[PATCH v3] ima: add the ability to query the cached hash of a given file

[KP Singh]

On 13-Jan 10:42, Florent Revest wrote:
> From: Florent Revest <revest at google.com>
> 
> This allows other parts of the kernel (perhaps a stacked LSM allowing
> system monitoring, eg. the proposed KRSI LSM [1]) to retrieve the hash
> of a given file from IMA if it's present in the iint cache.
> 
> It's true that the existence of the hash means that it's also in the
> audit logs or in /sys/kernel/security/ima/ascii_runtime_measurements,
> but it can be difficult to pull that information out for every
> subsequent exec. This is especially true if a given host has been up
> for a long time and the file was first measured a long time ago.
> 
> It should be kept in mind that this function gives access to cached
> entries which can be removed, for instance on security_inode_free().
> 
> This is based on Peter Moody's patch:
>  https://sourceforge.net/p/linux-ima/mailman/message/33036180/
> 
> [1] https://lkml.org/lkml/2019/9/10/393
> 
> Signed-off-by: Florent Revest <revest at google.com>

Reviewed-by: KP Singh <kpsingh at chromium.org>

> ---
>  include/linux/ima.h               |  6 ++++
>  security/integrity/ima/ima_main.c | 49 +++++++++++++++++++++++++++++++
>  2 files changed, 55 insertions(+)
> 
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index f4644c54f648..1659217e9b60 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -23,6 +23,7 @@ extern int ima_read_file(struct file *file, enum kernel_read_file_id id);
>  extern int ima_post_read_file(struct file *file, void *buf, loff_t size,
>  			      enum kernel_read_file_id id);
>  extern void ima_post_path_mknod(struct dentry *dentry);
> +extern int ima_file_hash(struct file *file, char *buf, size_t buf_size);
>  extern void ima_kexec_cmdline(const void *buf, int size);
>  
>  #ifdef CONFIG_IMA_KEXEC
> @@ -91,6 +92,11 @@ static inline void ima_post_path_mknod(struct dentry *dentry)
>  	return;
>  }
>  
> +static inline int ima_file_hash(struct file *file, char *buf, size_t buf_size)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
>  static inline void ima_kexec_cmdline(const void *buf, int size) {}
>  #endif /* CONFIG_IMA */
>  
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 2272c3255c7d..9fe949c6a530 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -445,6 +445,55 @@ int ima_file_check(struct file *file, int mask)
>  }
>  EXPORT_SYMBOL_GPL(ima_file_check);
>  
> +/**
> + * ima_file_hash - return the stored measurement if a file has been hashed and
> + * is in the iint cache.
> + * @file: pointer to the file
> + * @buf: buffer in which to store the hash
> + * @buf_size: length of the buffer
> + *
> + * On success, return the hash algorithm (as defined in the enum hash_algo).
> + * If buf is not NULL, this function also outputs the hash into buf.
> + * If the hash is larger than buf_size, then only buf_size bytes will be copied.
> + * It generally just makes sense to pass a buffer capable of holding the largest
> + * possible hash: IMA_MAX_DIGEST_SIZE.
> + * The file hash returned is based on the entire file, including the appended
> + * signature.
> + *
> + * If IMA is disabled or if no measurement is available, return -EOPNOTSUPP.
> + * If the parameters are incorrect, return -EINVAL.
> + */
> +int ima_file_hash(struct file *file, char *buf, size_t buf_size)
> +{
> +	struct inode *inode;
> +	struct integrity_iint_cache *iint;
> +	int hash_algo;
> +
> +	if (!file)
> +		return -EINVAL;
> +
> +	if (!ima_policy_flag)
> +		return -EOPNOTSUPP;
> +
> +	inode = file_inode(file);
> +	iint = integrity_iint_find(inode);
> +	if (!iint)
> +		return -EOPNOTSUPP;
> +
> +	mutex_lock(&iint->mutex);
> +	if (buf) {
> +		size_t copied_size;
> +
> +		copied_size = min_t(size_t, iint->ima_hash->length, buf_size);
> +		memcpy(buf, iint->ima_hash->digest, copied_size);
> +	}
> +	hash_algo = iint->ima_hash->algo;
> +	mutex_unlock(&iint->mutex);
> +
> +	return hash_algo;
> +}
> +EXPORT_SYMBOL_GPL(ima_file_hash);
> +
>  /**
>   * ima_post_create_tmpfile - mark newly created tmpfile as new
>   * @file : newly created tmpfile
> -- 
> 2.25.0.rc1.283.g88dfdc4193-goog
>