Incorrect implementation of `ccv_bfloat_to_double`

[hgarrereyn]

Hi, it looks like ccv_bfloat_to_double is implemented incorrectly.

Specifically, this is the current implementation:

ccv/lib/ccv_util.c

Lines 1819 to 1830 in a1a8e06

	void ccv_bfloat_to_double(const uint16_t* h, double* f, size_t len)
	{
	int i;
	for (i = 0; i < len; i++)
	{
	union {
	float v;
	uint16_t h[2];
	} u = { .h = { 0, h[1] } };
	f[i] = u.v;
	}
	}

Inside the loop, it always unconditionally accesses h[1] (i.e. the same element is repeatedly converted to a double), instead of indexing using the index variable (i.e. h[i]).

For cases where there is only one element, this causes a buffer OOB read (see below testcase), but also this would likely cause functional issues.

(found via automated fuzzing)

testcase.cpp

#include <cstdint>
extern "C" {
#include "/fuzz/install/include/ccv.h"
}
int main() {
  uint16_t b[1] = { 0x1234 };
  double d[1];
  ccv_bfloat_to_double(b, d, 1); // Triggers ASan OOB read in ccv_bfloat_to_double
  return 0;
}

crash report

{
  "Date": "2025-09-29T02:18:53.599961+00:00",
  "Uname": "Linux 795a6a8d1cfb 5.15.0-156-generic #166-Ubuntu SMP Sat Aug 9 00:02:46 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux",
  "OS": "Ubuntu",
  "OSRelease": "22.04",
  "Architecture": "amd64",
  "ExecutablePath": "/tmp/tmpsldzq0as/reproducer",
  "ProcEnviron": [
    "LIBAFL_EDGES_MAP_SIZE=800000",
    "PWD=/fuzz/workspace",
    "CXX=gf_libafl_cxx",
    "GRAPHFUZZ_USE_ASAN=1",
    "HOME=/root",
    "ASAN_OPTIONS=hard_rss_limit_mb=1024:detect_leaks=0",
    "TERM=xterm-256color",
    "SHLVL=1",
    "LD_LIBRARY_PATH=/fuzz/install/lib",
    "PATH=/root/.cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
    "CC=gf_libafl_cc",
    "DEBIAN_FRONTEND=noninteractive",
    "OLDPWD=/fuzz/src/lib",
    "_=/usr/local/bin/agfi"
  ],
  "ProcCmdline": "/tmp/tmpsldzq0as/reproducer",
  "Stdin": "",
  "ProcStatus": [],
  "ProcMaps": [],
  "ProcFiles": [],
  "NetworkConnections": [],
  "CrashSeverity": {
    "Type": "NOT_EXPLOITABLE",
    "ShortDescription": "stack-buffer-overflow(read)",
    "Description": "Stack buffer overflow",
    "Explanation": "The target reads data past the end, or before the beginning, of the intended stack buffer."
  },
  "Stacktrace": [
    "    #0 0x5555556948d8 in ccv_bfloat_to_double /fuzz/src/lib/ccv_util.c",
    "    #1 0x55555565c4d2 in main /tmp/tmpsldzq0as/reproducer.cpp:8:3",
    "    #2 0x7ffff5619d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16",
    "    #3 0x7ffff5619e3f in __libc_start_main csu/../csu/libc-start.c:392:3",
    "    #4 0x5555555812f4 in _start (/tmp/tmpsldzq0as/reproducer+0x2d2f4) (BuildId: dd32829972587349cd60b31f65a72ede5f40cb37)"
  ],
  "Registers": {},
  "Disassembly": [],
  "Package": "",
  "PackageVersion": "",
  "PackageArchitecture": "",
  "PackageDescription": "",
  "AsanReport": [
    "==219==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffff3600022 at pc 0x5555556948d9 bp 0x7fffffffea10 sp 0x7fffffffea08",
    "READ of size 2 at 0x7ffff3600022 thread T0",
    "    #0 0x5555556948d8 in ccv_bfloat_to_double /fuzz/src/lib/ccv_util.c",
    "    #1 0x55555565c4d2 in main /tmp/tmpsldzq0as/reproducer.cpp:8:3",
    "    #2 0x7ffff5619d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16",
    "    #3 0x7ffff5619e3f in __libc_start_main csu/../csu/libc-start.c:392:3",
    "    #4 0x5555555812f4 in _start (/tmp/tmpsldzq0as/reproducer+0x2d2f4) (BuildId: dd32829972587349cd60b31f65a72ede5f40cb37)",
    "",
    "Address 0x7ffff3600022 is located in stack of thread T0 at offset 34 in frame",
    "    #0 0x55555565c3ff in main /tmp/tmpsldzq0as/reproducer.cpp:5",
    "",
    "  This frame has 2 object(s):",
    "    [32, 34) 'b' (line 6) <== Memory access at offset 34 overflows this variable",
    "    [48, 56) 'd' (line 7)",
    "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 /fuzz/src/lib/ccv_util.c in ccv_bfloat_to_double",
    "Shadow bytes around the buggy address:",
    "  0x7ffff35ffd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff35ffe00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff35ffe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff35fff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff35fff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "=>0x7ffff3600000: f1 f1 f1 f1[02]f2 00 f3 f3 f3 f3 f3 00 00 00 00",
    "  0x7ffff3600080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff3600100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff3600180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff3600200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "  0x7ffff3600280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00",
    "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",
    "==219==ABORTING"
  ],
  "MsanReport": [],
  "UbsanReport": [],
  "LuaReport": [],
  "PythonReport": [],
  "GoReport": [],
  "JavaReport": [],
  "RustReport": [],
  "JsReport": [],
  "CSharpReport": [],
  "CrashLine": "/fuzz/src/lib/ccv_util.c",
  "Source": []
}