wallet: check descriptor cache xpub length before decoding #35440

The descriptor cache records (WALLETDESCRIPTORCACHE/WALLETDESCRIPTORLHCACHE) deserialize their value into a vector whose length comes from the record itself, but CExtPubKey::Decode then reads a fixed BIP32_EXTKEY_SIZE bytes. A record encoding a shorter xpub makes Decode read past the vector (caught as a container-overflow under ASAN). Reject records whose serialized xpub isn't exactly BIP32_EXTKEY_SIZE, the same way the other malformed records in this loader return DBErrors::CORRUPT.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35440.

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LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• WalletDescriptor wallet_descriptor(descriptor, 0, 0, 0, 0) in src/wallet/test/walletload_tests.cpp

^{2026-06-13 10:43:52}

is there a reason as to why no tests were needed for these changes ?

Was this LLM generated? What are the steps to test this? What is the output before and after the changes here?

ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_1FAEFB6177B4672DEE07F9D3AFC62588CCD2631EDCF22E8CCC1FB35B501C9C86

No, it's not LLM-generated. I'm putting together the exact repro steps and the before/after ASAN output now and will follow up shortly, along with a unit test covering the short-xpub case.

@maflcko Added a unit test. covering the short-xpub case for both the descriptor cache and last-hardened cache.

Steps and before/after, sorry for the delay.

Build with the sanitisers the ASan CI job uses and run the new case:

cmake -B build -DSANITIZERS=address,undefined
cmake --build build --target test_bitcoin
build/bin/test_bitcoin --run_test=walletload_tests/wallet_load_descriptor_cache_invalid_xpub_size

The test writes a descriptor plus a single cache record whose serialised xpub is one byte short of BIP32_EXTKEY_SIZE (73 bytes), for both walletdescriptorcache and walletdescriptorlhcache.

• expected: the loader rejects the record and returns DBErrors::CORRUPT.
• before: the record deserialises fine (ser_xpub.size() == 73), then CExtPubKey::Decode(ser_xpub.data()) reads a fixed 74 bytes, one past the end of the vector. ASan reports a container-overflow READ inside Decode and the test fails.
• after: the ser_xpub.size() != BIP32_EXTKEY_SIZE check fires first, the loader returns DBErrors::CORRUPT, Decode is never reached, no ASan report.

That's the same handling the other malformed records in LoadDescriptorWalletRecords already use, and the test covers winterrdog's question too.