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

[Roberto Sassu]

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.

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