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.
This is so the call doesn't sleep. 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 experimental, and has the risk of crashing the
system, we print a warning on first invocation, and the process name
on subsequent invocations.
It was tested with the tracex7 program on x86-64.
Signed-off-by: Sargun Dhillon <sar...@sargun.me>
Cc: Alexei Starovoitov <a...@kernel.org>
Cc: Daniel Borkmann <dan...@iogearbox.net>
---
include/uapi/linux/bpf.h | 10 ++++++++++
kernel/trace/bpf_trace.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++
samples/bpf/bpf_helpers.h | 2 ++
3 files changed, 60 insertions(+)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 2b7076f..da218fe 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -365,6 +365,16 @@ enum bpf_func_id {
*/
BPF_FUNC_get_current_task,
+ /**
+ * bpf_probe_write_user(void *dst, void *src, int len)
+ * safely attempt to write to a location
+ * @dst: destination address in userspace
+ * @src: source address on stack
+ * @len: number of bytes to copy
+ * Return: 0 on success or negative error
+ */
+ BPF_FUNC_probe_write_user,
+
__BPF_FUNC_MAX_ID,
};
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index a12bbd3..b69629b 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -81,6 +81,52 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
.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 a user context which it 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.
+ *
+ * In order to avoid this we check the current segment to verify that
+ * it is USER_DS. This avoid odd architectures and user threads that
+ * temporarily switch to 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("bpf_probe_write_user: %s[%d] installing program
with helper: it 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 +408,8 @@ static const struct bpf_func_proto *tracing_func_proto(enum
bpf_func_id func_id)
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;
}
diff --git a/samples/bpf/bpf_helpers.h b/samples/bpf/bpf_helpers.h
index 84e3fd9..217c8d5 100644
--- a/samples/bpf/bpf_helpers.h
+++ b/samples/bpf/bpf_helpers.h
@@ -41,6 +41,8 @@ static int (*bpf_perf_event_output)(void *ctx, void *map, int
index, void *data,
(void *) BPF_FUNC_perf_event_output;
static int (*bpf_get_stackid)(void *ctx, void *map, int flags) =
(void *) BPF_FUNC_get_stackid;
+static int (*bpf_probe_write_user)(void *dst, void *src, int size) =
+ (void *) BPF_FUNC_probe_write_user;
/* llvm builtin functions that eBPF C program may use to
* emit BPF_LD_ABS and BPF_LD_IND instructions
--
2.7.4
