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    Subject[RFC v1 14/14] krsi: Pin arg pages only when needed
    Date
    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;
    +}
    +
    +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,
    --
    2.20.1
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