From: Daniel Borkmann daniel@iogearbox.net

commit 08389d888287c3823f80b0216766b71e17f0aba5 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.1620765... [fllinden@amazon.com: backported to 4.14] Signed-off-by: Frank van der Linden fllinden@amazon.com --- Documentation/sysctl/kernel.txt | 21 +++++++++++++++++++++ init/Kconfig | 10 ++++++++++ kernel/bpf/syscall.c | 3 ++- kernel/sysctl.c | 29 +++++++++++++++++++++++++---- 4 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index 694968c7523c..3c8f5bfdf6da 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -91,6 +91,7 @@ show up in /proc/sys/kernel: - sysctl_writes_strict - tainted - threads-max +- unprivileged_bpf_disabled - unknown_nmi_panic - watchdog - watchdog_thresh @@ -999,6 +1000,26 @@ 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 diff --git a/init/Kconfig b/init/Kconfig index be58f0449c68..c87858c434cc 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1378,6 +1378,16 @@ config ADVISE_SYSCALLS applications use these syscalls, you can disable this option to save space.

+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 diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c index 21073682061d..59d44f1ad958 100644 --- a/kernel/bpf/syscall.c +++ b/kernel/bpf/syscall.c @@ -37,7 +37,8 @@ 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) diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 74fc3a9d1923..c9a3e61c88f8 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -242,6 +242,28 @@ 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[]; @@ -1201,10 +1223,9 @@ 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)