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SubjectRe: [RFC v1 14/14] krsi: Pin arg pages only when needed
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On 9/15/19 2:40 AM, KP Singh wrote:
> On 15-Sep 00:33, Yonghong Song wrote:
>>
>>
>> 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?
>
> It depends, if the user does not choose to use log environment
> variables or use the the value as a part of their MAC policy, the
> pinning of the pages is not needed.

Thanks. I just want to know whether we want to optimize such cases.
I am not a security expert, so I am okay with whatever decision you
made.

>
> Also, the pinning is needed since eBPF helpers cannot run a non-atomic
> context. It would not be needed if "sleepable eBPF" becomes a thing.

Indeed. 'sleepable BPF' might be also a good thing for realtime linux.
Some works need to address bpf area spin lock, per cpu variables, etc.
before that can happen.

>
> - KP
>
>>
>>> +}
>>> +
>>> +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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