AES: Add function pointer trampoline to avoid delocator issue

On AArch64, the delocator can patch up the computation of function
pointers only if the pointers can be computed with a PC-relative
offset in the range (-1MB, 1MB).

For the function pointer computations in aes/mode_wrappers.c,
this bounds condition is about to be violated by further code
additions to AWS-LC, as witnessed in AES-unrelated PRs.

This commit preventatively fixes the issue by adding function
pointer trampolines to crypto/fipsmodule/aes/mode_wrappers.c:
These are stub functions immediately branching into the desired
assembly routines, but close enough to the C code computing their
address to ensure that their addresses will be computable using
a PC-relative offset.

This fix is similar to #2165.

Mid/Long-term, it should be considered whether the delocator
can introduce the necessary indirections automatically.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker

crypto/fipsmodule/aes/mode_wrappers.c

@@ -54,6 +54,29 @@
 #include "../modes/internal.h"
 #include "../cipher/internal.h"
 
+// The following wrappers ensure that the delocator can handle the
+// function pointer calculation in AES_ctr128_encrypt. Without it,
+// on AArch64 there is risk of the calculations requiring a PC-relative
+// offset outside of the range (-1MB,1MB) addressable using `ADR`.
+//
+// We don't add a wrapper for aes_hw_ctr32_encrypt_blocks which is
+// used for X86 only and does not seem to face the same issue.
+#if defined(VPAES)
+#if defined(VPAES_CTR32)
+static inline void vpaes_ctr32_encrypt_blocks_wrapper(const uint8_t *in,
+                                                      uint8_t *out, size_t len,
+                                                      const AES_KEY *key,
+                                                      const uint8_t ivec[16]) {
+  vpaes_ctr32_encrypt_blocks(in, out, len, key, ivec);
+}
+#else // VPAES_CTR32
+static inline void vpaes_encrypt_wrapper(const uint8_t *in, uint8_t *out,
+                                         const AES_KEY *key) {
+  vpaes_encrypt(in, out, key);
+}
+#endif // !VPAES_CTR32
+#endif // VPAES
+
 void AES_ctr128_encrypt(const uint8_t *in, uint8_t *out, size_t len,
                         const AES_KEY *key, uint8_t ivec[AES_BLOCK_SIZE],
                         uint8_t ecount_buf[AES_BLOCK_SIZE], unsigned int *num) {
@@ -65,10 +88,10 @@ void AES_ctr128_encrypt(const uint8_t *in, uint8_t *out, size_t len,
     // TODO(davidben): On ARM, where |BSAES| is additionally defined, this could
     // use |vpaes_ctr32_encrypt_blocks_with_bsaes|.
     CRYPTO_ctr128_encrypt_ctr32(in, out, len, key, ivec, ecount_buf, num,
-                                vpaes_ctr32_encrypt_blocks);
+                                vpaes_ctr32_encrypt_blocks_wrapper);
 #else
     CRYPTO_ctr128_encrypt(in, out, len, key, ivec, ecount_buf, num,
-                          vpaes_encrypt);
+                          vpaes_encrypt_wrapper);
 #endif
   } else {
     CRYPTO_ctr128_encrypt_ctr32(in, out, len, key, ivec, ecount_buf, num,