bpf: Add bpf_probe_write_user BPF helper to be called in tracers
This allows user memory to be written to during the course of a kprobe.
It shouldn't be used to implement any kind of security mechanism
because of TOC-TOU attacks, but rather to debug, divert, and
manipulate execution of semi-cooperative processes.
Although it uses probe_kernel_write, we limit the address space
the probe can write into by checking the space with access_ok.
We do this as opposed to calling copy_to_user directly, in order
to avoid sleeping. In addition we ensure the threads's current fs
/ segment is USER_DS and the thread isn't exiting nor a kernel thread.
Given this feature is meant for experiments, and it has a risk of
crashing the system, and running programs, we print a warning on
when a proglet that attempts to use this helper is installed,
along with the pid and process name.
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index a12bbd32..b20438f 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -81,6 +81,49 @@
.arg3_type = ARG_ANYTHING,
};
+static u64 bpf_probe_write_user(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+ void *unsafe_ptr = (void *) (long) r1;
+ void *src = (void *) (long) r2;
+ int size = (int) r3;
+
+ /*
+ * Ensure we're in user context which is safe for the helper to
+ * run. This helper has no business in a kthread.
+ *
+ * access_ok() should prevent writing to non-user memory, but in
+ * some situations (nommu, temporary switch, etc) access_ok() does
+ * not provide enough validation, hence the check on KERNEL_DS.
+ */
+
+ if (unlikely(in_interrupt() ||
+ current->flags & (PF_KTHREAD | PF_EXITING)))
+ return -EPERM;
+ if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
+ return -EPERM;
+ if (!access_ok(VERIFY_WRITE, unsafe_ptr, size))
+ return -EPERM;
+
+ return probe_kernel_write(unsafe_ptr, src, size);
+}
+
+static const struct bpf_func_proto bpf_probe_write_user_proto = {
+ .func = bpf_probe_write_user,
+ .gpl_only = true,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_ANYTHING,
+ .arg2_type = ARG_PTR_TO_STACK,
+ .arg3_type = ARG_CONST_STACK_SIZE,
+};
+
+static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
+{
+ pr_warn_ratelimited("%s[%d] is installing a program with bpf_probe_write_user helper that may corrupt user memory!",
+ current->comm, task_pid_nr(current));
+
+ return &bpf_probe_write_user_proto;
+}
+
/*
* limited trace_printk()
* only %d %u %x %ld %lu %lx %lld %llu %llx %p %s conversion specifiers allowed
@@ -362,6 +405,8 @@
return &bpf_get_smp_processor_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();
default:
return NULL;
}