Stack exhaustion and crashes with malformed input

Start by reproducing the crashes with task rc:infinite_loop.rc list and task rc:path_expand_segv.rc list. Then inspect src/libshared/src/FS.cpp, especially Path::expand at line 265, and trace the calls from Directory::create. Done means both malformed inputs no longer cause stack exhaustion or a segmentation fault.

Whilst fuzzing with AFL I observed two unique crashes with similar input. Provided to task rc:$file list. As the crash occurred inside libshared I'm reporting the problem here. Please let me know if I should file the bug in the taskwarrior repository instead.

This appears to be an infinite loop through Directory::create which ends up exhausting the stack:

infinite_loop.rc:

data.location=/\

gdb-peda$ run rc:infinite_loop.rc list
Starting program: /home/kali/fuzzing/sessions/taskwarrior/task rc:infinite_loop.rc list
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Warning: AFL++ tools will need to set AFL_MAP_SIZE to 70464 to be able to run this instrumented program!

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0xf9a3
RBX: 0x0
RCX: 0x7ebb00 --> 0x0
RDX: 0xcd
RSI: 0x7fffff7ff090 --> 0x7fffff7ff0a0 --> 0x0
RDI: 0x7fffff7ff048 --> 0x7fffff7ff058 --> 0x0
RBP: 0x7fffff7ff058 --> 0x0
RSP: 0x7fffff7fef90
RIP: 0x6ffb2c (<_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+60>:     mov    QWORD PTR [rsp+0x8],rbx)
R8 : 0x9ff050 --> 0x9fee10 --> 0x9fed50 --> 0xa03ad0 --> 0x9ff7d0 --> 0xa06cd0 (--> ...)
R9 : 0x3
R10: 0xfffffffffffff9dd
R11: 0x246
R12: 0x7fffff7ff0f0 --> 0x0
R13: 0x7fffff7ff090 --> 0x7fffff7ff0a0 --> 0x0
R14: 0x7fffff7ff048 --> 0x7fffff7ff058 --> 0x0
R15: 0x7fffff7ff0a0 --> 0x0
EFLAGS: 0x10282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x6ffb22 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+50>:        mov    QWORD PTR [rdi],rbp
   0x6ffb25 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+53>:        mov    r15,QWORD PTR [rsi]
   0x6ffb28 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+56>:        mov    rbx,QWORD PTR [rsi+0x8]
=> 0x6ffb2c <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+60>:        mov    QWORD PTR [rsp+0x8],rbx
   0x6ffb31 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+65>:        cmp    rbx,0xf
   0x6ffb35 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+69>:        jbe    0x6ffb93 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+163>
   0x6ffb37 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+71>:        movsxd rax,DWORD PTR [rip+0xe42ba]        # 0x7e3df8
   0x6ffb3e <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+78>:        mov    rcx,QWORD PTR [rip+0xa5c03]        # 0x7a5748 <__afl_area_ptr>
[------------------------------------stack-------------------------------------]
Invalid $SP address: 0x7fffff7fef90
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string (this=0x7fffff7ff048, Python Exception <class 'gdb.error'> There is no member named _M_dataplus.:
__str=) at /usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/basic_string.h:451
451           { _M_construct(__str._M_data(), __str._M_data() + __str.length()); }
gdb-peda$

This file seems to trigger a crash inside Path::expand:

path_expand_segv.rc:

data.location=/o

gdb-peda$ run rc:path_expand_segv.rc  list
Starting program: /home/kali/fuzzing/sessions/taskwarrior/task rc:path_expand_segv.rc  list
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Warning: AFL++ tools will need to set AFL_MAP_SIZE to 70464 to be able to run this instrumented program!

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
RAX: 0xf993
RBX: 0x7fffff7ff0b0 --> 0x7fffff7ff0c0 --> 0x7fffff7ff100 --> 0x1
RCX: 0x7ebb00 --> 0x0
RDX: 0xf
RSI: 0x7fffff7ff080 --> 0x7fffff7ff090 --> 0x0
RDI: 0x7fffff7ff038 --> 0x0
RBP: 0x0
RSP: 0x7fffff7ff000 --> 0x7fffff7ff0e0 --> 0x0
RIP: 0x6ffaf9 (<_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+9>:      push   rbx)
R8 : 0x9ff050 --> 0x9fee10 --> 0x9fed50 --> 0xa03ad0 --> 0x9ff7d0 --> 0xa06cd0 (--> ...)
R9 : 0x2
R10: 0xfffffffffffff9dd
R11: 0x246
R12: 0x7fffff7ff0e0 --> 0x0
R13: 0x9ec001 --> 0x9100000000000000
R14: 0x7fffff7ff080 --> 0x7fffff7ff090 --> 0x0
R15: 0x7fffff7ff0c0 --> 0x7fffff7ff100 --> 0x1
EFLAGS: 0x10206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x6ffaf3 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+3>: push   r14
   0x6ffaf5 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+5>: push   r13
   0x6ffaf7 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+7>: push   r12
=> 0x6ffaf9 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+9>: push   rbx
   0x6ffafa <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+10>:        sub    rsp,0x78
   0x6ffafe <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+14>:        movsxd rax,DWORD PTR [rip+0xe42eb]        # 0x7e3df0
   0x6ffb05 <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+21>:        mov    rcx,QWORD PTR [rip+0xa5c3c]        # 0x7a5748 <__afl_area_ptr>
   0x6ffb0c <_ZN4Path6expandERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+28>:        mov    dl,BYTE PTR [rcx+rax*1]
[------------------------------------stack-------------------------------------]
0000| 0x7fffff7ff000 --> 0x7fffff7ff0e0 --> 0x0
0008| 0x7fffff7ff008 --> 0x9ec001 --> 0x9100000000000000
0016| 0x7fffff7ff010 --> 0x7fffff7ff080 --> 0x7fffff7ff090 --> 0x0
0024| 0x7fffff7ff018 --> 0x7fffff7ff0c0 --> 0x7fffff7ff100 --> 0x1
0032| 0x7fffff7ff020 --> 0x0
0040| 0x7fffff7ff028 --> 0x6ff894 (<_ZN4PathC2ERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE+100>:     mov    rsi,QWORD PTR [rsp+0x8])
0048| 0x7fffff7ff030 --> 0x0
0056| 0x7fffff7ff038 --> 0x0
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x00000000006ffaf9 in Path::expand (Python Exception <class 'gdb.error'> There is no member named _M_dataplus.:
in=) at /home/kali/fuzzing/victims/taskwarrior/src/libshared/src/FS.cpp:265
265     {
gdb-peda$