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[KernelCache] Automatically unwrap kernel caches within a full IMG4 container #7981

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Merged
bdash merged 1 commit into from
Feb 23, 2026
Merged

[KernelCache] Automatically unwrap kernel caches within a full IMG4 container #7981

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67 changes: 43 additions & 24 deletions view/kernelcache/core/transformers/KernelCacheTransforms.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ using namespace BinaryNinja;
#include "libimg4/img4.h"
#include "liblzfse/lzfse.h"
#include <algorithm>
#include <optional>

class IMG4PayloadTransform : public Transform
{
Expand All @@ -20,19 +21,40 @@ class IMG4PayloadTransform : public Transform
{
}

virtual bool Decode(const DataBuffer& input, DataBuffer& output, const std::map<std::string, DataBuffer>& params) override
// Attempt to decode a full IMG4 container first, falling back to decoding as a bare IM4P payload.
// Returns the IM4P payload, which contains pointers into the input buffer.
static std::optional<Img4Payload> DecodePayload(const uint8_t* data, size_t length)
{
DERItem item = {};
item.data = (DERByte *)input.GetData();
item.length = static_cast<DERSize>(std::min(input.GetLength(), (size_t)std::numeric_limits<DERSize>::max()));
DERSize derLen = static_cast<DERSize>(std::min(length, (size_t)std::numeric_limits<DERSize>::max()));

DERItem der = {};
der.data = (DERByte*)data;
der.length = derLen;

// Try unwrapping a full IMG4 container to get the inner IM4P item
DERItem img4Items[4] = {};
if (DERImg4Decode(&der, img4Items) == DR_Success)
der = img4Items[1];

// Parse the IM4P payload. We ignore DR_DecodeError as it is returned if the payload
// has trailing fields, such as the optional "PAYP" (payload properties) item.
Img4Payload payload = {};
if (auto result = DERImg4DecodePayload(&item, &payload); (result != DR_Success) && (result != DR_DecodeError))
return false;
if (auto result = DERImg4DecodePayload(&der, &payload); result != DR_Success && result != DR_DecodeError)
return std::nullopt;

if (!payload.payload.data || !payload.payload.length)
return false;
return std::nullopt;

output = DataBuffer(payload.payload.data, payload.payload.length);
return payload;
}

virtual bool Decode(const DataBuffer& input, DataBuffer& output, const std::map<std::string, DataBuffer>& params) override
{
auto payload = DecodePayload((const uint8_t*)input.GetData(), input.GetLength());
if (!payload)
return false;

output = DataBuffer(payload->payload.data, payload->payload.length);
return true;
}

Expand All @@ -46,32 +68,27 @@ class IMG4PayloadTransform : public Transform
if (!dataPtr || !dataLength)
return false;

DERItem item = {};
item.data = (DERByte *)dataPtr;
item.length = static_cast<DERSize>(std::min(dataLength, (size_t)std::numeric_limits<DERSize>::max()));
Img4Payload payload = {};
if (auto result = DERImg4DecodePayload(&item, &payload); (result != DR_Success) && (result != DR_DecodeError))
return false;
if (!payload.payload.data || !payload.payload.length)
auto payload = DecodePayload(dataPtr, dataLength);
if (!payload)
return false;

// Synthesize name: <type>[.<version>]
std::string filename = "";
if (payload.type.data && payload.type.length)
filename = std::string((const char*)payload.type.data, payload.type.length);
if (payload.version.data && payload.version.length)
if (payload->type.data && payload->type.length)
filename = std::string((const char*)payload->type.data, payload->type.length);
if (payload->version.data && payload->version.length)
{
if (!filename.empty())
filename += ".";
filename += std::string((const char*)payload.version.data, payload.version.length);
filename += std::string((const char*)payload->version.data, payload->version.length);
}

if (payload.keybag.data && payload.keybag.length)
if (payload->keybag.data && payload->keybag.length)
{
LogWarn("IMG4 payload contains keybag, which is not currently supported.");
}

context->SetChild(DataBuffer(payload.payload.data, payload.payload.length), filename, TransformSuccess, "", true);
context->SetChild(DataBuffer(payload->payload.data, payload->payload.length), filename, TransformSuccess, "", true);

return true;
}
Expand All @@ -91,7 +108,9 @@ class IMG4PayloadTransform : public Transform
v.insert(v.end(), p, p + len);
}

// TODO fix/support round-tripping of optional fields (type, desc)
// TODO fix/support round-tripping. Encode always produces a bare IM4P, but Decode
// accepts both bare IM4P and full IMG4 containers. Type/desc are also lost unless
// passed via params.
virtual bool Encode(const DataBuffer& input, DataBuffer& output, const std::map<std::string, DataBuffer>& params) override
{
// type (exactly 4 chars)
Expand Down Expand Up @@ -190,7 +209,7 @@ class IMG4PayloadTransform : public Transform
if (seqLen > (inputLength - offset))
return false;

// parse up to the first 5 elements to find the magic "IM4P"
// parse up to the first 5 elements to find the magic "IM4P" or "IMG4"
for (int i = 0; i < 5 && offset < seqEnd; ++i)
{
if (offset >= headerLength)
Expand All @@ -208,7 +227,7 @@ class IMG4PayloadTransform : public Transform
offset += elementLenHdr;
if (offset + elementLen > headerLength)
return false;
if ((tag == 0x16) && (elementLen == 4) && memcmp(data + offset, "IM4P", 4) == 0)
if (tag == 0x16 && elementLen == 4 && (memcmp(data + offset, "IM4P", 4) == 0 || memcmp(data + offset, "IMG4", 4) == 0))
return true;
offset += elementLen;
}
Expand Down