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authorDaniel Borkmann <daniel@iogearbox.net>2021-08-09 12:43:17 +0200
committerDaniel Borkmann <daniel@iogearbox.net>2021-08-10 10:10:10 +0200
commit51e1bb9eeaf7868db56e58f47848e364ab4c4129 (patch)
treefcc65c981be80c0ff7393bcc1dea857909416450
parent71330842ff93ae67a066c1fa68d75672527312fa (diff)
downloadlinux-51e1bb9eeaf7868db56e58f47848e364ab4c4129.tar.bz2
bpf: Add lockdown check for probe_write_user helper
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>
-rw-r--r--include/linux/security.h1
-rw-r--r--kernel/trace/bpf_trace.c5
-rw-r--r--security/security.c1
3 files changed, 5 insertions, 2 deletions
diff --git a/include/linux/security.h b/include/linux/security.h
index 724d7a4a0c91..5b7288521300 100644
--- 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,
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 1836591197a5..fdd14072fc3b 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -990,12 +990,13 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
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:
diff --git a/security/security.c b/security/security.c
index 6b83ab4e9d66..9ffa9e9c5c55 100644
--- a/security/security.c
+++ b/security/security.c
@@ -58,6 +58,7 @@ const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
[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",