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Subject[PATCH 18/27] bpf: Restrict kernel image access functions when the kernel is locked down
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From: Chun-Yi Lee <jlee@suse.com>

There are some bpf functions can be used to read kernel memory:
bpf_probe_read, bpf_probe_write_user and bpf_trace_printk. These allow
private keys in kernel memory (e.g. the hibernation image signing key) to
be read by an eBPF program. Prohibit those functions when the kernel is
locked down.

Signed-off-by: Chun-Yi Lee <jlee@suse.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: netdev@vger.kernel.org
---

kernel/trace/bpf_trace.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index dc498b605d5d..35e85a3fdb37 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -65,6 +65,11 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
{
int ret;

+ if (kernel_is_locked_down("BPF")) {
+ memset(dst, 0, size);
+ return -EPERM;
+ }
+
ret = probe_kernel_read(dst, unsafe_ptr, size);
if (unlikely(ret < 0))
memset(dst, 0, size);
@@ -84,6 +89,9 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
BPF_CALL_3(bpf_probe_write_user, void *, unsafe_ptr, const void *, src,
u32, size)
{
+ if (kernel_is_locked_down("BPF"))
+ return -EPERM;
+
/*
* Ensure we're in user context which is safe for the helper to
* run. This helper has no business in a kthread.
@@ -143,6 +151,9 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
if (fmt[--fmt_size] != 0)
return -EINVAL;

+ if (kernel_is_locked_down("BPF"))
+ return __trace_printk(1, fmt, 0, 0, 0);
+
/* check format string for allowed specifiers */
for (i = 0; i < fmt_size; i++) {
if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i]))
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