Re: [PATCH 4/9] ima: Move ima_file_free() into LSM

[Roberto Sassu]

On Tue, 2022-10-18 at 17:02 +0200, Christian Brauner wrote:
> On Thu, Oct 13, 2022 at 03:36:49PM -0700, Kees Cook wrote:
> > The file_free_security hook already exists for managing
> > notification of
> > released files. Use the LSM hook instead of open-coded stacking.
> > 
> > Cc: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > Cc: Dmitry Kasatkin <dmitry.kasatkin@xxxxxxxxx>
> > Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> > Cc: James Morris <jmorris@xxxxxxxxx>
> > Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> > Cc: Petr Vorel <pvorel@xxxxxxx>
> > Cc: Jonathan McDowell <noodles@xxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Takashi Iwai <tiwai@xxxxxxx>
> > Cc: linux-integrity@xxxxxxxxxxxxxxx
> > Cc: linux-security-module@xxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  fs/file_table.c                   | 1 -
> >  include/linux/ima.h               | 6 ------
> >  security/integrity/ima/ima_main.c | 3 ++-
> >  3 files changed, 2 insertions(+), 8 deletions(-)
> > 
> > diff --git a/fs/file_table.c b/fs/file_table.c
> > index 99c6796c9f28..fa707d221a43 100644
> > --- a/fs/file_table.c
> > +++ b/fs/file_table.c
> > @@ -311,7 +311,6 @@ static void __fput(struct file *file)
> >  	eventpoll_release(file);
> >  	locks_remove_file(file);
> >  
> > -	ima_file_free(file);
> >  	if (unlikely(file->f_flags & FASYNC)) {
> >  		if (file->f_op->fasync)
> >  			file->f_op->fasync(-1, file, 0);
> > diff --git a/include/linux/ima.h b/include/linux/ima.h
> > index 6dc5143f89f2..9f18df366064 100644
> > --- a/include/linux/ima.h
> > +++ b/include/linux/ima.h
> > @@ -19,7 +19,6 @@ extern enum hash_algo
> > ima_get_current_hash_algo(void);
> >  extern int ima_file_check(struct file *file, int mask);
> >  extern void ima_post_create_tmpfile(struct user_namespace
> > *mnt_userns,
> >  				    struct inode *inode);
> > -extern void ima_file_free(struct file *file);
> >  extern void ima_post_path_mknod(struct user_namespace *mnt_userns,
> >  				struct dentry *dentry);
> >  extern int ima_file_hash(struct file *file, char *buf, size_t
> > buf_size);
> > @@ -56,11 +55,6 @@ static inline void
> > ima_post_create_tmpfile(struct user_namespace *mnt_userns,
> >  {
> >  }
> >  
> > -static inline void ima_file_free(struct file *file)
> > -{
> > -	return;
> > -}
> > -
> >  static inline void ima_post_path_mknod(struct user_namespace
> > *mnt_userns,
> >  				       struct dentry *dentry)
> >  {
> > diff --git a/security/integrity/ima/ima_main.c
> > b/security/integrity/ima/ima_main.c
> > index b3b79d030a67..94379ba40b58 100644
> > --- a/security/integrity/ima/ima_main.c
> > +++ b/security/integrity/ima/ima_main.c
> > @@ -183,7 +183,7 @@ static void ima_check_last_writer(struct
> > integrity_iint_cache *iint,
> >   *
> >   * Flag files that changed, based on i_version
> >   */
> > -void ima_file_free(struct file *file)
> > +static void ima_file_free(struct file *file)
> >  {
> >  	struct inode *inode = file_inode(file);
> >  	struct integrity_iint_cache *iint;
> > @@ -1085,6 +1085,7 @@ static struct security_hook_list ima_hooks[]
> > __lsm_ro_after_init = {
> >  	LSM_HOOK_INIT(bprm_check_security, ima_bprm_check),
> >  	LSM_HOOK_INIT(mmap_file, ima_file_mmap),
> >  	LSM_HOOK_INIT(file_mprotect, ima_file_mprotect),
> > +	LSM_HOOK_INIT(file_free_security, ima_file_free),
> 
> This doesn't work afaict. If the file is opened for writing ima may
> update xattrs. But by the time security_file_free() is called
> put_file_access() has already been called which will have given up
> write
> access to the file's mount.
> 
> So you would have to - just one of the possibilities - have to move
> security_file_free() out of file_free() and into the old
> ima_file_free()
> location. But that might cause semantic changes for other LSMs.

Hi

I also did this work before. In my implementation, I created a new
security hook called security_file_pre_free().

https://github.com/robertosassu/linux/commit/692c9d36fff865435b23b3cb765d31f3584f6263

If useful, the whole patch set is available at:

https://github.com/robertosassu/linux/commits/ima-evm-lsm-v1-devel-v3

Roberto