NtQueryObject returns wrong insufficient required size via WOW64, why?

I am using the NT native API NtQueryObject()/ZwQueryObject() from user mode (and I am aware of the risks in general and I have written kernel mode drivers for Windows in the past in my professional capacity).

Generally when one uses the typical "query information" function (of which there are a few) the protocol is first to ask with a too small buffer to retrieve the required size with STATUS_INFO_LENGTH_MISMATCH, then allocate a buffer of said size and query again -- this time using the buffer and previously returned size.

In order to get the list of object types (67 on my build) on the system I am doing just that:

ULONG Size = 0;
NTSTATUS Status = NtQueryObject(NULL, ObjectTypesInformation, &Size, sizeof(Size), &Size);

And in Size I get 8280 (WOW64) and 8968 (x64). I then proceed to allocate the buffer with calloc() and query again:

ULONG Size2 = 0;
BYTE* Buf = (BYTE*)::calloc(1, Size);
Status = NtQueryObject(NULL, ObjectTypesInformation, Buf, Size, &Size2);

NB: ObjectTypesInformation is 3. It isn't declared in winternl.h, but Nebbett (as ObjectAllTypesInformation) and others describe it. Since I am not querying for a particular object's traits but the system-wide list of object types, I pass NULL for the object handle.

Curiously on WOW64, i.e. 32-bit, the value in Size2 upon return from the second query is 16 Bytes (= 8296) bigger than the previously returned required size.

As far as alignment is concerned, I'd expect at most 8 Bytes for this sort of thing and indeed neither 8280 nor 8296 are at a 16 Byte alignment boundary, but on an 8 Byte one.

Certainly I can add some slack space on top of the returned required size (e.g. ALIGN_UP to the next 32 Byte alignment boundary), but this seems highly irregular to be honest. And I'd rather want to understand what's going on than to implement a workaround that breaks, because I miss something crucial.

The practical issue for the code is that in Debug configurations it tells me there's a corrupted heap somewhere, upon freeing Buf. Which suggests that NtQueryObject() was indeed writing these extra 16 Bytes beyond the buffer I provided.

Question: Any idea why it is doing that?

As usual for NT native API the sources of information are scarce. The x64 version of the exact same code returns the exact number of bytes required. So my thinking here is that WOW64 is the issue. A somewhat cursory look into wow64.dll with IDA didn't reveal any immediate points for suspicion regarding what goes wrong in translating the results to 32-bit here.

PS: Windows 10 (10.0.19043, ntdll.dll "timestamp" 77755782)

~~PPS: this may be related: https://wj32.org/wp/2012/11/30/obquerytypeinfo-and-ntqueryobject-buffer-overrun-in-windows-8/ (archived)~~ Tested it, by checking that OBJECT_TYPE_INFORMATION::TypeName.Length + sizeof(WCHAR) == OBJECT_TYPE_INFORMATION::TypeName.MaximumLength in all returned items, which was the case.

Solution

Alright, I think I figured out the issue with the help of WinDbg and a thorough look at wow64.dll using IDA.

NB: the wow64.dll I have has the same build number, but differs slightly in data only (checksum, security directory entry, pieces from version resources). The code is identical, which was to be expected, given deterministic builds and how they affect the PE timestamp.

There's an internal function called whNtQueryObject_SpecialQueryCase (according to PDBs), which covers the ObjectTypesInformation class queries.

For the above wow64.dll I used the following points of interest in WinDbg, from a 32 bit program which calls NtQueryObject(NULL, ObjectTypesInformation, ...) (the program itself is irrelevant, though):

0:000> .load wow64exts
0:000> bp wow64!whNtQueryObject_SpecialQueryCase+B0E0
0:000> bp wow64!whNtQueryObject_SpecialQueryCase+B14E
0:000> bp wow64!whNtQueryObject_SpecialQueryCase+B1A7
0:000> bp wow64!whNtQueryObject_SpecialQueryCase+B24A
0:000> bp wow64!whNtQueryObject_SpecialQueryCase+B252

Explanation of the above points of interest:

+B0E0: computing length required for 64 bit query, based on passed length for 32 bit
+B14E: call to NtQueryObject()
+B1A7: loop body for copying 64 to 32 bit buffer contents, after successful NtQueryObject() call
+B24A: computing written length by subtracting current (last + 1) entry from base buffer address
+B252: downsizing returned (64 bit) required length to 32 bit

The logic of this function in regards to just ObjectTypesInformation is roughly as follows:

Common steps

Take the ObjectInformationLength (32 bit query!) argument and size it up to fit the 64 bit info
Align the retrieved size up to the next 16 byte boundary
If necessary allocate the resulting amount from some PEB::ProcessHeap and store in TLS slot 3; otherwise using this as a scratch space
Call NtQueryObject() passing the buffer and length from the two previous steps

The length passed to NtQueryObject() is the one from step 1, not the one aligned to a 16 byte boundary. There seems to be some sort of header to this scratch space, so perhaps that's where the 16 byte alignment comes from?

Case 1: buffer size too small (here: 4), just querying required length

The up-sized length in this case equals 4, which is too small and consequently NtQueryObject() returns STATUS_INFO_LENGTH_MISMATCH. Required size is reported as 8968.

Down-size from the 64 bit required length to 32 bit and end up 16 bytes too short
Return the status from NtQueryObject() and the down-sized required length form the previous step

Case 2: buffer size supposedly (!) sufficient

Copy OBJECT_TYPES_INFORMATION::NumberOfTypes from queried buffer to 32 bit one
Step to the first entry (OBJECT_TYPE_INFORMATION) of source (64 bit) and target (32 bit) buffer, 8 and 4 byte aligned respectively
For for each entry up to OBJECT_TYPES_INFORMATION::NumberOfTypes:
- Copy UNICODE_STRING::Length and UNICODE_STRING::MaximumLength for TypeName member
- memcpy() UNICODE_STRING::Length bytes from source to target UNICODE_STRING::Buffer (target entry + sizeof(OBJECT_TYPE_INFORMATION32)
- Add terminating zero (WCHAR) past the memcpy'd string
- Copy the individual members past the TypeName from 64 to 32 bit struct
- Compute pointer of next entry by aligning UNICODE_STRING::MaximumLength up to an 8 byte boundary (i.e. the ULONG_PTR alignment mentioned in the other answer) + sizeof(OBJECT_TYPE_INFORMATION64) (already 8 byte aligned!)
  - The next target entry (32 bit) gets 4 byte aligned instead
At the end compute required (32 bit) length by subtracting the value we arrived at for the "next" entry (i.e. one past the last) from the base address of the buffer passed by the WOW64 program (32 bit) to NtQueryObject()
- In my debugged scenario these were: 0x008ce050 - 0x008cbfe8 = 0x00002068 (= 8296), which is 16 bytes larger than the buffer length we were told during case 1 (8280)!

The issue

That crucial last step differs between merely querying and actually getting the buffer filled. There is no further bounds checking in that loop I described for case 2.

And this means it will just overrun the passed buffer and return a written length bigger than the buffer length passed to it.

Possible solutions and workarounds

I'll have to approach this mathematically after some sleep, the workaround is obviously to top up the required length returned from case 1 in order to avoid the buffer overrun. The easiest method is to use my up_size_from_32bit() from the example below and use that on the returned required size. This way you are allocating enough for the 64 bit buffer, while querying the 32 bit one. This should never overrun during the copy loop.

However, the fix in wow64.dll is a little more involved, I guess. While adding bounds checking to the loop would help avert the overrun, it would mean that the caller would have to query for the required size twice, because the first time around it lies to us.

Which means the query-only case (1) would have to allocate that internal buffer after querying the required length for 64 bit, then get it filled and then walk the entries (just like the copy loop), skipping over the last entry to compute the required length the same as it is now done after the copy loop.

Example program demonstrating the "static" computation by `wow64.dll`

Build for x64, just the way wow64.dll was!

#define WIN32_LEAN_AND_MEAN
#include <Windows.h>
#include <cstdio>

typedef struct
{
    ULONG JustPretending[24];
} OBJECT_TYPE_INFORMATION32;

typedef struct
{
    ULONG JustPretending[26];
} OBJECT_TYPE_INFORMATION64;

constexpr ULONG size_delta_3264 = sizeof(OBJECT_TYPE_INFORMATION64) - sizeof(OBJECT_TYPE_INFORMATION32);

constexpr ULONG down_size_to_32bit(ULONG len)
{
    return len - size_delta_3264 * ((len - 4) / sizeof(OBJECT_TYPE_INFORMATION64));
}

constexpr ULONG up_size_from_32bit(ULONG len)
{
    return len + size_delta_3264 * ((len - 4) / sizeof(OBJECT_TYPE_INFORMATION32));
}

// Trying to mimic the wdm.h macro
constexpr size_t align_up_by(size_t address, size_t alignment)
{
    return (address + (alignment - 1)) & ~(alignment - 1);
}

constexpr auto u32 = 8280UL;
constexpr auto u64 = 8968UL;
constexpr auto from_64 = down_size_to_32bit(u64);
constexpr auto from_32 = up_size_from_32bit(u32);
constexpr auto from_32_16_byte_aligned = (ULONG)align_up_by(from_32, 16);

int wmain()
{
    wprintf(L"32 to 64 bit: %u -> %u -(16-byte-align)-> %u\n", u32, from_32, from_32_16_byte_aligned);
    wprintf(L"64 to 32 bit: %u -> %u\n", u64, from_64);
    return 0;
}

static_assert(sizeof(OBJECT_TYPE_INFORMATION32) == 96, "Size for 64 bit struct does not match.");
static_assert(sizeof(OBJECT_TYPE_INFORMATION64) == 104, "Size for 64 bit struct does not match.");
static_assert(u32 == from_64, "Must match (from 64 to 32 bit)");
static_assert(u64 == from_32, "Must match (from 32 to 64 bit)");
static_assert(from_32_16_byte_aligned % 16 == 0, "16 byte alignment failed");
static_assert(from_32_16_byte_aligned > from_32, "We're aligning up");

This does not mimic the computation that happens in case 2, though.