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Subject[PATCH bpf-next v1 07/13] bpf: lsm: Implement attach, detach and execution.
Date
From: KP Singh <kpsingh@google.com>

A user space program can attach an eBPF program by:

hook_fd = open("/sys/kernel/security/bpf/bprm_check_security",
O_RDWR|O_CLOEXEC)
prog_fd = bpf(BPF_PROG_LOAD, ...)
bpf(BPF_PROG_ATTACH, hook_fd, prog_fd)

The following permissions are required to attach a program to a hook:

- CAP_SYS_ADMIN to load eBPF programs
- CAP_MAC_ADMIN (to update the policy of an LSM)
- The securityfs file being a valid hook and writable (O_RDWR)

When such an attach call is received, the attachment logic looks up the
dentry and appends the program to the bpf_prog_array.

The BPF programs are stored in a bpf_prog_array and writes to the array
are guarded by a mutex. The eBPF programs are executed as a part of the
LSM hook they are attached to. If any of the eBPF programs return
an error (-ENOPERM) the action represented by the hook is denied.

Signed-off-by: KP Singh <kpsingh@google.com>
---
MAINTAINERS | 1 +
include/linux/bpf_lsm.h | 13 ++++
kernel/bpf/syscall.c | 5 +-
security/bpf/lsm_fs.c | 19 +++++-
security/bpf/ops.c | 134 ++++++++++++++++++++++++++++++++++++++++
5 files changed, 169 insertions(+), 3 deletions(-)

diff --git a/MAINTAINERS b/MAINTAINERS
index 3b82d8ff21fb..681ae39bb2f0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3181,6 +3181,7 @@ L: linux-security-module@vger.kernel.org
L: bpf@vger.kernel.org
S: Maintained
F: security/bpf/
+F: include/linux/bpf_lsm.h

BROADCOM B44 10/100 ETHERNET DRIVER
M: Michael Chan <michael.chan@broadcom.com>
diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
index 76f81e642dc2..c029f2b8d6fd 100644
--- a/include/linux/bpf_lsm.h
+++ b/include/linux/bpf_lsm.h
@@ -7,6 +7,19 @@

#ifdef CONFIG_SECURITY_BPF
extern int bpf_lsm_fs_initialized;
+int bpf_lsm_attach(const union bpf_attr *attr, struct bpf_prog *prog);
+int bpf_lsm_detach(const union bpf_attr *attr);
+#else
+static inline int bpf_lsm_attach(const union bpf_attr *attr,
+ struct bpf_prog *prog)
+{
+ return -EINVAL;
+}
+
+static inline int bpf_lsm_detach(const union bpf_attr *attr)
+{
+ return -EINVAL;
+}
#endif /* CONFIG_SECURITY_BPF */

#endif /* _LINUX_BPF_LSM_H */
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4fcaf6042c07..8897b774973f 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <linux/bpf_trace.h>
#include <linux/bpf_lirc.h>
+#include <linux/bpf_lsm.h>
#include <linux/btf.h>
#include <linux/syscalls.h>
#include <linux/slab.h>
@@ -2132,7 +2133,7 @@ static int bpf_prog_attach(const union bpf_attr *attr)
ret = lirc_prog_attach(attr, prog);
break;
case BPF_PROG_TYPE_LSM:
- ret = -EINVAL;
+ ret = bpf_lsm_attach(attr, prog);
break;
case BPF_PROG_TYPE_FLOW_DISSECTOR:
ret = skb_flow_dissector_bpf_prog_attach(attr, prog);
@@ -2200,6 +2201,8 @@ static int bpf_prog_detach(const union bpf_attr *attr)
case BPF_CGROUP_SETSOCKOPT:
ptype = BPF_PROG_TYPE_CGROUP_SOCKOPT;
break;
+ case BPF_LSM_MAC:
+ return bpf_lsm_detach(attr);
default:
return -EINVAL;
}
diff --git a/security/bpf/lsm_fs.c b/security/bpf/lsm_fs.c
index 49165394ef7d..b271e9582d0f 100644
--- a/security/bpf/lsm_fs.c
+++ b/security/bpf/lsm_fs.c
@@ -9,6 +9,8 @@
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/types.h>
+#include <linux/filter.h>
+#include <linux/bpf.h>
#include <linux/security.h>
#include <linux/bpf_lsm.h>

@@ -28,6 +30,19 @@ bool is_bpf_lsm_hook_file(struct file *f)

static void __init free_hook(struct bpf_lsm_hook *h)
{
+ struct bpf_prog_array_item *item;
+ /*
+ * This function is __init so we are guaranteed that there will be
+ * no concurrent access.
+ */
+ struct bpf_prog_array *progs = rcu_dereference_raw(h->progs);
+
+ if (progs) {
+ for (item = progs->items; item->prog; item++)
+ bpf_prog_put(item->prog);
+ bpf_prog_array_free(progs);
+ }
+
securityfs_remove(h->h_dentry);
h->h_dentry = NULL;
}
@@ -36,8 +51,8 @@ static int __init init_hook(struct bpf_lsm_hook *h, struct dentry *parent)
{
struct dentry *h_dentry;

- h_dentry = securityfs_create_file(h->name, 0600, parent,
- NULL, &hook_ops);
+ h_dentry = securityfs_create_file(h->name, 0600,
+ parent, NULL, &hook_ops);
if (IS_ERR(h_dentry))
return PTR_ERR(h_dentry);

diff --git a/security/bpf/ops.c b/security/bpf/ops.c
index 2fa3ebdf598d..eb8a8db28109 100644
--- a/security/bpf/ops.c
+++ b/security/bpf/ops.c
@@ -4,11 +4,145 @@
* Copyright 2019 Google LLC.
*/

+#include <linux/err.h>
+#include <linux/types.h>
#include <linux/filter.h>
#include <linux/bpf.h>
+#include <linux/security.h>
+#include <linux/bpf_lsm.h>
+
+#include "bpf_lsm.h"
+#include "fs.h"
+
+static struct bpf_lsm_hook *get_hook_from_fd(int fd)
+{
+ struct bpf_lsm_hook *h;
+ struct fd f;
+ int ret;
+
+ /*
+ * Only CAP_MAC_ADMIN users are allowed to make changes to LSM hooks
+ */
+ if (!capable(CAP_MAC_ADMIN))
+ return ERR_PTR(-EPERM);
+
+
+ f = fdget(fd);
+ if (!f.file)
+ return ERR_PTR(-EBADF);
+
+
+ if (!is_bpf_lsm_hook_file(f.file)) {
+ ret = -EINVAL;
+ goto error;
+ }
+
+ /*
+ * It's wrong to attach the program to the hook if the file is not
+ * opened for a writing. Note that, this is an EBADF and not an EPERM
+ * because the file has been opened with an incorrect mode.
+ */
+ if (!(f.file->f_mode & FMODE_WRITE)) {
+ ret = -EBADF;
+ goto error;
+ }
+
+ /*
+ * The securityfs dentry never disappears, so we don't need to take a
+ * reference to it.
+ */
+ h = file_dentry(f.file)->d_fsdata;
+ if (WARN_ON(!h)) {
+ ret = -EINVAL;
+ goto error;
+ }
+ fdput(f);
+ return h;
+
+error:
+ fdput(f);
+ return ERR_PTR(ret);
+}
+
+int bpf_lsm_attach(const union bpf_attr *attr, struct bpf_prog *prog)
+{
+ struct bpf_prog_array *old_array;
+ struct bpf_prog_array *new_array;
+ struct bpf_lsm_hook *h;
+ int ret = 0;
+
+ h = get_hook_from_fd(attr->target_fd);
+ if (IS_ERR(h))
+ return PTR_ERR(h);
+
+ mutex_lock(&h->mutex);
+ old_array = rcu_dereference_protected(h->progs,
+ lockdep_is_held(&h->mutex));
+
+ ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
+ if (ret < 0)
+ goto unlock;
+
+ rcu_assign_pointer(h->progs, new_array);
+ bpf_prog_array_free(old_array);
+
+unlock:
+ mutex_unlock(&h->mutex);
+ return ret;
+}
+
+int bpf_lsm_detach(const union bpf_attr *attr)
+{
+ struct bpf_prog_array *old_array, *new_array;
+ struct bpf_prog *prog;
+ struct bpf_lsm_hook *h;
+ int ret = 0;
+
+ if (attr->attach_flags)
+ return -EINVAL;
+
+ h = get_hook_from_fd(attr->target_fd);
+ if (IS_ERR(h))
+ return PTR_ERR(h);
+
+ prog = bpf_prog_get_type(attr->attach_bpf_fd,
+ BPF_PROG_TYPE_LSM);
+ if (IS_ERR(prog))
+ return PTR_ERR(prog);
+
+ mutex_lock(&h->mutex);
+ old_array = rcu_dereference_protected(h->progs,
+ lockdep_is_held(&h->mutex));
+
+ ret = bpf_prog_array_copy(old_array, prog, NULL, &new_array);
+ if (ret)
+ goto unlock;
+
+ rcu_assign_pointer(h->progs, new_array);
+ bpf_prog_array_free(old_array);
+unlock:
+ bpf_prog_put(prog);
+ mutex_unlock(&h->mutex);
+ return ret;
+}

const struct bpf_prog_ops lsm_prog_ops = {
};

+static const struct bpf_func_proto *get_bpf_func_proto(enum bpf_func_id
+ func_id, const struct bpf_prog *prog)
+{
+ switch (func_id) {
+ case BPF_FUNC_map_lookup_elem:
+ return &bpf_map_lookup_elem_proto;
+ case BPF_FUNC_get_current_pid_tgid:
+ return &bpf_get_current_pid_tgid_proto;
+ default:
+ return NULL;
+ }
+}
+
const struct bpf_verifier_ops lsm_verifier_ops = {
+ .get_func_proto = get_bpf_func_proto,
+ .is_valid_access = btf_ctx_access,
};
--
2.20.1
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