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

PATCH Superseded Ard Biesheuvel 2 participants 2 messages Jun 5, 2021 – Jun 9, 2021 Crypto API

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Ard Biesheuvel

author

2021-06-05 06:59 UTC #0 lore raw

PATCH v2 1/1 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, which makes the problem go away.

Cc Sami Tolvanen <[email protected]>

Signed-off-by Ard Biesheuvel <[email protected]>

---
v2: add code comment to explain why the function needs to remain out of line

 crypto/shash.c                 | 20 +++++++++++++++++---
 include/crypto/internal/hash.h |  8 +-------
 2 files changed, 18 insertions(+), 10 deletions(-)

crypto/shash.c +17 -3

index 2e3433ad9762..36579c37e27d 100644
@@ -20,12 +20,26 @@
 
 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)
+{
+	/*
+	 * Function pointer comparisons such as the one below will not work as
+	 * expected when CFI is enabled, and the comparison involves an
+	 * exported symbol: as indirect function calls are routed via CFI stubs
+	 * that are private to each module, the pointer values may be different
+	 * even if they refer to the same function.
+	 *
+	 * Therefore, this function must remain out of line.
+	 */
+	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

Eric Biggers

2021-06-09 22:34 UTC #1 lore raw

Quoted 43 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.
Cc Sami Tolvanen <[email protected]>
Signed-off-by Ard Biesheuvel <[email protected]>
---
v2: add code comment to explain why the function needs to remain out of line
 crypto/shash.c                 | 20 +++++++++++++++++---
 include/crypto/internal/hash.h |  8 +-------
 2 files changed, 18 insertions(+), 10 deletions(-)
crypto/shash.c +7 -3
index 2e3433ad9762..36579c37e27d 100644
@@ -20,12 +20,26 @@
 
 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)
+{
+	/*
+	 * Function pointer comparisons such as the one below will not work as

crypto/shash.c line 42

Quoted 10 lines

crypto/shash.c +10 -0

+	 * expected when CFI is enabled, and the comparison involves an
+	 * exported symbol: as indirect function calls are routed via CFI stubs
+	 * that are private to each module, the pointer values may be different
+	 * even if they refer to the same function.
+	 *
+	 * Therefore, this function must remain out of line.
+	 */
+	return alg->setkey != shash_no_setkey;
+}
+EXPORT_SYMBOL_GPL(crypto_shash_alg_has_setkey);

Looks okay, but the comment isn't great since it doesn't get to the point until the last sentence. Also it talks about an exported symbol, which is confusing because shash_no_setkey won't actually be exported anymore. I'd prefer something like the following:

/*
* 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);