bpf: Add kconfig knob for disabling unpriv bpf by default

Add a kconfig knob which allows for unprivileged bpf to be disabled by default.
If set, the knob sets /proc/sys/kernel/unprivileged_bpf_disabled to value of 2.

This still allows a transition of 2 -> {0,1} through an admin. Similarly,
this also still keeps 1 -> {1} behavior intact, so that once set to permanently
disabled, it cannot be undone aside from a reboot.

We've also added extra2 with max of 2 for the procfs handler, so that an admin
still has a chance to toggle between 0 <-> 2.

Either way, as an additional alternative, applications can make use of CAP_BPF
that we added a while ago.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/74ec548079189e4e4dffaeb42b8987bb3c852eee.1620765074.git.daniel@iogearbox.net

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 1d56a6b..24ab20d 100644 (file)
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1457,11 +1457,22 @@ unprivileged_bpf_disabled
 =========================
 
 Writing 1 to this entry will disable unprivileged calls to ``bpf()``;
-once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` will return
-``-EPERM``.
+once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` or ``CAP_BPF``
+will return ``-EPERM``. Once set to 1, this can't be cleared from the
+running kernel anymore.
 
-Once set, this can't be cleared.
+Writing 2 to this entry will also disable unprivileged calls to ``bpf()``,
+however, an admin can still change this setting later on, if needed, by
+writing 0 or 1 to this entry.
 
+If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then this
+entry will default to 2 instead of 0.
+
+= =============================================================
+0 Unprivileged calls to ``bpf()`` are enabled
+1 Unprivileged calls to ``bpf()`` are disabled without recovery
+2 Unprivileged calls to ``bpf()`` are disabled
+= =============================================================
 
 watchdog
 ========

diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
index b4edaef..26b591e 100644 (file)
--- a/kernel/bpf/Kconfig
+++ b/kernel/bpf/Kconfig
@@ -61,6 +61,16 @@ config BPF_JIT_DEFAULT_ON
        def_bool ARCH_WANT_DEFAULT_BPF_JIT || BPF_JIT_ALWAYS_ON
        depends on HAVE_EBPF_JIT && BPF_JIT
 
+config BPF_UNPRIV_DEFAULT_OFF
+       bool "Disable unprivileged BPF by default"
+       depends on BPF_SYSCALL
+       help
+         Disables unprivileged BPF by default by setting the corresponding
+         /proc/sys/kernel/unprivileged_bpf_disabled knob to 2. An admin can
+         still reenable it by setting it to 0 later on, or permanently
+         disable it by setting it to 1 (from which no other transition to
+         0 is possible anymore).
+
 source "kernel/bpf/preload/Kconfig"
 
 config BPF_LSM

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 941ca06..ea04b0d 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -50,7 +50,8 @@ static DEFINE_SPINLOCK(map_idr_lock);
 static DEFINE_IDR(link_idr);
 static DEFINE_SPINLOCK(link_idr_lock);
 
-int sysctl_unprivileged_bpf_disabled __read_mostly;
+int sysctl_unprivileged_bpf_disabled __read_mostly =
+       IS_BUILTIN(CONFIG_BPF_UNPRIV_DEFAULT_OFF) ? 2 : 0;
 
 static const struct bpf_map_ops * const bpf_map_types[] = {
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index f91d327..6df7c81 100644 (file)
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -225,7 +225,27 @@ static int bpf_stats_handler(struct ctl_table *table, int write,
        mutex_unlock(&bpf_stats_enabled_mutex);
        return ret;
 }
-#endif
+
+static int bpf_unpriv_handler(struct ctl_table *table, int write,
+                             void *buffer, size_t *lenp, loff_t *ppos)
+{
+       int ret, unpriv_enable = *(int *)table->data;
+       bool locked_state = unpriv_enable == 1;
+       struct ctl_table tmp = *table;
+
+       if (write && !capable(CAP_SYS_ADMIN))
+               return -EPERM;
+
+       tmp.data = &unpriv_enable;
+       ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+       if (write && !ret) {
+               if (locked_state && unpriv_enable != 1)
+                       return -EPERM;
+               *(int *)table->data = unpriv_enable;
+       }
+       return ret;
+}
+#endif /* CONFIG_BPF_SYSCALL && CONFIG_SYSCTL */
 
 /*
  * /proc/sys support
@@ -2600,10 +2620,9 @@ static struct ctl_table kern_table[] = {
                .data           = &sysctl_unprivileged_bpf_disabled,
                .maxlen         = sizeof(sysctl_unprivileged_bpf_disabled),
                .mode           = 0644,
-               /* only handle a transition from default "0" to "1" */
-               .proc_handler   = proc_dointvec_minmax,
-               .extra1         = SYSCTL_ONE,
-               .extra2         = SYSCTL_ONE,
+               .proc_handler   = bpf_unpriv_handler,
+               .extra1         = SYSCTL_ZERO,
+               .extra2         = &two,
        },
        {
                .procname       = "bpf_stats_enabled",