lib/crypto: chacha20poly1305: Statically check fixed array lengths

Several parameters of the chacha20poly1305 functions require arrays of
an exact length. Use the new at_least keyword to instruct gcc and
clang to statically check that the caller is passing an object of at
least that length.

Here it is in action, with this faulty patch to wireguard's cookie.h:

     struct cookie_checker {
     	u8 secret[NOISE_HASH_LEN];
    -	u8 cookie_encryption_key[NOISE_SYMMETRIC_KEY_LEN];
    +	u8 cookie_encryption_key[NOISE_SYMMETRIC_KEY_LEN - 1];
     	u8 message_mac1_key[NOISE_SYMMETRIC_KEY_LEN];

If I try compiling this code, I get this helpful warning:

  CC      drivers/net/wireguard/cookie.o
drivers/net/wireguard/cookie.c: In function ‘wg_cookie_message_create’:
drivers/net/wireguard/cookie.c:193:9: warning: ‘xchacha20poly1305_encrypt’ reading 32 bytes from a region of size 31 [-Wstringop-overread]
  193 |         xchacha20poly1305_encrypt(dst->encrypted_cookie, cookie, COOKIE_LEN,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  194 |                                   macs->mac1, COOKIE_LEN, dst->nonce,
      |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  195 |                                   checker->cookie_encryption_key);
      |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireguard/cookie.c:193:9: note: referencing argument 7 of type ‘const u8 *’ {aka ‘const unsigned char *’}
In file included from drivers/net/wireguard/messages.h:10,
                 from drivers/net/wireguard/cookie.h:9,
                 from drivers/net/wireguard/cookie.c:6:
include/crypto/chacha20poly1305.h:28:6: note: in a call to function ‘xchacha20poly1305_encrypt’
   28 | void xchacha20poly1305_encrypt(u8 *dst, const u8 *src, const size_t src_len,

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20251123054819.2371989-4-Jason@zx2c4.com
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

diff --git a/include/crypto/chacha20poly1305.h b/include/crypto/chacha20poly1305.h
index d2ac3ff7dc1ec1beb4a924187c733934f3089d73..0f71b037702da2378b1d24d34659852e7f48cb12 100644 (file)
--- a/include/crypto/chacha20poly1305.h
+++ b/include/crypto/chacha20poly1305.h
@@ -18,32 +18,33 @@ enum chacha20poly1305_lengths {
 void chacha20poly1305_encrypt(u8 *dst, const u8 *src, const size_t src_len,
                              const u8 *ad, const size_t ad_len,
                              const u64 nonce,
-                             const u8 key[CHACHA20POLY1305_KEY_SIZE]);
+                             const u8 key[at_least CHACHA20POLY1305_KEY_SIZE]);
 
 bool __must_check
 chacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
                         const u8 *ad, const size_t ad_len, const u64 nonce,
-                        const u8 key[CHACHA20POLY1305_KEY_SIZE]);
+                        const u8 key[at_least CHACHA20POLY1305_KEY_SIZE]);
 
 void xchacha20poly1305_encrypt(u8 *dst, const u8 *src, const size_t src_len,
                               const u8 *ad, const size_t ad_len,
-                              const u8 nonce[XCHACHA20POLY1305_NONCE_SIZE],
-                              const u8 key[CHACHA20POLY1305_KEY_SIZE]);
+                              const u8 nonce[at_least XCHACHA20POLY1305_NONCE_SIZE],
+                              const u8 key[at_least CHACHA20POLY1305_KEY_SIZE]);
 
 bool __must_check xchacha20poly1305_decrypt(
-       u8 *dst, const u8 *src, const size_t src_len, const u8 *ad,
-       const size_t ad_len, const u8 nonce[XCHACHA20POLY1305_NONCE_SIZE],
-       const u8 key[CHACHA20POLY1305_KEY_SIZE]);
+       u8 *dst, const u8 *src, const size_t src_len,
+       const u8 *ad, const size_t ad_len,
+       const u8 nonce[at_least XCHACHA20POLY1305_NONCE_SIZE],
+       const u8 key[at_least CHACHA20POLY1305_KEY_SIZE]);
 
 bool chacha20poly1305_encrypt_sg_inplace(struct scatterlist *src, size_t src_len,
                                         const u8 *ad, const size_t ad_len,
                                         const u64 nonce,
-                                        const u8 key[CHACHA20POLY1305_KEY_SIZE]);
+                                        const u8 key[at_least CHACHA20POLY1305_KEY_SIZE]);
 
 bool chacha20poly1305_decrypt_sg_inplace(struct scatterlist *src, size_t src_len,
                                         const u8 *ad, const size_t ad_len,
                                         const u64 nonce,
-                                        const u8 key[CHACHA20POLY1305_KEY_SIZE]);
+                                        const u8 key[at_least CHACHA20POLY1305_KEY_SIZE]);
 
 bool chacha20poly1305_selftest(void);
 

diff --git a/lib/crypto/chacha20poly1305.c b/lib/crypto/chacha20poly1305.c
index 0b49d6aedefdd9cb819b49a1e8883785f79e530d..212ce33562afd51284d996c62ea63ca6cd6556cb 100644 (file)
--- a/lib/crypto/chacha20poly1305.c
+++ b/lib/crypto/chacha20poly1305.c
@@ -89,7 +89,7 @@ __chacha20poly1305_encrypt(u8 *dst, const u8 *src, const size_t src_len,
 void chacha20poly1305_encrypt(u8 *dst, const u8 *src, const size_t src_len,
                              const u8 *ad, const size_t ad_len,
                              const u64 nonce,
-                             const u8 key[CHACHA20POLY1305_KEY_SIZE])
+                             const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
 {
        struct chacha_state chacha_state;
        u32 k[CHACHA_KEY_WORDS];
@@ -111,8 +111,8 @@ EXPORT_SYMBOL(chacha20poly1305_encrypt);
 
 void xchacha20poly1305_encrypt(u8 *dst, const u8 *src, const size_t src_len,
                               const u8 *ad, const size_t ad_len,
-                              const u8 nonce[XCHACHA20POLY1305_NONCE_SIZE],
-                              const u8 key[CHACHA20POLY1305_KEY_SIZE])
+                              const u8 nonce[at_least XCHACHA20POLY1305_NONCE_SIZE],
+                              const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
 {
        struct chacha_state chacha_state;
 
@@ -170,7 +170,7 @@ __chacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
 bool chacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
                              const u8 *ad, const size_t ad_len,
                              const u64 nonce,
-                             const u8 key[CHACHA20POLY1305_KEY_SIZE])
+                             const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
 {
        struct chacha_state chacha_state;
        u32 k[CHACHA_KEY_WORDS];
@@ -195,8 +195,8 @@ EXPORT_SYMBOL(chacha20poly1305_decrypt);
 
 bool xchacha20poly1305_decrypt(u8 *dst, const u8 *src, const size_t src_len,
                               const u8 *ad, const size_t ad_len,
-                              const u8 nonce[XCHACHA20POLY1305_NONCE_SIZE],
-                              const u8 key[CHACHA20POLY1305_KEY_SIZE])
+                              const u8 nonce[at_least XCHACHA20POLY1305_NONCE_SIZE],
+                              const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
 {
        struct chacha_state chacha_state;
 
@@ -211,7 +211,7 @@ bool chacha20poly1305_crypt_sg_inplace(struct scatterlist *src,
                                       const size_t src_len,
                                       const u8 *ad, const size_t ad_len,
                                       const u64 nonce,
-                                      const u8 key[CHACHA20POLY1305_KEY_SIZE],
+                                      const u8 key[at_least CHACHA20POLY1305_KEY_SIZE],
                                       int encrypt)
 {
        const u8 *pad0 = page_address(ZERO_PAGE(0));
@@ -335,7 +335,7 @@ bool chacha20poly1305_crypt_sg_inplace(struct scatterlist *src,
 bool chacha20poly1305_encrypt_sg_inplace(struct scatterlist *src, size_t src_len,
                                         const u8 *ad, const size_t ad_len,
                                         const u64 nonce,
-                                        const u8 key[CHACHA20POLY1305_KEY_SIZE])
+                                        const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
 {
        return chacha20poly1305_crypt_sg_inplace(src, src_len, ad, ad_len,
                                                 nonce, key, 1);
@@ -345,7 +345,7 @@ EXPORT_SYMBOL(chacha20poly1305_encrypt_sg_inplace);
 bool chacha20poly1305_decrypt_sg_inplace(struct scatterlist *src, size_t src_len,
                                         const u8 *ad, const size_t ad_len,
                                         const u64 nonce,
-                                        const u8 key[CHACHA20POLY1305_KEY_SIZE])
+                                        const u8 key[at_least CHACHA20POLY1305_KEY_SIZE])
 {
        if (unlikely(src_len < POLY1305_DIGEST_SIZE))
                return false;