[PATCH 5.13 3/8] bpf: Add lockdown check for probe_write_user helper

Greg Kroah-Hartman
2021-08-13T15:21:15Z

From: Daniel Borkmann <daniel@iogearbox.net>

commit 51e1bb9eeaf7868db56e58f47848e364ab4c4129 upstream.

Back then, commit 96ae52279594 ("bpf: Add bpf_probe_write_user BPF helper
to be called in tracers") added the bpf_probe_write_user() helper in order
to allow to override user space memory. Its original goal was to have a
facility to "debug, divert, and manipulate execution of semi-cooperative
processes" under CAP_SYS_ADMIN. Write to kernel was explicitly disallowed
since it would otherwise tamper with its integrity.

One use case was shown in cf9b1199de27 ("samples/bpf: Add test/example of
using bpf_probe_write_user bpf helper") where the program DNATs traffic
at the time of connect(2) syscall, meaning, it rewrites the arguments to
a syscall while they're still in userspace, and before the syscall has a
chance to copy the argument into kernel space. These days we have better
mechanisms in BPF for achieving the same (e.g. for load-balancers), but
without having to write to userspace memory.

Of course the bpf_probe_write_user() helper can also be used to abuse
many other things for both good or bad purpose. Outside of BPF, there is
a similar mechanism for ptrace(2) such as PTRACE_PEEK{TEXT,DATA} and
PTRACE_POKE{TEXT,DATA}, but would likely require some more effort.
Commit 96ae52279594 explicitly dedicated the helper for experimentation
purpose only. Thus, move the helper's availability behind a newly added
LOCKDOWN_BPF_WRITE_USER lockdown knob so that the helper is disabled under
the "integrity" mode. More fine-grained control can be implemented also
from LSM side with this change.

Fixes: 96ae52279594 ("bpf: Add bpf_probe_write_user BPF helper to be called in tracers")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/security.h | 1 +
kernel/trace/bpf_trace.c | 5 +++--
security/security.c | 1 +
3 files changed, 5 insertions(+), 2 deletions(-)

--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -120,6 +120,7 @@ enum lockdown_reason {
LOCKDOWN_MMIOTRACE,
LOCKDOWN_DEBUGFS,
LOCKDOWN_XMON_WR,
+ LOCKDOWN_BPF_WRITE_USER,
LOCKDOWN_INTEGRITY_MAX,
LOCKDOWN_KCORE,
LOCKDOWN_KPROBES,
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -990,12 +990,13 @@ bpf_tracing_func_proto(enum bpf_func_id
return &bpf_get_numa_node_id_proto;
case BPF_FUNC_perf_event_read:
return &bpf_perf_event_read_proto;
- case BPF_FUNC_probe_write_user:
- return bpf_get_probe_write_proto();
case BPF_FUNC_current_task_under_cgroup:
return &bpf_current_task_under_cgroup_proto;
case BPF_FUNC_get_prandom_u32:
return &bpf_get_prandom_u32_proto;
+ case BPF_FUNC_probe_write_user:
+ return security_locked_down(LOCKDOWN_BPF_WRITE_USER) < 0 ?
+ NULL : bpf_get_probe_write_proto();
case BPF_FUNC_probe_read_user:
return &bpf_probe_read_user_proto;
case BPF_FUNC_probe_read_kernel:
--- a/security/security.c
+++ b/security/security.c
@@ -58,6 +58,7 @@ const char *const lockdown_reasons[LOCKD
[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
[LOCKDOWN_DEBUGFS] = "debugfs access",
[LOCKDOWN_XMON_WR] = "xmon write access",
+ [LOCKDOWN_BPF_WRITE_USER] = "use of bpf to write user RAM",
[LOCKDOWN_INTEGRITY_MAX] = "integrity",
[LOCKDOWN_KCORE] = "/proc/kcore access",
[LOCKDOWN_KPROBES] = "use of kprobes",