Stack buffer overflow (write) while loading map in CMapLayers::LoadEnvPoints, maplayers.cpp:184

[mmmds]

The client crashes when an invalid map (14.map.zip) is loaded. Such a map can be delivered to the client by a malicious server.

Tested on Ubuntu 20.04 x86_64, Teeworlds version: f35da54b6f2c5f9fd4bbd4559f887ccf8f8cc526

Compilation with ASAN:

export CXXFLAGS="-ggdb -O0 -fsanitize=address -fno-omit-frame-pointer"
export CFLAGS="-ggdb -O0 -fsanitize=address -fno-omit-frame-pointer"
cmake ..
make

Run and connect to a server that delivers an invalid map:

./teeworlds "connect 127.0.0.1" "gfx_fullscreen 0" "gfx_screen_height 240" "gfx_screen_width 360"
[...]
=================================================================
==33805==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffffffd8f78 at pc 0x5555556fae80 bp 0x7ffffffd8e10 sp 0x7ffffffd8e00
WRITE of size 4 at 0x7ffffffd8f78 thread T0
    #0 0x5555556fae7f in CMapLayers::LoadEnvPoints(CLayers const*, array<CEnvPoint, allocator_default<CEnvPoint> >&) /home/osboxes/teeworlds/teeworlds-asan/src/game/client/components/maplayers.cpp:184
    #1 0x5555556fcf3b in CMapLayers::OnMapLoad() /home/osboxes/teeworlds/teeworlds-asan/src/game/client/components/maplayers.cpp:117
    #2 0x555555839981 in CGameClient::OnConnected() /home/osboxes/teeworlds/teeworlds-asan/src/game/client/gameclient.cpp:459
    #3 0x5555555f6755 in CClient::ProcessServerPacket(CNetChunk*) /home/osboxes/teeworlds/teeworlds-asan/src/engine/client/client.cpp:1283
    #4 0x5555555f91ce in CClient::PumpNetwork() /home/osboxes/teeworlds/teeworlds-asan/src/engine/client/client.cpp:1596
    #5 0x5555555f9596 in CClient::Update() /home/osboxes/teeworlds/teeworlds-asan/src/engine/client/client.cpp:1766
    #6 0x5555555fc1f0 in CClient::Run() /home/osboxes/teeworlds/teeworlds-asan/src/engine/client/client.cpp:2108
    #7 0x5555555d24e5 in main /home/osboxes/teeworlds/teeworlds-asan/src/engine/client/client.cpp:2704
    #8 0x7ffff68e10b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #9 0x5555555dc48d in _start (/home/osboxes/teeworlds/teeworlds-asan/build/teeworlds+0x8848d)

Address 0x7ffffffd8f78 is located in stack of thread T0 at offset 184 in frame
    #0 0x5555556f949f in CMapLayers::LoadEnvPoints(CLayers const*, array<CEnvPoint, allocator_default<CEnvPoint> >&) /home/osboxes/teeworlds/teeworlds-asan/src/game/client/components/maplayers.cpp:135

  This frame has 5 object(s):
    [32, 36) 'Start' (line 141)
    [48, 52) 'Num' (line 141)
    [64, 68) 'Start' (line 151)
    [80, 84) 'Num' (line 151)
    [96, 184) 'p' (line 173) <== Memory access at offset 184 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/osboxes/teeworlds/teeworlds-asan/src/game/client/components/maplayers.cpp:184 in CMapLayers::LoadEnvPoints(CLayers const*, array<CEnvPoint, allocator_default<CEnvPoint> >&)
Shadow bytes around the buggy address:
  0x10007fff3190: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff31a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff31b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff31c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff31d0: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 f8 f2 f8 f2
=>0x10007fff31e0: 04 f2 04 f2 00 00 00 00 00 00 00 00 00 00 00[f3]
  0x10007fff31f0: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff3200: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x10007fff3210: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f3
  0x10007fff3220: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff3230: 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 f2
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==33805==ABORTING

Compilation without ASAN:

export CXXFLAGS="-ggdb -O0"
export CFLAGS="-ggdb -O0"
cmake ..
make

Run and connect to a server that delivers an invalid map:

./teeworlds "connect 127.0.0.1" "gfx_fullscreen 0" "gfx_screen_height 240" "gfx_screen_width 360"
[...]
Segmentation fault (core dumped)

[mmmds]

The bug can be abused to write arbitrary value on a stack and control an instruction pointer leading to arbitrary code execution.

$ gdb --args ./teeworlds 'connect 127.0.0.1'
[...]
Thread 1 "teeworlds" received signal SIGSEGV, Segmentation fault.
0x000000aabbccddee in ?? ()
(gdb) i r
rax 0x6 6
rbx 0xeeeeeeddddddddcc -1229783011548078644
rcx 0x555556004390 93825003438992
rdx 0x46 70
rsi 0x0 0
rdi 0x555556004390 93825003438992
rbp 0xee 0xee
rsp 0x7ffffffddad0 0x7ffffffddad0
r8 0x4d0 1232
r9 0x7ffff7861ff0 140737346150384
r10 0x555555a3c010 93824997376016
r11 0x7ffff7861be0 140737346149344
r12 0x0 0
r13 0x0 0
r14 0x0 0
r15 0x0 0
rip 0xaabbccddee 0xaabbccddee
eflags 0x210202 [ IF RF ID ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0

Additionally, the binaries available on GitHub do not have stack protection enabled. On the contrary, build available in Ubuntu 20.04 repository has the protection enabled.

[carnil]

CVE-2021-43518 appears to have been assigned for this issue