On 15-Sep 00:33, Yonghong Song wrote:

On 9/10/19 12:55 PM, KP Singh wrote:

quoted

From: KP Singh <redacted>

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 <redacted>
---
  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

quoted

+}
+
+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,