crypto: shash - avoid comparing pointers to exported functions under CFI

crypto_shash_alg_has_setkey() is implemented by testing whether the
.setkey() member of a struct shash_alg points to the default version,
called shash_no_setkey(). As crypto_shash_alg_has_setkey() is a static
inline, this requires shash_no_setkey() to be exported to modules.

Unfortunately, when building with CFI, function pointers are routed
via CFI stubs which are private to each module (or to the kernel proper)
and so this function pointer comparison may fail spuriously.

Let's fix this by turning crypto_shash_alg_has_setkey() into an out of
line function.

Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/shash.c

@@ -20,12 +20,24 @@
 
 static const struct crypto_type crypto_shash_type;
 
-int shash_no_setkey(struct crypto_shash *tfm, const u8 *key,
-		    unsigned int keylen)
+static int shash_no_setkey(struct crypto_shash *tfm, const u8 *key,
+			   unsigned int keylen)
 {
 	return -ENOSYS;
 }
-EXPORT_SYMBOL_GPL(shash_no_setkey);
+
+/*
+ * Check whether an shash algorithm has a setkey function.
+ *
+ * For CFI compatibility, this must not be an inline function.  This is because
+ * when CFI is enabled, modules won't get the same address for shash_no_setkey
+ * (if it were exported, which inlining would require) as the core kernel will.
+ */
+bool crypto_shash_alg_has_setkey(struct shash_alg *alg)
+{
+	return alg->setkey != shash_no_setkey;
+}
+EXPORT_SYMBOL_GPL(crypto_shash_alg_has_setkey);
 
 static int shash_setkey_unaligned(struct crypto_shash *tfm, const u8 *key,
 				  unsigned int keylen)

include/crypto/internal/hash.h

@@ -75,13 +75,7 @@ void crypto_unregister_ahashes(struct ahash_alg *algs, int count);
 int ahash_register_instance(struct crypto_template *tmpl,
 			    struct ahash_instance *inst);
 
-int shash_no_setkey(struct crypto_shash *tfm, const u8 *key,
-		    unsigned int keylen);
-
-static inline bool crypto_shash_alg_has_setkey(struct shash_alg *alg)
-{
-	return alg->setkey != shash_no_setkey;
-}
+bool crypto_shash_alg_has_setkey(struct shash_alg *alg);
 
 static inline bool crypto_shash_alg_needs_key(struct shash_alg *alg)
 {