Re: [RFC v1 14/14] krsi: Pin arg pages only when needed

[Yonghong Song <yhs@fb.com>]

On 9/10/19 12:55 PM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
> 
> Adds a callback which is called when a new program is attached
> to a hook. The callback registered by the process_exection hook
> checks if a program that has calls to a helper that requires pages to
> be pinned (eg. krsi_get_env_var).
> 
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>   include/linux/krsi.h              |  1 +
>   security/krsi/include/hooks.h     |  5 ++-
>   security/krsi/include/krsi_init.h |  7 ++++
>   security/krsi/krsi.c              | 62 ++++++++++++++++++++++++++++---
>   security/krsi/ops.c               | 10 ++++-
>   5 files changed, 77 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/krsi.h b/include/linux/krsi.h
> index c7d1790d0c1f..e443d0309764 100644
> --- a/include/linux/krsi.h
> +++ b/include/linux/krsi.h
> @@ -7,6 +7,7 @@
>   
>   #ifdef CONFIG_SECURITY_KRSI
>   int krsi_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog);
> +extern const struct bpf_func_proto krsi_get_env_var_proto;
>   #else
>   static inline int krsi_prog_attach(const union bpf_attr *attr,
>   				   struct bpf_prog *prog)
> diff --git a/security/krsi/include/hooks.h b/security/krsi/include/hooks.h
> index e070c452b5de..38293125ff99 100644
> --- a/security/krsi/include/hooks.h
> +++ b/security/krsi/include/hooks.h
> @@ -8,7 +8,7 @@
>    *
>    * Format:
>    *
> - *   KRSI_HOOK_INIT(TYPE, NAME, LSM_HOOK, KRSI_HOOK_FN)
> + *   KRSI_HOOK_INIT(TYPE, NAME, LSM_HOOK, KRSI_HOOK_FN, CALLBACK)
>    *
>    * KRSI adds one layer of indirection between the name of the hook and the name
>    * it exposes to the userspace in Security FS to prevent the userspace from
> @@ -18,4 +18,5 @@
>   KRSI_HOOK_INIT(PROCESS_EXECUTION,
>   	       process_execution,
>   	       bprm_check_security,
> -	       krsi_process_execution)
> +	       krsi_process_execution,
> +	       krsi_process_execution_cb)
> diff --git a/security/krsi/include/krsi_init.h b/security/krsi/include/krsi_init.h
> index 6152847c3b08..99801d5b273a 100644
> --- a/security/krsi/include/krsi_init.h
> +++ b/security/krsi/include/krsi_init.h
> @@ -31,6 +31,8 @@ struct krsi_ctx {
>   	};
>   };
>   
> +typedef int (*krsi_prog_attach_t) (struct bpf_prog_array *);
> +
>   /*
>    * The LSM creates one file per hook.
>    *
> @@ -61,6 +63,11 @@ struct krsi_hook {
>   	 * The eBPF programs that are attached to this hook.
>   	 */
>   	struct bpf_prog_array __rcu	*progs;
> +	/*
> +	 * The attach callback is called before a new program is attached
> +	 * to the hook and is passed the updated bpf_prog_array as an argument.
> +	 */
> +	krsi_prog_attach_t attach_callback;
>   };
>   
>   extern struct krsi_hook krsi_hooks_list[];
> diff --git a/security/krsi/krsi.c b/security/krsi/krsi.c
> index 00a7150c1b22..a4443d7aa150 100644
> --- a/security/krsi/krsi.c
> +++ b/security/krsi/krsi.c
> @@ -5,15 +5,65 @@
>   #include <linux/bpf.h>
>   #include <linux/binfmts.h>
>   #include <linux/highmem.h>
> +#include <linux/krsi.h>
>   #include <linux/mm.h>
>   
>   #include "krsi_init.h"
>   
> +/*
> + * need_arg_pages is only updated in bprm_check_security_cb
> + * when a mutex on krsi_hook for bprm_check_security is already
> + * held. need_arg_pages avoids pinning pages when no program
> + * that needs them is attached to the hook.
> + */
> +static bool need_arg_pages;
> +
> +/*
> + * Checks if the instruction is a BPF_CALL to an eBPF helper located
> + * at the given address.
> + */
> +static inline bool bpf_is_call_to_func(struct bpf_insn *insn,
> +				       void *func_addr)
> +{
> +	u8 opcode = BPF_OP(insn->code);
> +
> +	if (opcode != BPF_CALL)
> +		return false;
> +
> +	if (insn->src_reg == BPF_PSEUDO_CALL)
> +		return false;
> +
> +	/*
> +	 * The BPF verifier updates the value of insn->imm from the
> +	 * enum bpf_func_id to the offset of the address of helper
> +	 * from the __bpf_call_base.
> +	 */
> +	return __bpf_call_base + insn->imm == func_addr;

In how many cases, krsi program does not have krsi_get_env_var() helper?

> +}
> +
> +static int krsi_process_execution_cb(struct bpf_prog_array *array)
> +{
> +	struct bpf_prog_array_item *item = array->items;
> +	struct bpf_prog *p;
> +	const struct bpf_func_proto *proto = &krsi_get_env_var_proto;
> +	int i;
> +
> +	while ((p = READ_ONCE(item->prog))) {
> +		for (i = 0; i < p->len; i++) {
> +			if (bpf_is_call_to_func(&p->insnsi[i], proto->func))
> +				need_arg_pages = true;
> +		}
> +		item++;
> +	}
> +	return 0;
> +}
> +
>   struct krsi_hook krsi_hooks_list[] = {
> -	#define KRSI_HOOK_INIT(TYPE, NAME, H, I) \
> +	#define KRSI_HOOK_INIT(TYPE, NAME, H, I, CB) \
>   		[TYPE] = { \
>   			.h_type = TYPE, \
>   			.name = #NAME, \
> +			.attach_callback = CB, \
>   		},
>   	#include "hooks.h"
>   	#undef KRSI_HOOK_INIT
> @@ -75,9 +125,11 @@ static int krsi_process_execution(struct linux_binprm *bprm)
>   		.bprm = bprm,
>   	};
>   
> -	ret = pin_arg_pages(&ctx.bprm_ctx);
> -	if (ret < 0)
> -		goto out_arg_pages;
> +	if (READ_ONCE(need_arg_pages)) {
> +		ret = pin_arg_pages(&ctx.bprm_ctx);
> +		if (ret < 0)
> +			goto out_arg_pages;
> +	}
>   
>   	ret = krsi_run_progs(PROCESS_EXECUTION, &ctx);
>   	kfree(ctx.bprm_ctx.arg_pages);
> @@ -87,7 +139,7 @@ static int krsi_process_execution(struct linux_binprm *bprm)
>   }
>   
>   static struct security_hook_list krsi_hooks[] __lsm_ro_after_init = {
> -	#define KRSI_HOOK_INIT(T, N, HOOK, IMPL) LSM_HOOK_INIT(HOOK, IMPL),
> +	#define KRSI_HOOK_INIT(T, N, HOOK, IMPL, CB) LSM_HOOK_INIT(HOOK, IMPL),
>   	#include "hooks.h"
>   	#undef KRSI_HOOK_INIT
>   };
> diff --git a/security/krsi/ops.c b/security/krsi/ops.c
> index 1db94dfaac15..2de682371eff 100644
> --- a/security/krsi/ops.c
> +++ b/security/krsi/ops.c
> @@ -139,6 +139,14 @@ int krsi_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>   		goto unlock;
>   	}
>   
> +	if (h->attach_callback) {
> +		ret = h->attach_callback(new_array);
> +		if (ret < 0) {
> +			bpf_prog_array_free(new_array);
> +			goto unlock;
> +		}
> +	}
> +
>   	rcu_assign_pointer(h->progs, new_array);
>   	bpf_prog_array_free(old_array);
>   
> @@ -278,7 +286,7 @@ BPF_CALL_5(krsi_get_env_var, struct krsi_ctx *, ctx, char *, name, u32, n_size,
>   	return get_env_var(ctx, name, dest, n_size, size);
>   }
>   
> -static const struct bpf_func_proto krsi_get_env_var_proto = {
> +const struct bpf_func_proto krsi_get_env_var_proto = {
>   	.func = krsi_get_env_var,
>   	.gpl_only = true,
>   	.ret_type = RET_INTEGER,
>