[PATCH bpf-next v2 4/5] bpf: Add ECDSA signature verification kfuncs

Daniel Hodges
2025-12-05T17:42:59Z

Add context-based ECDSA signature verification kfuncs:
- bpf_ecdsa_ctx_create(): Creates reusable ECDSA context with public key
- bpf_ecdsa_verify(): Verifies signatures using the context
- bpf_ecdsa_ctx_acquire(): Increments context reference count
- bpf_ecdsa_ctx_release(): Releases context with RCU safety

The ECDSA implementation supports NIST curves (P-256, P-384, P-521) and
uses the kernel's crypto_sig API. Public keys must be in uncompressed
format (0x04 || x || y), and signatures are in r || s format.

Signed-off-by: Daniel Hodges <git@danielhodges.dev>
---
kernel/bpf/crypto.c | 230 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 230 insertions(+)

diff --git a/kernel/bpf/crypto.c b/kernel/bpf/crypto.c
index 95625c7ffb1a..3a3741cf657f 100644
--- a/kernel/bpf/crypto.c
+++ b/kernel/bpf/crypto.c
@@ -9,6 +9,7 @@
#include <linux/scatterlist.h>
#include <linux/skbuff.h>
#include <crypto/skcipher.h>
+#include <crypto/sig.h>

struct bpf_crypto_type_list {
const struct bpf_crypto_type *type;
@@ -57,6 +58,21 @@ struct bpf_crypto_ctx {
refcount_t usage;
};

+#if IS_ENABLED(CONFIG_CRYPTO_ECDSA)
+/**
+ * struct bpf_ecdsa_ctx - refcounted BPF ECDSA context structure
+ * @tfm: The crypto_sig transform for ECDSA operations
+ * @rcu: The RCU head used to free the context with RCU safety
+ * @usage: Object reference counter. When the refcount goes to 0, the
+ * memory is released with RCU safety.
+ */
+struct bpf_ecdsa_ctx {
+ struct crypto_sig *tfm;
+ struct rcu_head rcu;
+ refcount_t usage;
+};
+#endif
+
int bpf_crypto_register_type(const struct bpf_crypto_type *type)
{
struct bpf_crypto_type_list *node;
@@ -399,12 +415,206 @@ __bpf_kfunc int bpf_crypto_hash(struct bpf_crypto_ctx *ctx,
}
#endif /* CONFIG_CRYPTO_HASH2 */

+#if IS_ENABLED(CONFIG_CRYPTO_ECDSA)
+/**
+ * bpf_ecdsa_ctx_create() - Create a BPF ECDSA verification context
+ * @algo_name: bpf_dynptr to the algorithm name (e.g., "p1363(ecdsa-nist-p256)")
+ * @public_key: bpf_dynptr to the public key in uncompressed format (0x04 || x || y)
+ * Must be 65 bytes for P-256, 97 for P-384, 133 for P-521
+ * @err: Pointer to store error code on failure
+ *
+ * Creates an ECDSA verification context that can be reused for multiple
+ * signature verifications. This function uses GFP_KERNEL allocation and
+ * can only be called from sleepable BPF programs. Uses bpf_dynptr to ensure
+ * safe memory access without risk of page faults.
+ */
+__bpf_kfunc struct bpf_ecdsa_ctx *
+bpf_ecdsa_ctx_create(const struct bpf_dynptr *algo_name,
+ const struct bpf_dynptr *public_key, int *err)
+{
+ const struct bpf_dynptr_kern *algo_kern = (struct bpf_dynptr_kern *)algo_name;
+ const struct bpf_dynptr_kern *key_kern = (struct bpf_dynptr_kern *)public_key;
+ struct bpf_ecdsa_ctx *ctx;
+ const char *algo_ptr;
+ const u8 *key_ptr;
+ u32 algo_len, key_len;
+ char algo[64];
+ int ret;
+
+ if (!err)
+ return NULL;
+
+ algo_len = __bpf_dynptr_size(algo_kern);
+ key_len = __bpf_dynptr_size(key_kern);
+
+ if (algo_len == 0 || algo_len >= sizeof(algo)) {
+ *err = -EINVAL;
+ return NULL;
+ }
+
+ if (key_len < 65) {
+ *err = -EINVAL;
+ return NULL;
+ }
+
+ algo_ptr = __bpf_dynptr_data(algo_kern, algo_len);
+ if (!algo_ptr) {
+ *err = -EINVAL;
+ return NULL;
+ }
+
+ key_ptr = __bpf_dynptr_data(key_kern, key_len);
+ if (!key_ptr) {
+ *err = -EINVAL;
+ return NULL;
+ }
+
+ if (key_ptr[0] != 0x04) {
+ *err = -EINVAL;
+ return NULL;
+ }
+
+ memcpy(algo, algo_ptr, algo_len);
+ algo[algo_len] = '\0';
+
+ ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+ if (!ctx) {
+ *err = -ENOMEM;
+ return NULL;
+ }
+
+ ctx->tfm = crypto_alloc_sig(algo, 0, 0);
+ if (IS_ERR(ctx->tfm)) {
+ *err = PTR_ERR(ctx->tfm);
+ kfree(ctx);
+ return NULL;
+ }
+
+ ret = crypto_sig_set_pubkey(ctx->tfm, key_ptr, key_len);
+ if (ret) {
+ *err = ret;
+ crypto_free_sig(ctx->tfm);
+ kfree(ctx);
+ return NULL;
+ }
+
+ refcount_set(&ctx->usage, 1);
+ *err = 0;
+ return ctx;
+}
+
+/**
+ * bpf_ecdsa_verify() - Verify ECDSA signature using pre-allocated context
+ * @ctx: ECDSA context created by bpf_ecdsa_ctx_create()
+ * @message: bpf_dynptr to the message hash to verify. Must be a trusted pointer.
+ * @signature: bpf_dynptr to the ECDSA signature in r || s format. Must be a trusted pointer.
+ * Must be 64 bytes for P-256, 96 for P-384, 132 for P-521
+ *
+ * Verifies an ECDSA signature using a pre-allocated context. This function
+ * does not allocate memory and can be used in non-sleepable BPF programs.
+ * Uses bpf_dynptr to ensure safe memory access without risk of page faults.
+ */
+__bpf_kfunc int bpf_ecdsa_verify(struct bpf_ecdsa_ctx *ctx,
+ const struct bpf_dynptr *message,
+ const struct bpf_dynptr *signature)
+{
+ const struct bpf_dynptr_kern *msg_kern = (struct bpf_dynptr_kern *)message;
+ const struct bpf_dynptr_kern *sig_kern = (struct bpf_dynptr_kern *)signature;
+ const u8 *msg_ptr, *sig_ptr;
+ u32 msg_len, sig_len;
+
+ if (!ctx)
+ return -EINVAL;
+
+ msg_len = __bpf_dynptr_size(msg_kern);
+ sig_len = __bpf_dynptr_size(sig_kern);
+
+ if (msg_len == 0 || sig_len == 0)
+ return -EINVAL;
+
+ msg_ptr = __bpf_dynptr_data(msg_kern, msg_len);
+ if (!msg_ptr)
+ return -EINVAL;
+
+ sig_ptr = __bpf_dynptr_data(sig_kern, sig_len);
+ if (!sig_ptr)
+ return -EINVAL;
+
+ return crypto_sig_verify(ctx->tfm, sig_ptr, sig_len, msg_ptr, msg_len);
+}
+
+__bpf_kfunc struct bpf_ecdsa_ctx *
+bpf_ecdsa_ctx_acquire(struct bpf_ecdsa_ctx *ctx)
+{
+ if (!refcount_inc_not_zero(&ctx->usage))
+ return NULL;
+ return ctx;
+}
+
+static void ecdsa_free_cb(struct rcu_head *head)
+{
+ struct bpf_ecdsa_ctx *ctx = container_of(head, struct bpf_ecdsa_ctx, rcu);
+
+ crypto_free_sig(ctx->tfm);
+ kfree(ctx);
+}
+
+__bpf_kfunc void bpf_ecdsa_ctx_release(struct bpf_ecdsa_ctx *ctx)
+{
+ if (refcount_dec_and_test(&ctx->usage))
+ call_rcu(&ctx->rcu, ecdsa_free_cb);
+}
+
+/**
+ * bpf_ecdsa_keysize() - Get the key size for ECDSA context
+ * @ctx: ECDSA context
+ *
+ * Returns: Key size in bits, or negative error code on failure
+ */
+__bpf_kfunc int bpf_ecdsa_keysize(struct bpf_ecdsa_ctx *ctx)
+{
+ if (!ctx)
+ return -EINVAL;
+
+ return crypto_sig_keysize(ctx->tfm);
+}
+
+/**
+ * bpf_ecdsa_digestsize() - Get the maximum digest size for ECDSA context
+ * @ctx: ECDSA context
+ */
+__bpf_kfunc int bpf_ecdsa_digestsize(struct bpf_ecdsa_ctx *ctx)
+{
+ if (!ctx)
+ return -EINVAL;
+
+ return crypto_sig_digestsize(ctx->tfm);
+}
+
+/**
+ * bpf_ecdsa_maxsize() - Get the maximum signature size for ECDSA context
+ * @ctx: ECDSA context
+ */
+__bpf_kfunc int bpf_ecdsa_maxsize(struct bpf_ecdsa_ctx *ctx)
+{
+ if (!ctx)
+ return -EINVAL;
+
+ return crypto_sig_maxsize(ctx->tfm);
+}
+#endif /* CONFIG_CRYPTO_ECDSA */
+
__bpf_kfunc_end_defs();

BTF_KFUNCS_START(crypt_init_kfunc_btf_ids)
BTF_ID_FLAGS(func, bpf_crypto_ctx_create, KF_ACQUIRE | KF_RET_NULL | KF_SLEEPABLE)
BTF_ID_FLAGS(func, bpf_crypto_ctx_release, KF_RELEASE)
BTF_ID_FLAGS(func, bpf_crypto_ctx_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
+#if IS_ENABLED(CONFIG_CRYPTO_ECDSA)
+BTF_ID_FLAGS(func, bpf_ecdsa_ctx_create, KF_ACQUIRE | KF_RET_NULL | KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_ecdsa_ctx_release, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_ecdsa_ctx_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
+#endif
BTF_KFUNCS_END(crypt_init_kfunc_btf_ids)

static const struct btf_kfunc_id_set crypt_init_kfunc_set = {
@@ -418,6 +628,12 @@ BTF_ID_FLAGS(func, bpf_crypto_encrypt, KF_RCU)
#if IS_ENABLED(CONFIG_CRYPTO_HASH2)
BTF_ID_FLAGS(func, bpf_crypto_hash, KF_RCU)
#endif
+#if IS_ENABLED(CONFIG_CRYPTO_ECDSA)
+BTF_ID_FLAGS(func, bpf_ecdsa_verify, 0)
+BTF_ID_FLAGS(func, bpf_ecdsa_keysize, 0)
+BTF_ID_FLAGS(func, bpf_ecdsa_digestsize, 0)
+BTF_ID_FLAGS(func, bpf_ecdsa_maxsize, 0)
+#endif
BTF_KFUNCS_END(crypt_kfunc_btf_ids)

static const struct btf_kfunc_id_set crypt_kfunc_set = {
@@ -428,6 +644,10 @@ static const struct btf_kfunc_id_set crypt_kfunc_set = {
BTF_ID_LIST(bpf_crypto_dtor_ids)
BTF_ID(struct, bpf_crypto_ctx)
BTF_ID(func, bpf_crypto_ctx_release)
+#if IS_ENABLED(CONFIG_CRYPTO_ECDSA)
+BTF_ID(struct, bpf_ecdsa_ctx)
+BTF_ID(func, bpf_ecdsa_ctx_release)
+#endif

static int __init crypto_kfunc_init(void)
{
@@ -437,6 +657,12 @@ static int __init crypto_kfunc_init(void)
.btf_id = bpf_crypto_dtor_ids[0],
.kfunc_btf_id = bpf_crypto_dtor_ids[1]
},
+#if IS_ENABLED(CONFIG_CRYPTO_ECDSA)
+ {
+ .btf_id = bpf_crypto_dtor_ids[2],
+ .kfunc_btf_id = bpf_crypto_dtor_ids[3]
+ },
+#endif
};

ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &crypt_kfunc_set);
@@ -445,6 +671,10 @@ static int __init crypto_kfunc_init(void)
ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &crypt_kfunc_set);
ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL,
&crypt_init_kfunc_set);
+ ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS,
+ &crypt_init_kfunc_set);
+ ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP,
+ &crypt_init_kfunc_set);
return ret ?: register_btf_id_dtor_kfuncs(bpf_crypto_dtors,
ARRAY_SIZE(bpf_crypto_dtors),
THIS_MODULE);
--
2.51.0