> having less return places makes the code easier to handle.

That was in a case where you did more than just "return". Nonetheless,
I think it's a matter of style that's not worth debating. Do as you will.

>
> Thanks
>
> Roberto
>
>>> -	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 67410e085205..88452e45025c 100644
>>> --- a/include/linux/lsm_hook_defs.h
>>> +++ b/include/linux/lsm_hook_defs.h
>>> @@ -149,6 +149,8 @@ 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 664df46b22a9..922ea7709bae 100644
>>> --- a/include/linux/security.h
>>> +++ b/include/linux/security.h
>>> @@ -380,6 +380,7 @@ 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,
>>> @@ -940,6 +941,10 @@ 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 ce3bc7642e18..8aa6e9f316dd 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -2452,6 +2452,20 @@ 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