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

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.

- 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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