Description

gnrc_sock_dns doesn't perform sufficient sanity checks on the DNS response it receives from the configured DNS server.

Here is a non-exhaustive list of issues:

1. The QDCOUNT contained in the DNS response is not verified. This causes an out-of-bounds buffer access in the _skip_hostname function if QDCOUNT is set to a value larger than the buf len supplied to _parse_dns_reply.
2. The RDLENGTH bounds-check for the answer section is incorrect for two reasons: (a) buf + len is the first invalid address, so >= needs to be used as a comparison operator (b) The result of bufpos + addrlen might cause a pointer overflow (especially due to the fact that addrlen is attacker controlled). If pointer overflows wrap around (undefined behaviour) this would allow an attacker to circumvent the bounds-check and exposes a buffer overflow vulnerability since the attacker controlled addrlen is later used in memcpy(addr_out, bufpos, addrlen), potentially allowing a code execution.
3. The size of the caller allocated buffer addr_out is not passed to the _parse_dns_reply function at all. This makes checking whether the attacker controlled address actually fits in the buffer impossible and allows an easy buffer overflow and potential code execution.

All of these are especially critical due to the fact that DNS responses can easily be spoofed, especially since all spoofing protection mechanisms of DNS were not implemented. So an attacker doesn't even need to control the configured DNS server in order to exploit this.

Steps to reproduce the issue

1. Flash an unmodified version of tests/gnrc_sock_dns to your RIOT node.
2. Adjust your radvd.conf on your border router and add a RDNSS definition.

Causing a crash

Constantly send a DNS response with an excessive qdcount on the computer associated with the IP-Address you configured in the radvd RDNSS definition. For example

while sleep 1; do
    echo AACEAwkmAAAAAAAAKioqKioqKioqKioqKioqKioqKio= | \
        base64 -d | \
        socat fd:0 udp6-sendto:'[address-of-riot-node]':49152,sourceport=53
done

Remote code execution

This is (obviously) highly platform specific. We did this with BOARD=pba-d-01-kw2x. We wrote some ARM assembler code which toggles the LED and stored the machine code for it in the RDATA field of the answer section in the DNS response, thereby overflowing the addr buffer in the main stack frame. Our payload exactly fits into the stack frame of the main function and overwrites the return address of that function, jumping to the addr buffer and executing our payload.

Exploit written by @pyropeter. See: https://github.com/beduino-project/exploit-riot-dns

Versions

RIOT-Version: 5e03f58
Build environment:

Operating System Environment
-----------------------------
       Operating System: "Arch Linux" 
                 Kernel: Linux 4.19.4-arch1-1-ARCH x86_64 unknown

Installed compiler toolchains
-----------------------------
             native gcc: gcc (GCC) 8.2.1 20181127
      arm-none-eabi-gcc: arm-none-eabi-gcc (Arch Repository) 8.2.0
                avr-gcc: avr-gcc (GCC) 8.2.0
       mips-mti-elf-gcc: missing
             msp430-gcc: missing
   riscv-none-embed-gcc: missing
                  clang: clang version 7.0.0 (tags/RELEASE_700/final)

Installed compiler libs
-----------------------
   arm-none-eabi-newlib: "3.0.0"
    mips-mti-elf-newlib: missing
riscv-none-embed-newlib: missing
               avr-libc: "2.0.0" ("20150208")

Installed development tools
---------------------------
                  cmake: cmake version 3.13.1
               cppcheck: missing
                doxygen: 1.8.14
                 flake8: missing
                    git: git version 2.19.2
                   make: GNU Make 4.2.1
                openocd: Open On-Chip Debugger 0.10.0+dev-00436-g0fdf48f1 (2018-06-18-18:19)
                 python: Python 3.7.1
                python2: Python 2.7.15
                python3: Python 3.7.1
             coccinelle: missing