Re: [PATCH bpf-next 1/3] ima: Implement ima_inode_hash

[Yonghong Song <yhs@fb.com>]

On 11/20/20 5:17 AM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
> 
> This is in preparation to add a helper for BPF LSM programs to use
> IMA hashes when attached to LSM hooks. There are LSM hooks like
> inode_unlink which do not have a struct file * argument and cannot
> use the existing ima_file_hash API.
> 
> An inode based API is, therefore, useful in LSM based detections like an
> executable trying to delete itself which rely on the inode_unlink LSM
> hook.
> 
> Moreover, the ima_file_hash function does nothing with the struct file
> pointer apart from calling file_inode on it and converting it to an
> inode.
> 
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>   include/linux/ima.h               |  6 +++
>   scripts/bpf_helpers_doc.py        |  1 +
>   security/integrity/ima/ima_main.c | 74 ++++++++++++++++++++++---------
>   3 files changed, 59 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index 8fa7bcfb2da2..7233a2751754 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -29,6 +29,7 @@ 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 int ima_inode_hash(struct inode *inode, char *buf, size_t buf_size);
>   extern void ima_kexec_cmdline(int kernel_fd, const void *buf, int size);
>   
>   #ifdef CONFIG_IMA_KEXEC
> @@ -115,6 +116,11 @@ static inline int ima_file_hash(struct file *file, char *buf, size_t buf_size)
>   	return -EOPNOTSUPP;
>   }
>   
> +static inline int ima_inode_hash(struct inode *inode, char *buf, size_t buf_size)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
>   static inline void ima_kexec_cmdline(int kernel_fd, const void *buf, int size) {}
>   #endif /* CONFIG_IMA */
>   
> diff --git a/scripts/bpf_helpers_doc.py b/scripts/bpf_helpers_doc.py
> index c5bc947a70ad..add7fcb32dcd 100755
> --- a/scripts/bpf_helpers_doc.py
> +++ b/scripts/bpf_helpers_doc.py
> @@ -478,6 +478,7 @@ class PrinterHelpers(Printer):
>               'struct tcp_request_sock',
>               'struct udp6_sock',
>               'struct task_struct',
> +            'struct inode',

This change (bpf_helpers_doc.py) belongs to patch #2.

>               'struct path',
>               'struct btf_ptr',
>       }
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 2d1af8899cab..1dd2123b5b43 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -501,37 +501,17 @@ 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)
> +static int __ima_inode_hash(struct inode *inode, char *buf, size_t buf_size)
>   {
> -	struct inode *inode;
>   	struct integrity_iint_cache *iint;
>   	int hash_algo;
>   
> -	if (!file)
> +	if (!inode)
>   		return -EINVAL;

Based on original code, for ima_file_hash(), inode cannot be NULL,
so I prefer to remove this change here and add !inode test in
ima_inode_hash.


>   
>   	if (!ima_policy_flag)
>   		return -EOPNOTSUPP;
>   
> -	inode = file_inode(file);
>   	iint = integrity_iint_find(inode);
>   	if (!iint)
>   		return -EOPNOTSUPP;
> @@ -558,8 +538,58 @@ int ima_file_hash(struct file *file, char *buf, size_t buf_size)
>   
>   	return hash_algo;
>   }
> +
> +/**
> + * 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)
> +{
> +	if (!file)
> +		return -EINVAL;
> +
> +	return __ima_inode_hash(file_inode(file), buf, buf_size);
> +}
>   EXPORT_SYMBOL_GPL(ima_file_hash);
>   
> +/**
> + * ima_inode_hash - return the stored measurement if the inode has been hashed
> + * and is in the iint cache.
> + * @inode: pointer to the inode
> + * @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 hash returned is based on the entire contents, 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_inode_hash(struct inode *inode, char *buf, size_t buf_size)
> +{

Add
	if (!inode)
		return -EINVAL;


> +	return __ima_inode_hash(inode, buf, buf_size);
> +}
> +EXPORT_SYMBOL_GPL(ima_inode_hash);
> +
>   /**
>    * ima_post_create_tmpfile - mark newly created tmpfile as new
>    * @file : newly created tmpfile
>