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SubjectRe: [RFC v1 14/14] krsi: Pin arg pages only when needed
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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,
>
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