Re: [PATCH bpf-next 2/3] bpf: Add a BPF helper for getting the IMA hash of an inode

[Yonghong Song <yhs@fb.com>]

On 11/20/20 5:17 AM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
> 
> Provide a wrapper function to get the IMA hash of an inode. This helper
> is useful in fingerprinting files (e.g executables on execution) and
> using these fingerprints in detections like an executable unlinking
> itself.
> 
> Since the ima_inode_hash can sleep, it's only allowed for sleepable
> LSM hooks.
> 
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>   include/uapi/linux/bpf.h       | 11 +++++++++++
>   kernel/bpf/bpf_lsm.c           | 26 ++++++++++++++++++++++++++
>   scripts/bpf_helpers_doc.py     |  1 +
>   tools/include/uapi/linux/bpf.h | 11 +++++++++++
>   4 files changed, 49 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 3ca6146f001a..dd5b8622bb89 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3807,6 +3807,16 @@ union bpf_attr {
>    * 		See: **clock_gettime**\ (**CLOCK_MONOTONIC_COARSE**)
>    * 	Return
>    * 		Current *ktime*.
> + *
> + * long bpf_ima_inode_hash(struct inode *inode, void *dst, u32 size)
> + *	Description
> + *		Returns the stored IMA hash of the *inode* (if it's avaialable).
> + *		If the hash is larger than *size*, then only *size*
> + *		bytes will be copied to *dst*
> + *	Return > + *		The **hash_algo** of is returned on success,

of => if?

> + *		**-EOPNOTSUP** if IMA is disabled and **-EINVAL** if

and => or

> + *		invalid arguments are passed.
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -3970,6 +3980,7 @@ union bpf_attr {
>   	FN(get_current_task_btf),	\
>   	FN(bprm_opts_set),		\
>   	FN(ktime_get_coarse_ns),	\
> +	FN(ima_inode_hash),		\
>   	/* */
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index b4f27a874092..51c36f61339e 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -15,6 +15,7 @@
>   #include <net/bpf_sk_storage.h>
>   #include <linux/bpf_local_storage.h>
>   #include <linux/btf_ids.h>
> +#include <linux/ima.h>
>   
>   /* For every LSM hook that allows attachment of BPF programs, declare a nop
>    * function where a BPF program can be attached.
> @@ -75,6 +76,29 @@ const static struct bpf_func_proto bpf_bprm_opts_set_proto = {
>   	.arg2_type	= ARG_ANYTHING,
>   };
>   
> +BPF_CALL_3(bpf_ima_inode_hash, struct inode *, inode, void *, dst, u32, size)
> +{
> +	return ima_inode_hash(inode, dst, size);
> +}
> +
> +static bool bpf_ima_inode_hash_allowed(const struct bpf_prog *prog)
> +{
> +	return bpf_lsm_is_sleepable_hook(prog->aux->attach_btf_id);
> +}
> +
> +BTF_ID_LIST_SINGLE(bpf_ima_inode_hash_btf_ids, struct, inode)
> +
> +const static struct bpf_func_proto bpf_ima_inode_hash_proto = {
> +	.func		= bpf_ima_inode_hash,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_BTF_ID,
> +	.arg1_btf_id	= &bpf_ima_inode_hash_btf_ids[0],
> +	.arg2_type	= ARG_PTR_TO_UNINIT_MEM,
> +	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,

I know ARG_CONST_SIZE_OR_ZERO provides some flexibility and may
make verifier easier to verify programs. But beyond that did
you see any real use case user will pass a zero size buf to
get hash value?

> +	.allowed	= bpf_ima_inode_hash_allowed,
> +};
> +
>   static const struct bpf_func_proto *
>   bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   {
> @@ -97,6 +121,8 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   		return &bpf_task_storage_delete_proto;
>   	case BPF_FUNC_bprm_opts_set:
>   		return &bpf_bprm_opts_set_proto;
> +	case BPF_FUNC_ima_inode_hash:
> +		return &bpf_ima_inode_hash_proto;
>   	default:
>   		return tracing_prog_func_proto(func_id, prog);
>   	}
> diff --git a/scripts/bpf_helpers_doc.py b/scripts/bpf_helpers_doc.py
> index add7fcb32dcd..cb16687acb66 100755
> --- a/scripts/bpf_helpers_doc.py
> +++ b/scripts/bpf_helpers_doc.py
> @@ -430,6 +430,7 @@ class PrinterHelpers(Printer):
>               'struct tcp_request_sock',
>               'struct udp6_sock',
>               'struct task_struct',
> +            'struct inode',
>   
>               'struct __sk_buff',
>               'struct sk_msg_md',
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 3ca6146f001a..dd5b8622bb89 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -3807,6 +3807,16 @@ union bpf_attr {
>    * 		See: **clock_gettime**\ (**CLOCK_MONOTONIC_COARSE**)
>    * 	Return
>    * 		Current *ktime*.
> + *
> + * long bpf_ima_inode_hash(struct inode *inode, void *dst, u32 size)
> + *	Description
> + *		Returns the stored IMA hash of the *inode* (if it's avaialable).
> + *		If the hash is larger than *size*, then only *size*
> + *		bytes will be copied to *dst*
> + *	Return
> + *		The **hash_algo** of is returned on success,

of => if?

> + *		**-EOPNOTSUP** if IMA is disabled and **-EINVAL** if

and => or.

> + *		invalid arguments are passed.
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -3970,6 +3980,7 @@ union bpf_attr {
>   	FN(get_current_task_btf),	\
>   	FN(bprm_opts_set),		\
>   	FN(ktime_get_coarse_ns),	\
> +	FN(ima_inode_hash),		\
>   	/* */
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>