bpf: Add kconfig knob for disabling unpriv bpf by default

commit 08389d8882 upstream.

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
[fllinden@amazon.com: backported to 4.19]
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/sysctl/kernel.txt

@@ -94,6 +94,7 @@ show up in /proc/sys/kernel:
 - sysctl_writes_strict
 - tainted
 - threads-max
+- unprivileged_bpf_disabled
 - unknown_nmi_panic
 - watchdog
 - watchdog_thresh
@@ -1041,6 +1042,26 @@ available RAM pages threads-max is reduced accordingly.
 
 ==============================================================
 
+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 set to 1, this can't be cleared from the running kernel
+anymore.
+
+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
+
+==============================================================
+
 unknown_nmi_panic:
 
 The value in this file affects behavior of handling NMI. When the

init/Kconfig

@@ -1474,6 +1474,16 @@ config BPF_JIT_ALWAYS_ON
 	  Enables BPF JIT and removes BPF interpreter to avoid
 	  speculative execution of BPF instructions by the interpreter
 
+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).
+
 config USERFAULTFD
 	bool "Enable userfaultfd() system call"
 	select ANON_INODES

kernel/bpf/syscall.c

@@ -48,7 +48,8 @@ static DEFINE_SPINLOCK(prog_idr_lock);
 static DEFINE_IDR(map_idr);
 static DEFINE_SPINLOCK(map_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, _ops)

kernel/sysctl.c

@@ -250,6 +250,28 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
 
 #endif
 
+#ifdef CONFIG_BPF_SYSCALL
+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
+
 static struct ctl_table kern_table[];
 static struct ctl_table vm_table[];
 static struct ctl_table fs_table[];
@@ -1220,10 +1242,9 @@ 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		= &one,
-		.extra2		= &one,
+		.proc_handler	= bpf_unpriv_handler,
+		.extra1		= &zero,
+		.extra2		= &two,
 	},
 #endif
 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)