[PATCH] crypto: shash - stop comparing function pointers to avoid breaking CFI

[PATCH] crypto: shash - stop comparing function pointers to avoid breaking CFI

[Ard Biesheuvel]

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, which makes the problem go away.

Cc: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 crypto/shash.c                 | 11 ++++++++---
 include/crypto/internal/hash.h |  8 +-------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/crypto/shash.c b/crypto/shash.c
index 2e3433ad9762..e0ddfcd13ac7 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -20,12 +20,17 @@
 
 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);
+
+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)
diff --git a/include/crypto/internal/hash.h b/include/crypto/internal/hash.h
index 0a288dddcf5b..25806141db59 100644
--- a/include/crypto/internal/hash.h
+++ b/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)
 {
-- 
2.30.2

Re: [PATCH] crypto: shash - stop comparing function pointers to avoid breaking CFI

[Herbert Xu]

On Fri, Jun 04, 2021 at 09:00:09PM +0200, Ard Biesheuvel wrote:
> 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, which makes the problem go away.
> 
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  crypto/shash.c                 | 11 ++++++++---
>  include/crypto/internal/hash.h |  8 +-------
>  2 files changed, 9 insertions(+), 10 deletions(-)

I think this deserves a comment in the code.

> +bool crypto_shash_alg_has_setkey(struct shash_alg *alg)
> +{
> +       return alg->setkey != shash_no_setkey;
> +}
> +EXPORT_SYMBOL_GPL(crypto_shash_alg_has_setkey);

This would be a good spot for the comment so someone doesn't try
to make it inline again later.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt