Re: kernel crash in mknod

[Christian Brauner <brauner@kernel.org>]

On Thu, Mar 28, 2024 at 12:53:40PM +0200, Roberto Sassu wrote:
> On 3/26/2024 12:40 PM, Christian Brauner wrote:
> > > we can change the parameter of security_path_post_mknod() from
> > > dentry to inode?
> > 
> > If all current callers only operate on the inode then it seems the best
> > to only pass the inode. If there's some reason someone later needs a
> > dentry the hook can always be changed.
> 
> Ok, so the crash is likely caused by:
> 
> void security_path_post_mknod(struct mnt_idmap *idmap, struct dentry
> *dentry)
> {
>         if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> 
> I guess we can also simply check if there is an inode attached to the
> dentry, to minimize the changes. I can do both.
> 
> More technical question, do I need to do extra checks on the dentry before
> calling security_path_post_mknod()?

Why do you need the dentry? The two users I see are ima in [1] and evm in [2].
Both of them don't care about the dentry. They only care about the
inode. So why is that hook not just:

diff --git a/security/security.c b/security/security.c
index 7e118858b545..025689a7e912 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1799,11 +1799,11 @@ EXPORT_SYMBOL(security_path_mknod);
  *
  * Update inode security field after a file has been created.
  */
-void security_path_post_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
+void security_inode_post_mknod(struct mnt_idmap *idmap, struct inode *inode)
 {
-       if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+       if (unlikely(IS_PRIVATE(inode)))
                return;
-       call_void_hook(path_post_mknod, idmap, dentry);
+       call_void_hook(path_post_mknod, idmap, inode);
 }

 /**

And one another thing I'd like to point out is that the security hook is
called "security_path_post_mknod()" while the evm and ima hooks are
called evm_post_path_mknod() and ima_post_path_mknod() respectively. In
other words:

git grep _path_post_mknod() doesn't show the implementers of that hook
which is rather unfortunate. It would be better if the pattern were:

<specific LSM>_$some_$ordered_$words()

[1]:
static void evm_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
{
        struct inode *inode = d_backing_inode(dentry);
        struct evm_iint_cache *iint = evm_iint_inode(inode);

        if (!S_ISREG(inode->i_mode))
                return;

        if (iint)
                iint->flags |= EVM_NEW_FILE;
}

[2]:
static void ima_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
{
        struct ima_iint_cache *iint;
        struct inode *inode = dentry->d_inode;
        int must_appraise;

        if (!ima_policy_flag || !S_ISREG(inode->i_mode))
                return;

        must_appraise = ima_must_appraise(idmap, inode, MAY_ACCESS,
                                          FILE_CHECK);
        if (!must_appraise)
                return;

        /* Nothing to do if we can't allocate memory */
        iint = ima_inode_get(inode);
        if (!iint)
                return;

        /* needed for re-opening empty files */
        iint->flags |= IMA_NEW_FILE;
}



> 
> Thanks
> 
> Roberto
> 
> > For bigger changes it's also worthwhile if the object that's passed down
> > into the hook-based LSM layer is as specific as possible. If someone
> > does a change that affects lifetime rules of mounts then any hook that
> > takes a struct path argument that's unused means going through each LSM
> > that implements the hook only to find out it's not actually used.
> > Similar for dentry vs inode imho.
>