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From: Greg Kroah-Hartman <>
Cc: Greg Kroah-Hartman <>,, Daniel Borkmann <>,
	Andrii Nakryiko <>
Subject: [PATCH 5.13 3/8] bpf: Add lockdown check for probe_write_user helper
Date: Fri, 13 Aug 2021 17:07:40 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Daniel Borkmann <>

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 <>
Acked-by: Andrii Nakryiko <>
Signed-off-by: Greg Kroah-Hartman <>
 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 {
--- 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",

  parent reply	other threads:[~2021-08-13 15:21 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-13 15:07 [PATCH 5.13 0/8] 5.13.11-rc1 review Greg Kroah-Hartman
2021-08-13 15:07 ` [PATCH 5.13 1/8] firmware: tee_bnxt: Release TEE shm, session, and context during kexec Greg Kroah-Hartman
2021-08-13 15:07 ` [PATCH 5.13 2/8] bpf: Add _kernel suffix to internal lockdown_bpf_read Greg Kroah-Hartman
2021-08-13 15:07 ` Greg Kroah-Hartman [this message]
2021-08-13 15:07 ` [PATCH 5.13 4/8] ALSA: pcm: Fix mmap breakage without explicit buffer setup Greg Kroah-Hartman
2021-08-13 15:07 ` [PATCH 5.13 5/8] ALSA: hda/realtek: fix mute/micmute LEDs for HP ProBook 650 G8 Notebook PC Greg Kroah-Hartman
2021-08-13 15:07 ` [PATCH 5.13 6/8] ALSA: hda: Add quirk for ASUS Flow x13 Greg Kroah-Hartman
2021-08-13 15:07 ` [PATCH 5.13 7/8] ppp: Fix generating ppp unit id when ifname is not specified Greg Kroah-Hartman
2021-08-13 15:07 ` [PATCH 5.13 8/8] ovl: prevent private clone if bind mount is not allowed Greg Kroah-Hartman
2021-08-13 23:23 ` [PATCH 5.13 0/8] 5.13.11-rc1 review Shuah Khan
2021-08-14  4:12 ` Fox Chen
2021-08-14  5:56 ` Naresh Kamboju
2021-08-14 18:16 ` Guenter Roeck

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