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

[KP Singh <kpsingh@chromium.org>]

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,
> >