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Subject[PATCH v5 10/11] crypto: ahash: Remove VLA usage for AHASH_REQUEST_ON_STACK
Date
In the quest to remove all stack VLA usage from the kernel[1], this caps
the ahash request size similar to the other limits and adds a sanity
check at initialization. AHASH_REQUEST_ON_STACK is special, though: it
is only ever used for shash-wrapped ahash, so its size is bounded only
by non-async hashes. A manual inspection of this shows the largest to be:
sizeof(struct shash_desc) + SHASH_MAX_DESCSIZE

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
---
crypto/shash.c | 9 ++++++++-
include/crypto/hash.h | 10 +++++++++-
2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/crypto/shash.c b/crypto/shash.c
index 8d4746b14dd5..e344560458cb 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -355,6 +355,7 @@ int crypto_init_shash_ops_async(struct crypto_tfm *tfm)
struct crypto_ahash *crt = __crypto_ahash_cast(tfm);
struct crypto_shash **ctx = crypto_tfm_ctx(tfm);
struct crypto_shash *shash;
+ size_t reqsize;

if (!crypto_mod_get(calg))
return -EAGAIN;
@@ -365,6 +366,12 @@ int crypto_init_shash_ops_async(struct crypto_tfm *tfm)
return PTR_ERR(shash);
}

+ reqsize = sizeof(struct shash_desc) + crypto_shash_descsize(shash);
+ if (WARN_ON(reqsize > AHASH_MAX_REQSIZE)) {
+ crypto_mod_put(calg);
+ return -EINVAL;
+ }
+
*ctx = shash;
tfm->exit = crypto_exit_shash_ops_async;

@@ -383,7 +390,7 @@ int crypto_init_shash_ops_async(struct crypto_tfm *tfm)
if (alg->import)
crt->import = shash_async_import;

- crt->reqsize = sizeof(struct shash_desc) + crypto_shash_descsize(shash);
+ crt->reqsize = reqsize;

return 0;
}
diff --git a/include/crypto/hash.h b/include/crypto/hash.h
index 4fcd0e2368cd..2181fb38eab9 100644
--- a/include/crypto/hash.h
+++ b/include/crypto/hash.h
@@ -67,9 +67,17 @@ struct ahash_request {
#define AHASH_MAX_DIGESTSIZE 512
#define AHASH_MAX_STATESIZE 512

+/*
+ * HASH_REQUEST_ON_STACK must only be used when wrapping shash (i.e.
+ * allocated with a CRYPTO_ALG_ASYNC mask), and is generally discouraged
+ * (just use shash directly). This is really only ever needed when using
+ * scatter/gather input sources.
+ */
+#define AHASH_MAX_REQSIZE (sizeof(struct shash_desc) + \
+ SHASH_MAX_DESCSIZE)
#define AHASH_REQUEST_ON_STACK(name, ahash) \
char __##name##_desc[sizeof(struct ahash_request) + \
- crypto_ahash_reqsize(ahash)] CRYPTO_MINALIGN_ATTR; \
+ AHASH_MAX_REQSIZE] CRYPTO_MINALIGN_ATTR; \
struct ahash_request *name = (void *)__##name##_desc

/**
--
2.17.1
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