Re: [PATCH v2 3/7] ima: Ensure lock is held when setting iint pointer in inode security blob

[Mimi Zohar <zohar@linux.ibm.com>]

On Thu, 2024-11-28 at 11:06 +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
> 
> IMA stores a pointer of the ima_iint_cache structure, containing integrity
> metadata, in the inode security blob. However, check and assignment of this
> pointer is not atomic, and it might happen that two tasks both see that the
> iint pointer is NULL and try to set it, causing a memory leak.
> 
> Ensure that the iint check and assignment is guarded, by adding a lockdep
> assertion in ima_inode_get().

-> is guarded by the ima_iint_cache_lock mutex, ...

> 
> Consequently, guard the remaining ima_inode_get() calls, in
> ima_post_create_tmpfile() and ima_post_path_mknod(), to avoid the lockdep
> warnings.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  security/integrity/ima/ima_iint.c |  2 ++
>  security/integrity/ima/ima_main.c | 14 ++++++++++++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_iint.c b/security/integrity/ima/ima_iint.c
> index dcc32483d29f..fca9db293c79 100644
> --- a/security/integrity/ima/ima_iint.c
> +++ b/security/integrity/ima/ima_iint.c
> @@ -97,6 +97,8 @@ struct ima_iint_cache *ima_inode_get(struct inode *inode)
>  	if (!iint_lock)
>  		return NULL;
>  
> +	lockdep_assert_held(&iint_lock->mutex);
> +

lockdep_assert_held() doesn't actually "ensure" the lock is held, but emits a warning
when the lock is not held (if debugging is enabled).  Semantically "ensure" gives the
impression of enforcing.

Mimi

>  	iint = iint_lock->iint;
>  	if (iint)
>  		return iint;
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 05cfb04cd02b..1e474ff6a777 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -705,14 +705,19 @@ static void ima_post_create_tmpfile(struct mnt_idmap *idmap,
>  	if (!must_appraise)
>  		return;
>  
> +	ima_iint_lock(inode);
> +
>  	/* Nothing to do if we can't allocate memory */
>  	iint = ima_inode_get(inode);
> -	if (!iint)
> +	if (!iint) {
> +		ima_iint_unlock(inode);
>  		return;
> +	}
>  
>  	/* needed for writing the security xattrs */
>  	set_bit(IMA_UPDATE_XATTR, &iint->atomic_flags);
>  	iint->ima_file_status = INTEGRITY_PASS;
> +	ima_iint_unlock(inode);
>  }
>  
>  /**
> @@ -737,13 +742,18 @@ static void ima_post_path_mknod(struct mnt_idmap *idmap,
> struct dentry *dentry)
>  	if (!must_appraise)
>  		return;
>  
> +	ima_iint_lock(inode);
> +
>  	/* Nothing to do if we can't allocate memory */
>  	iint = ima_inode_get(inode);
> -	if (!iint)
> +	if (!iint) {
> +		ima_iint_unlock(inode);
>  		return;
> +	}
>  
>  	/* needed for re-opening empty files */
>  	iint->flags |= IMA_NEW_FILE;
> +	ima_iint_unlock(inode);
>  }
>  
>  /**