Re: [PATCH 1/1] tracing, bpf: Implement function bpf_probe_write

From: Sargun Dhillon
Date: Sun Jul 17 2016 - 06:19:43 EST



On Fri, 15 Jul 2016, Alexei Starovoitov wrote:

On Fri, Jul 15, 2016 at 07:16:01PM -0700, Sargun Dhillon wrote:


On Thu, 14 Jul 2016, Alexei Starovoitov wrote:

On Wed, Jul 13, 2016 at 01:31:57PM -0700, Sargun Dhillon wrote:


On Wed, 13 Jul 2016, Alexei Starovoitov wrote:

On Wed, Jul 13, 2016 at 03:36:11AM -0700, Sargun Dhillon wrote:
Provides BPF programs, attached to kprobes a safe way to write to
memory referenced by probes. This is done by making probe_kernel_write
accessible to bpf functions via the bpf_probe_write helper.

not quite :)

Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx>
---
include/uapi/linux/bpf.h | 3 +++
kernel/trace/bpf_trace.c | 20 ++++++++++++++++++++
samples/bpf/bpf_helpers.h | 2 ++
3 files changed, 25 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 406459b..355b565 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -313,6 +313,9 @@ enum bpf_func_id {
*/
BPF_FUNC_skb_get_tunnel_opt,
BPF_FUNC_skb_set_tunnel_opt,
+
+ BPF_FUNC_probe_write, /* int bpf_probe_write(void *dst, void *src,
int size) */
+

the patch is against some old kernel.
Please always make the patch against net-next tree and cc netdev list.

Sorry, I did this against Linus's tree, not net-next. Will fix.

+static u64 bpf_probe_write(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+ void *dst = (void *) (long) r1;
+ void *unsafe_ptr = (void *) (long) r2;
+ int size = (int) r3;
+
+ return probe_kernel_write(dst, unsafe_ptr, size);
+}

the patch is whitepsace mangled. Please see Documentation/networking/netdev-FAQ.txt
Also will fix.


the main issue though that we cannot simply allow bpf to do probe_write,
since it may crash the kernel.
What might be ok is to allow writing into memory of current
user space process only. This way bpf prog will keep kernel safety guarantees,
yet it will be able to modify user process memory when necessary.
Since bpf+tracing is root only, it doesn't pose security risk.



Doesn't probe_write prevent you from writing to protected memory and
generate an EFAULT? Or are you worried about the situation where a bpf
program writes to some other chunk of kernel memory, or writes bad data
to said kernel memory?

I guess when I meant "safe" -- it's safer than allowing arbitrary memcpy.
I don't see a good way to ensure safety otherwise as we don't know
which registers point to memory that it's reasonable for probes to
manipulate. It's not like skb_store_bytes where we can check the pointer
going in is the same pointer that's referenced, and with a super
restricted datatype.

exactly. probe_write can write anywhere in the kernel and that
will cause crashes. If we allow that bpf becomes no different than
kernel module.

Perhaps, it would be a good idea to describe an example where I used this:
#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <bcc/proto.h>


int trace_inet_stream_connect(struct pt_regs *ctx)
{
if (!PT_REGS_PARM2(ctx)) {
return 0;
}
struct sockaddr uaddr = {};
struct sockaddr_in *addr_in;
bpf_probe_read(&uaddr, sizeof(struct sockaddr), (void *)PT_REGS_PARM2(ctx));
if (uaddr.sa_family == AF_INET) {
// Simple cast causes LLVM weirdness
addr_in = &uaddr;
char fmt[] = "Connecting on port: %d\n";
bpf_trace_printk(fmt, sizeof(fmt), ntohs(addr_in->sin_port));
if (ntohs(addr_in->sin_port) == 80) {
addr_in->sin_port = htons(443);
bpf_probe_write((void *)PT_REGS_PARM2(ctx), &uaddr, sizeof(uaddr));
}
}
return 0;
};

There are two reasons I want to do this:
1) Debugging - sometimes, it makes sense to divert a program's syscalls in
order to allow for better debugging
2) Network Functions - I wrote a load balancer which intercepts
inet_stream_connect & tcp_set_state. We can manipulate the destination
address as neccessary at connect time. This also has the nice side effect
that getpeername() returns the real IP that a server is connected to, and
the performance is far better than doing "network load balancing"

(I realize this is a total hack, better approaches would be appreciated)

nice. interesting idea.
Have you considered ld_preload hack to do port rewrite?

We've thought about it. It wont really work for us, because we're doing this
to manipulate 3rd party runtimes, many of which are written in languages
that don't play nice with LD_PRELOAD. Go is the primary problem child in
this case. We also looked at using SECCOMP + ptrace, but again, not all
runtimes play nice with ptrace.

interesting!
I was about to suggest to hack write support into seccomp, since few
folks were interested in it as well. Why seccomp won't work?
You cannot have a centralized daemon that launches all the processes?

If we allowed manipulation of the current task's user memory by exposing
copy_to_user, that could also work if I attach the probe to sys_connect,
I could overwrite the address there before it gets copied into
kernel space, but that could lead to its own weirdness.

we cannot simply call copy_to_user from the bpf either,
but yeah, something semantically equivalent to copy_to_user should
solve your port rewriting case, right?
Could you explain little bit more on 'syscall divert' ideas?


If we had a "safe" copy_to_user which checked if BPF programs were running
in the user context, that would work right? I mean, you could still make
user programs crash, but that's better than making the kernel fall over. We
would need both copy_from_user, and copy_to_user. If you look at the example
program, it first checks what the user is connecting to -- so it'd have to
check the address the user is passing to the syscall.

yep. imo 'safe' copy_to_user is no different in user impact from seccomp.
Both can make user process inoperable if there is a bug in bpf program,
but both are always safe from kernel point of view.
there is no need in copy_from_user. bpf_probe_read can read user memory just
fine or you actually want to make even safer version of probe_read that
can only read current task memory instead of any memory?
Makes sense to me.

Syscall Divert:
The idea here is to try to "lie" to the program about the environment. A
specific example is where I want to bypass certain calls like prctl during
debugging, to allow certain instructions to execute. I don't always have
access to the source of the containerizer I'm running, and it's nice to turn
them off during debugging.

makes sense too.
so seccomp with write support is also not option here, because you actually
want to divert seccomp itself?


Yeah, the only difference with seccomp is that the impact is isolated to a group of processes versus the entire system. Though, kprobes require root, whereas seccomp is accessible to others. It would be really nice to be able to call bpf_probe_read from seccomp filters, and be able to filter on things like the endpoint that people are connecting to. As it stands right now, it would open up seccomp to a TOCTOU attack. I'm not sure


If seccomp had full eBPF, and write support that'd solve most of my use cases. As far as diverting diverting future syscall behaviour, if I could just intercept those, and rewrite seccomp filters that'd solve the problem too.

I'm thinking of submitting something along the lines of the following patch [WIP]. What do you think?
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index c4d9224..d435f7c 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -364,6 +364,17 @@ enum bpf_func_id {
*/
BPF_FUNC_get_current_task,

+ /**
+ * copy_to_user(to, from, len) - Copy a block of data into user space.
+ * @to: destination address in userspace
+ * @from: source address on stack
+ * @len: number of bytes to copy
+ * Return:
+ * Returns number of bytes that could not be copied.
+ * On success, this will be zero
+ */
+ BPF_FUNC_copy_to_user,
+
__BPF_FUNC_MAX_ID,
};

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index ebfbb7d..be89c148 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -81,6 +81,30 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
.arg3_type = ARG_ANYTHING,
};

+static u64 bpf_copy_to_user(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+ void *to = (void *) (long) r1;
+ void *from = (void *) (long) r2;
+ int size = (int) r3;
+
+ /* check if we're in a user context */
+ if (unlikely(in_interrupt()))
+ return -EINVAL;
+ if (unlikely(!current->pid))
+ return -EINVAL;
+
+ return copy_to_user(to, from, size);
+}
+
+static const struct bpf_func_proto bpf_copy_to_user_proto = {
+ .func = bpf_copy_to_user,
+ .gpl_only = false,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_ANYTHING,
+ .arg2_type = ARG_PTR_TO_RAW_STACK,
+ .arg3_type = ARG_CONST_STACK_SIZE,
+};
+
/*
* limited trace_printk()
* only %d %u %x %ld %lu %lx %lld %llu %llx %p %s conversion specifiers allowed
@@ -360,6 +384,8 @@ 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_copy_to_user:
+ return &bpf_copy_to_user_proto;
default:
return NULL;
}
diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index a98b780..be2bab9 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -14,6 +14,7 @@ hostprogs-y += tracex3
hostprogs-y += tracex4
hostprogs-y += tracex5
hostprogs-y += tracex6
+hostprogs-y += tracex7
hostprogs-y += trace_output
hostprogs-y += lathist
hostprogs-y += offwaketime
@@ -35,6 +36,7 @@ tracex3-objs := bpf_load.o libbpf.o tracex3_user.o
tracex4-objs := bpf_load.o libbpf.o tracex4_user.o
tracex5-objs := bpf_load.o libbpf.o tracex5_user.o
tracex6-objs := bpf_load.o libbpf.o tracex6_user.o
+tracex7-objs := bpf_load.o libbpf.o tracex7_user.o
trace_output-objs := bpf_load.o libbpf.o trace_output_user.o
lathist-objs := bpf_load.o libbpf.o lathist_user.o
offwaketime-objs := bpf_load.o libbpf.o offwaketime_user.o
@@ -54,6 +56,7 @@ always += tracex3_kern.o
always += tracex4_kern.o
always += tracex5_kern.o
always += tracex6_kern.o
+always += tracex7_kern.o
always += trace_output_kern.o
always += tcbpf1_kern.o
always += lathist_kern.o
@@ -78,6 +81,7 @@ HOSTLOADLIBES_tracex3 += -lelf
HOSTLOADLIBES_tracex4 += -lelf -lrt
HOSTLOADLIBES_tracex5 += -lelf
HOSTLOADLIBES_tracex6 += -lelf
+HOSTLOADLIBES_tracex7 += -lelf
HOSTLOADLIBES_trace_output += -lelf -lrt
HOSTLOADLIBES_lathist += -lelf
HOSTLOADLIBES_offwaketime += -lelf
diff --git a/samples/bpf/bpf_helpers.h b/samples/bpf/bpf_helpers.h
index 84e3fd9..a54a26c 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_copy_to_user)(void *to, void *from, int size) =
+ (void *) BPF_FUNC_copy_to_user;

/* llvm builtin functions that eBPF C program may use to
* emit BPF_LD_ABS and BPF_LD_IND instructions
diff --git a/samples/bpf/tracex7_kern.c b/samples/bpf/tracex7_kern.c
new file mode 100644
index 0000000..c341a0a
--- /dev/null
+++ b/samples/bpf/tracex7_kern.c
@@ -0,0 +1,53 @@
+/* Copyright (c) 2013-2015 PLUMgrid, http://plumgrid.com
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ */
+#include <linux/skbuff.h>
+#include <linux/netdevice.h>
+#include <uapi/linux/bpf.h>
+#include <linux/version.h>
+#include "bpf_helpers.h"
+
+
+struct bpf_map_def SEC("maps") dnat_map = {
+ .type = BPF_MAP_TYPE_HASH,
+ .key_size = sizeof(struct sockaddr_in),
+ .value_size = sizeof(struct sockaddr_in),
+ .max_entries = 256,
+};
+
+
+/* kprobe is NOT a stable ABI
+ * kernel functions can be removed, renamed or completely change semantics.
+ * Number of arguments and their positions can change, etc.
+ * In such case this bpf+kprobe example will no longer be meaningful
+ *
+ * This example sits on a syscall, and the syscall ABI is relatively stable
+ * of course, across platforms, and over time, the ABI may change.
+ */
+SEC("kprobe/sys_connect")
+int bpf_prog1(struct pt_regs *ctx)
+{
+ struct sockaddr_in new_addr, orig_addr = {};
+ struct sockaddr_in *mapped_addr;
+ void *sockaddr_arg = (void *)PT_REGS_PARM2(ctx);
+ int sockaddr_len = (int)PT_REGS_PARM3(ctx);
+
+ if (sockaddr_len > sizeof(orig_addr))
+ return 0;
+
+ if (bpf_probe_read(&orig_addr, sizeof(orig_addr), sockaddr_arg) != 0)
+ return 0;
+
+ mapped_addr = bpf_map_lookup_elem(&dnat_map, &orig_addr);
+ if (mapped_addr != NULL) {
+ memcpy(&new_addr, mapped_addr, sizeof(new_addr));
+ bpf_copy_to_user(sockaddr_arg, &new_addr, sizeof(new_addr));
+ }
+ return 0;
+}
+
+char _license[] SEC("license") = "GPL";
+u32 _version SEC("version") = LINUX_VERSION_CODE;
diff --git a/samples/bpf/tracex7_user.c b/samples/bpf/tracex7_user.c
new file mode 100644
index 0000000..167ee40
--- /dev/null
+++ b/samples/bpf/tracex7_user.c
@@ -0,0 +1,76 @@
+#include <stdio.h>
+#include <assert.h>
+#include <linux/bpf.h>
+#include <unistd.h>
+#include "libbpf.h"
+#include "bpf_load.h"
+#include <sys/socket.h>
+#include <string.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+int main(int ac, char **argv)
+{
+ int serverfd, serverconnfd, clientfd;
+ socklen_t sockaddr_len;
+ struct sockaddr serv_addr, mapped_addr, tmp_addr;
+ struct sockaddr_in *serv_addr_in, *mapped_addr_in, *tmp_addr_in;
+ char filename[256];
+ char *ip;
+
+ serv_addr_in = (struct sockaddr_in *)&serv_addr;
+ mapped_addr_in = (struct sockaddr_in *)&mapped_addr;
+ tmp_addr_in = (struct sockaddr_in *)&tmp_addr;
+
+ snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+
+ if (load_bpf_file(filename)) {
+ printf("%s", bpf_log_buf);
+ return 1;
+ }
+
+ assert((serverfd = socket(AF_INET, SOCK_STREAM, 0)) > 0);
+ assert((clientfd = socket(AF_INET, SOCK_STREAM, 0)) > 0);
+
+ /* Bind server to ephemeral port on lo */
+ memset(&serv_addr, 0, sizeof(serv_addr));
+ serv_addr_in->sin_family = AF_INET;
+ serv_addr_in->sin_port = 0;
+ serv_addr_in->sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+
+ assert(bind(serverfd, &serv_addr, sizeof(serv_addr)) == 0);
+
+ sockaddr_len = sizeof(serv_addr);
+ assert(getsockname(serverfd, &serv_addr, &sockaddr_len) == 0);
+ ip = inet_ntoa(serv_addr_in->sin_addr);
+ printf("Server bound to: %s:%d\n", ip, ntohs(serv_addr_in->sin_port));
+
+ memset(&mapped_addr, 0, sizeof(mapped_addr));
+ mapped_addr_in->sin_family = AF_INET;
+ mapped_addr_in->sin_port = htons(5555);
+ mapped_addr_in->sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+
+ assert(bpf_update_elem(map_fd[0], &mapped_addr, &serv_addr,
+ BPF_ANY) == 0);
+
+ assert(listen(serverfd, 5) == 0);
+
+ ip = inet_ntoa(mapped_addr_in->sin_addr);
+ printf("Client connecting to: %s:%d\n", ip,
+ ntohs(mapped_addr_in->sin_port));
+ assert(connect(clientfd, &mapped_addr, sizeof(mapped_addr)) == 0);
+
+ sockaddr_len = sizeof(tmp_addr);
+ ip = inet_ntoa(tmp_addr_in->sin_addr);
+ assert((serverconnfd = accept(serverfd, &tmp_addr, &sockaddr_len))
0);
+ printf("Server received connection from: %s:%d\n", ip,
+ ntohs(tmp_addr_in->sin_port));
+
+ sockaddr_len = sizeof(tmp_addr);
+ assert(getpeername(clientfd, &tmp_addr, &sockaddr_len) == 0);
+ ip = inet_ntoa(tmp_addr_in->sin_addr);
+ printf("Client's peer address: %s:%d\n", ip,
+ ntohs(tmp_addr_in->sin_port));
+
+ return 0;
+}