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

PATCH In review Ard Biesheuvel 2 participants 2 messages Jun 4, 2021 Crypto API

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Ard Biesheuvel

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PATCH 1/1 crypto: shash - stop comparing function pointers to avoid breaking 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, which makes the problem go away.

Cc Sami Tolvanen <[email protected]>
Signed-off-by Ard Biesheuvel <[email protected]>
---
 crypto/shash.c                 | 11 ++++++++---
 include/crypto/internal/hash.h |  8 +-------
 2 files changed, 9 insertions(+), 10 deletions(-)
crypto/shash.c +8 -3
index 2e3433ad9762..e0ddfcd13ac7 100644
@@ -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)
include/crypto/internal/hash.h +1 -7
index 0a288dddcf5b..25806141db59 100644
@@ -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)
 {
Signature · 1 line
2.30.2
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#1 lore raw
Quoted 12 lines · 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.

Quoted 6 lines
Cc Sami Tolvanen <[email protected]>
Signed-off-by Ard Biesheuvel <[email protected]>
---
 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.

crypto/shash.c line 33

Quoted 5 lines
crypto/shash.c +5 -0
+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,

Signature · 3 lines
Email: Herbert Xu <[email protected]>
Home Page: http://gondor.apana.org.au/~herbert/
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