Subject: Re: [RFC PATCH net-next] bpf: fix potential percpu map overcopy to user.

From: Daniel Borkmann <[email-masked]>
Date: Thu, 20 Oct 2016 18:04:38 +0200

On 10/19/2016 05:15 PM, Daniel Borkmann wrote:
> On 10/19/2016 07:31 AM, William Tu wrote:
>>> ...
>>>> -     if (copy_to_user(uvalue, value, value_size) != 0)
>>>> +     if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
>>>>                goto free_value;
>>>
>>> I think such approach won't actually fix anything. User space
>>> may lose some of the values and won't have any idea what was lost.
>>> I think we need to fix sample code to avoid using sysconf(_SC_NPROCESSORS_CONF)
>>> and use /sys/devices/system/cpu/possible instead.
>>> I would argue that glibc should be fixed as well since relying on
>>> ls -d /sys/devices/system/cpu/cpu[0-9]*|wc -l turned out to be incorrect.
>>
>> Thanks for the feedback. I think glibc is correct. The
>> _SC_NPROCESSORS_CONF presents the number of processors
>> configured/populated and is indeed "ls
>> /sys/devices/system/cpu/cpu[0-9]*|wc -l". This means the actual number
>> of CPUs installed on your system. On the other hand, the
>
> In glibc __get_nprocs_conf() seems to try a number of things, first it
> tries equivalent of /sys/devices/system/cpu/cpu[0-9]*|wc -l, if that fails,
> depending on the config, it either tries to count cpus in /proc/cpuinfo,
> or returns the _SC_NPROCESSORS_ONLN value instead. If /proc/cpuinfo has
> some issue, it returns just 1 worst case. _SC_NPROCESSORS_ONLN will parse
> /sys/devices/system/cpu/online, if that fails it looks into /proc/stat
> for cpuX entries, and if also that fails for some reason, /proc/cpuinfo
> is consulted (and returning 1 if unlikely all breaks down).
>
>> num_possible_cpus() includes both the installed CPUs and the empty CPU
>> socket/slot, in order to support CPU hotplug.
>
> Correct.
>
>> As a example, one of my dual socket motherboard with 1 CPU installed has
>> # /sys/devices/system/cpu/possible
>> 0-239
>> # /sys/devices/system/cpu/cpu[0-9]*|wc -l
>> 12
>> Note that these 12 cpus could be online/offline by
>> # echo 1/0 > /sys/devices/system/cpu/cpuX/online
>> Even if it is offline, the entry is still there.
>>
>> Thinking about another solution, maybe we should use
>> "num_present_cpus()" which means the configured/populated CPUs and the
>> value is the same as sysconf(_SC_NPROCESSORS_CONF). Consider:
>> 1) cpuX is online/offline: the num_present_cpus() remains the same.
>> 2) new cpu is hotplug into the empty socket: the num_present_cpus()
>> gets updates, and also the sysconf(_SC_NPROCESSORS_CONF).
>>
>> +++ b/kernel/bpf/syscall.c
>> @@ -297,7 +297,7 @@ static int map_lookup_elem(union bpf_attr *attr)
>>
>>          if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
>>              map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
>> -               value_size = round_up(map->value_size, 8) * num_possible_cpus();
>> +               value_size = round_up(map->value_size, 8) * num_present_cpus();
>>          else
>>                  value_size = map->value_size;
>
> But as you say in 2) that also has a chance of being racy on CPU hotplug
> compared to num_possible_cpus() which is fixed at boot time.
>
> Documentation/cputopology.txt +106 says /sys/devices/system/cpu/possible
> outputs cpu_possible_mask. That is the same as in num_possible_cpus(), so
> first step would be to fix the buggy example code, imho.
>
> What perhaps could be done in a second step to reduce overhead is an option
> for bpf(2) to pass in a cpu mask similarly as for sched_{get,set}affinity()
> syscalls, where user space can construct a mask via CPU_SET(3). For the
> syscall time, kernel would lock hot plugging via get_online_cpus() and
> put_online_cpus(), it would check whether passed CPUs are online to query
> and if so then it would copy the values into the user provided buffer. I'd
> think this might be useful in a number of ways anyway.

Maybe something like this as mentioned first step to fix the examples. Does
this work for you, William?

  tools/testing/selftests/bpf/test_maps.c | 33 ++++++++++++++++++++++++++++++---
  1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index ee384f0..d4832e8 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c
@@ -25,6 +25,33 @@

  static int map_flags;

+static unsigned int num_possible_cpus(void)
+{
+	static const char *fcpu = "/sys/devices/system/cpu/possible";
+	unsigned int val, possible_cpus = 0;
+	char buff[128];
+	FILE *fp;
+
+	fp = fopen(fcpu, "r");
+	if (!fp) {
+		printf("Failed to open %s: '%s'!\n", fcpu, strerror(errno));
+		exit(1);
+	}
+
+	while (fgets(buff, sizeof(buff), fp)) {
+		if (sscanf(buff, "%*u-%u", &val) == 1)
+			possible_cpus = val;
+	}
+
+	fclose(fp);
+	if (!possible_cpus) {
+		printf("Failed to retrieve # possible CPUs!\n");
+		exit(1);
+	}
+
+	return possible_cpus;
+}
+
  static void test_hashmap(int task, void *data)
  {
  	long long key, next_key, value;
@@ -110,7 +137,7 @@ static void test_hashmap(int task, void *data)

  static void test_hashmap_percpu(int task, void *data)
  {
-	unsigned int nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
+	unsigned int nr_cpus = num_possible_cpus();
  	long long value[nr_cpus];
  	long long key, next_key;
  	int expected_key_mask = 0;
@@ -258,7 +285,7 @@ static void test_arraymap(int task, void *data)

  static void test_arraymap_percpu(int task, void *data)
  {
-	unsigned int nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
+	unsigned int nr_cpus = num_possible_cpus();
  	int key, next_key, fd, i;
  	long values[nr_cpus];

@@ -313,7 +340,7 @@ static void test_arraymap_percpu(int task, void *data)

  static void test_arraymap_percpu_many_keys(void)
  {
-	unsigned int nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
+	unsigned int nr_cpus = num_possible_cpus();
  	unsigned int nr_keys = 20000;
  	long values[nr_cpus];
  	int key, fd, i;
-- 
1.9.3