From 839b60ced95f47224cbe6a517d03daac625b35bf Mon Sep 17 00:00:00 2001 From: Thomas Huth Date: Wed, 16 Sep 2026 11:50:15 +0200 Subject: [PATCH] lib/crypto: Add documentation about zeroization of key and context data Add a central document about zeroization in libcrypto so we don't have to repeat this information in the individual kernel docs of the zeroization functions all over the place. Signed-off-by: Thomas Huth Link: https://patch.msgid.link/20260916095022.604354-14-thuth@redhat.com Signed-off-by: Eric Biggers --- .../crypto/libcrypto-zeroization.rst | 150 ++++++++++++++++++ Documentation/crypto/libcrypto.rst | 1 + 2 files changed, 151 insertions(+) create mode 100644 Documentation/crypto/libcrypto-zeroization.rst diff --git a/Documentation/crypto/libcrypto-zeroization.rst b/Documentation/crypto/libcrypto-zeroization.rst new file mode 100644 index 000000000000..76b650671163 --- /dev/null +++ b/Documentation/crypto/libcrypto-zeroization.rst @@ -0,0 +1,150 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +Crypto Key Zeroization +====================== + +This document describes the conventions for zeroizing crypto structures in the +kernel. + +Note: the kernel follows traditional cryptographic terminology by using +the term "zeroizing" to mean erasing sensitive parameters to prevent +their disclosure if the system is later compromised. This distinguishes +it from zeroing memory for other purposes such as initialization. + +.. contents:: + +Overview +-------- + +Cryptographic key material and intermediate state (such as HMAC contexts) must +be zeroized after use to prevent sensitive data from lingering on the stack or +heap, where it could be leaked through memory disclosure vulnerabilities, +crash dumps, or cold-boot attacks. + +For memory that has been allocated with kmalloc() or a similar function, +kfree_sensitive() should be used instead of kfree() to release the memory. + +For other cases, the kernel provides memzero_explicit() for clearing the +memory. Unlike plain memset(), memzero_explicit() is guaranteed not +to be optimized away by the compiler, even when the memory being cleared +appears to be dead. + +The crypto library builds on memzero_explicit() by providing typed +zeroization helpers for each key and context structure. These helpers serve +two purposes: + +1. They make __cleanup() annotations possible, so that structures on + the stack are automatically zeroized when they go out of scope. + +2. They improve readability by replacing ``memzero_explicit(&key, sizeof(key))`` + with a self-documenting call like ``aes_zeroize_key(&key)``. + + +What to zeroize +--------------- + +The following types of structures hold sensitive material and should be +zeroized after use: + +- **Key structures** (e.g. ``struct aes_key``, ``struct hmac_sha256_key``): + contain expanded round keys or prepared key material. + +- **HMAC/MAC context structures** (e.g. ``struct hmac_sha256_ctx``, + ``struct aes_cmac_ctx``): contain inner and outer hash states derived from + the key. + +- **Hash context structures** (e.g. ``struct sha256_ctx``): may contain + sensitive data being hashed. + +Not all of these require explicit cleanup by callers. Many ``..._final()`` +functions already zeroize their context internally (see `Automatic vs. manual +zeroization`_ below). + + +Zeroization helpers +------------------- + +Each crypto structure that callers may need to zeroize should have a +corresponding inline helper function. The naming convention is:: + + _zeroize_(struct _ *p); + +For example:: + + void aes_zeroize_key(struct aes_key *key); + void aes_zeroize_enckey(struct aes_enckey *key); + void hmac_sha256_zeroize_ctx(struct hmac_sha256_ctx *ctx); + void aes_cmac_zeroize_key(struct aes_cmac_key *key); + void aes_cmac_zeroize_ctx(struct aes_cmac_ctx *ctx); + +Each helper is a ``static inline`` function in the algorithm's header that +wraps ``memzero_explicit()``, for example:: + + static inline void hmac_sha256_zeroize_ctx(struct hmac_sha256_ctx *ctx) + { + memzero_explicit(ctx, sizeof(*ctx)); + } + + +Using __cleanup for automatic zeroization +----------------------------------------- + +The preferred way to zeroize stack-allocated key and context structures is +with the __cleanup() attribute. This ensures zeroization happens on all +exit paths, including error returns and early exits. For example:: + + static int my_aesxts_setkey(..., const u8 *key, unsigned int len) + { + struct crypto_aes_ctx aes __cleanup(aes_zeroize_ctx); + ... + + /* Only half of the key data is cipher key */ + keylen = (len >> 1); + ret = aes_expandkey(&aes, key, keylen); + if (ret) + return ret; + + ... do something with the cipher key ... + + /* The other half is the tweak key */ + ret = aes_expandkey(&aes, (u8 *)(key + keylen), keylen); + if (ret) + return ret; /* <-- Could leak cipher key without __cleanup */ + + ... do something with the tweak key ... + + /* No need for memzero_explicit() at the end thanks to the __cleanup */ + return 0; + } + +Note that __cleanup() attributes should not be used in functions that use +"goto" statements. The benefit of cleanup helpers is the removal of "gotos", +and that "goto" statements can jump between scopes, so the expectation is +that usage of "goto" and cleanup helpers is never mixed in the same function. + + +Automatic vs. manual zeroization +-------------------------------- + +Many ``..._final()`` functions in the crypto library automatically zeroize +their context before returning. When this is the case, the kernel-doc for the +function documents it:: + + After finishing, this zeroizes @ctx. So the caller does not need to do it. + +In these cases, callers on simple code paths (where ``..._final()`` is always +reached) do not need to add __cleanup() or explicit zeroization. +However, __cleanup() is still recommended whenever there are error paths +that bypass ``..._final()``, as it ensures zeroization on all paths. + +For algorithms where ``..._final()`` does *not* zeroize the context (such as +the SHAKE XOFs, where ``shake_squeeze()`` can be called multiple times), +callers must explicitly zeroize the context by calling the appropriate helper +or using __cleanup(), for example:: + + struct shake_ctx ctx __cleanup(shake_zeroize_ctx); + + shake256_init(&ctx); + shake_update(&ctx, data, data_len); + shake_squeeze(&ctx, out, out_len); + /* ctx is automatically zeroized at end of scope */ diff --git a/Documentation/crypto/libcrypto.rst b/Documentation/crypto/libcrypto.rst index e911e0521597..9533c12caa79 100644 --- a/Documentation/crypto/libcrypto.rst +++ b/Documentation/crypto/libcrypto.rst @@ -165,4 +165,5 @@ API documentation libcrypto-signature libcrypto-unauth-encryption libcrypto-utils + libcrypto-zeroization sha3