Add bpf_crypto_shash module that registers a hash type with the BPF
crypto infrastructure, enabling BPF programs to access kernel hash
algorithms through a unified interface.

Update the bpf_crypto_type interface with hash-specific callbacks:
   - alloc_tfm: Allocates crypto_shash context with proper descriptor size
   - free_tfm: Releases hash transform and context memory
   - has_algo: Checks algorithm availability via crypto_has_shash()
   - hash: Performs single-shot hashing via crypto_shash_digest()
   - digestsize: Returns the output size for the hash algorithm
   - get_flags: Exposes transform flags to BPF programs

Update bpf_shash_ctx to contain crypto_shash transform and shash_desc
descriptor to accommodate algorithm-specific descriptor requirements.

Signed-off-by: Daniel Hodges <git@danielhodges.dev>
---
 crypto/Makefile           |  3 ++
 crypto/bpf_crypto_shash.c | 95 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+)
 create mode 100644 crypto/bpf_crypto_shash.c

diff --git a/crypto/Makefile b/crypto/Makefile
index 16a35649dd91..853dff375906 100644
--- a/crypto/Makefile
+++ b/crypto/Makefile
@@ -30,6 +30,9 @@ obj-$(CONFIG_CRYPTO_ECHAINIV) += echainiv.o
 crypto_hash-y += ahash.o
 crypto_hash-y += shash.o
 obj-$(CONFIG_CRYPTO_HASH2) += crypto_hash.o
+ifeq ($(CONFIG_BPF_SYSCALL),y)
+obj-$(CONFIG_CRYPTO_HASH2) += bpf_crypto_shash.o
+endif
 
 obj-$(CONFIG_CRYPTO_AKCIPHER2) += akcipher.o
 obj-$(CONFIG_CRYPTO_SIG2) += sig.o
diff --git a/crypto/bpf_crypto_shash.c b/crypto/bpf_crypto_shash.c
new file mode 100644
index 000000000000..95c178ec0ce8
--- /dev/null
+++ b/crypto/bpf_crypto_shash.c
@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+#include <linux/types.h>
+#include <linux/module.h>
+#include <linux/bpf_crypto.h>
+#include <crypto/hash.h>
+
+struct bpf_shash_ctx {
+	struct crypto_shash *tfm;
+	struct shash_desc desc;
+};
+
+static void *bpf_crypto_shash_alloc_tfm(const char *algo)
+{
+	struct bpf_shash_ctx *ctx;
+	struct crypto_shash *tfm;
+
+	tfm = crypto_alloc_shash(algo, 0, 0);
+	if (IS_ERR(tfm))
+		return tfm;
+
+	ctx = kzalloc(sizeof(*ctx) + crypto_shash_descsize(tfm), GFP_KERNEL);
+	if (!ctx) {
+		crypto_free_shash(tfm);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	ctx->tfm = tfm;
+	ctx->desc.tfm = tfm;
+
+	return ctx;
+}
+
+static void bpf_crypto_shash_free_tfm(void *tfm)
+{
+	struct bpf_shash_ctx *ctx = tfm;
+
+	crypto_free_shash(ctx->tfm);
+	kfree(ctx);
+}
+
+static int bpf_crypto_shash_has_algo(const char *algo)
+{
+	return crypto_has_shash(algo, 0, 0);
+}
+
+static int bpf_crypto_shash_hash(void *tfm, const u8 *data, u8 *out,
+				 unsigned int len)
+{
+	struct bpf_shash_ctx *ctx = tfm;
+
+	return crypto_shash_digest(&ctx->desc, data, len, out);
+}
+
+static unsigned int bpf_crypto_shash_digestsize(void *tfm)
+{
+	struct bpf_shash_ctx *ctx = tfm;
+
+	return crypto_shash_digestsize(ctx->tfm);
+}
+
+static u32 bpf_crypto_shash_get_flags(void *tfm)
+{
+	struct bpf_shash_ctx *ctx = tfm;
+
+	return crypto_shash_get_flags(ctx->tfm);
+}
+
+static const struct bpf_crypto_type bpf_crypto_shash_type = {
+	.alloc_tfm	= bpf_crypto_shash_alloc_tfm,
+	.free_tfm	= bpf_crypto_shash_free_tfm,
+	.has_algo	= bpf_crypto_shash_has_algo,
+	.hash		= bpf_crypto_shash_hash,
+	.digestsize	= bpf_crypto_shash_digestsize,
+	.get_flags	= bpf_crypto_shash_get_flags,
+	.owner		= THIS_MODULE,
+	.name		= "hash",
+};
+
+static int __init bpf_crypto_shash_init(void)
+{
+	return bpf_crypto_register_type(&bpf_crypto_shash_type);
+}
+
+static void __exit bpf_crypto_shash_exit(void)
+{
+	int err = bpf_crypto_unregister_type(&bpf_crypto_shash_type);
+
+	WARN_ON_ONCE(err);
+}
+
+module_init(bpf_crypto_shash_init);
+module_exit(bpf_crypto_shash_exit);
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Hash algorithm support for BPF");
-- 
2.51.0

> diff --git a/crypto/bpf_crypto_shash.c b/crypto/bpf_crypto_shash.c
> new file mode 100644
> index 000000000000..95c178ec0ce8
> --- /dev/null
> +++ b/crypto/bpf_crypto_shash.c
> @@ -0,0 +1,95 @@
[ ... ]
> +
> +static const struct bpf_crypto_type bpf_crypto_shash_type = {
> +	.alloc_tfm	= bpf_crypto_shash_alloc_tfm,
> +	.free_tfm	= bpf_crypto_shash_free_tfm,
> +	.has_algo	= bpf_crypto_shash_has_algo,
> +	.hash		= bpf_crypto_shash_hash,
> +	.digestsize	= bpf_crypto_shash_digestsize,
> +	.get_flags	= bpf_crypto_shash_get_flags,
> +	.owner		= THIS_MODULE,
> +	.name		= "hash",
> +};

The bpf_crypto_type implementation leaves several function pointers as
NULL (setkey, setauthsize, encrypt, decrypt, ivsize, statesize), which is
appropriate for plain hash algorithms. However, this creates an interface
contract mismatch with bpf_crypto_ctx_create() in kernel/bpf/crypto.c.

Looking at the caller bpf_crypto_ctx_create():

  Line 174-177:
    if (!params->key_len || params->key_len > sizeof(params->key)) {
        *err = -EINVAL;
        goto err_module_put;
    }

Can hash contexts be created? This check requires key_len to be non-zero,
but plain hash algorithms like SHA256 don't use keys. This prevents
creating hash contexts entirely.

  Line 186-189:
    ctx->tfm = type->alloc_tfm(params->algo);
    if (IS_ERR(ctx->tfm)) {
        *err = PTR_ERR(ctx->tfm);
        goto err_free_tfm;

Can bpf_crypto_shash_free_tfm handle ERR_PTR? When alloc_tfm fails and
returns ERR_PTR, the code stores it in ctx->tfm then jumps to err_free_tfm
which calls type->free_tfm(ctx->tfm). In bpf_crypto_shash_free_tfm(), the
code dereferences ctx->tfm without checking for ERR_PTR, which will crash:

    static void bpf_crypto_shash_free_tfm(void *tfm)
    {
        struct bpf_shash_ctx *ctx = tfm;
        crypto_free_shash(ctx->tfm);  /* dereferences ctx */
        kfree(ctx);
    }

  Line 198:
    *err = type->setkey(ctx->tfm, params->key, params->key_len);

Does this dereference NULL? The hash implementation doesn't provide setkey
(it's NULL in bpf_crypto_shash_type), but this line calls it
unconditionally.

  Line 207:
    ctx->siv_len = type->ivsize(ctx->tfm) + type->statesize(ctx->tfm);

Do these dereference NULL? The hash implementation doesn't provide ivsize
or statesize (both NULL), but this line calls them unconditionally.

Note: These issues are fixed in commit 14cc32f873e0 ("bpf: Add SHA hash
kfunc for cryptographic hashing") which makes the setkey, ivsize, and
statesize calls conditional, and fixes the ERR_PTR handling by changing
the error path from "goto err_free_tfm" to "goto err_free_ctx".


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