fscrypt: use smp_load_acquire() for ->i_crypt_info

Normally smp_store_release() or cmpxchg_release() is paired with
smp_load_acquire().  Sometimes smp_load_acquire() can be replaced with
the more lightweight READ_ONCE().  However, for this to be safe, all the
published memory must only be accessed in a way that involves the
pointer itself.  This may not be the case if allocating the object also
involves initializing a static or global variable, for example.

fscrypt_info includes various sub-objects which are internal to and are
allocated by other kernel subsystems such as keyrings and crypto.  So by
using READ_ONCE() for ->i_crypt_info, we're relying on internal
implementation details of these other kernel subsystems.

Remove this fragile assumption by using smp_load_acquire() instead.

(Note: I haven't seen any real-world problems here.  This change is just
fixing the code to be guaranteed correct and less fragile.)

Fixes: e37a784d8b ("fscrypt: use READ_ONCE() to access ->i_crypt_info")
Link: https://lore.kernel.org/r/20200721225920.114347-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/keysetup.c

@@ -518,7 +518,17 @@ int fscrypt_get_encryption_info(struct inode *inode)
 	if (res)
 		goto out;
 
+	/*
+	 * Multiple tasks may race to set ->i_crypt_info, so use
+	 * cmpxchg_release().  This pairs with the smp_load_acquire() in
+	 * fscrypt_get_info().  I.e., here we publish ->i_crypt_info with a
+	 * RELEASE barrier so that other tasks can ACQUIRE it.
+	 */
 	if (cmpxchg_release(&inode->i_crypt_info, NULL, crypt_info) == NULL) {
+		/*
+		 * We won the race and set ->i_crypt_info to our crypt_info.
+		 * Now link it into the master key's inode list.
+		 */
 		if (master_key) {
 			struct fscrypt_master_key *mk =
 				master_key->payload.data[0];
@@ -589,7 +599,7 @@ EXPORT_SYMBOL(fscrypt_free_inode);
  */
 int fscrypt_drop_inode(struct inode *inode)
 {
-	const struct fscrypt_info *ci = READ_ONCE(inode->i_crypt_info);
+	const struct fscrypt_info *ci = fscrypt_get_info(inode);
 	const struct fscrypt_master_key *mk;
 
 	/*

fs/crypto/policy.c

@@ -352,7 +352,7 @@ static int fscrypt_get_policy(struct inode *inode, union fscrypt_policy *policy)
 	union fscrypt_context ctx;
 	int ret;
 
-	ci = READ_ONCE(inode->i_crypt_info);
+	ci = fscrypt_get_info(inode);
 	if (ci) {
 		/* key available, use the cached policy */
 		*policy = ci->ci_policy;
@@ -641,7 +641,7 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child,
 	if (res < 0)
 		return res;
 
-	ci = READ_ONCE(parent->i_crypt_info);
+	ci = fscrypt_get_info(parent);
 	if (ci == NULL)
 		return -ENOKEY;
 

include/linux/fscrypt.h

@@ -74,10 +74,15 @@ struct fscrypt_operations {
 			    struct request_queue **devs);
 };
 
-static inline bool fscrypt_has_encryption_key(const struct inode *inode)
+static inline struct fscrypt_info *fscrypt_get_info(const struct inode *inode)
 {
-	/* pairs with cmpxchg_release() in fscrypt_get_encryption_info() */
+	/*
-	return READ_ONCE(inode->i_crypt_info) != NULL;
+	 * Pairs with the cmpxchg_release() in fscrypt_get_encryption_info().
+	 * I.e., another task may publish ->i_crypt_info concurrently, executing
+	 * a RELEASE barrier.  We need to use smp_load_acquire() here to safely
+	 * ACQUIRE the memory the other task published.
+	 */
+	return smp_load_acquire(&inode->i_crypt_info);
 }
 
 /**
@@ -234,9 +239,9 @@ static inline void fscrypt_set_ops(struct super_block *sb,
 }
 #else  /* !CONFIG_FS_ENCRYPTION */
 
-static inline bool fscrypt_has_encryption_key(const struct inode *inode)
+static inline struct fscrypt_info *fscrypt_get_info(const struct inode *inode)
 {
-	return false;
+	return NULL;
 }
 
 static inline bool fscrypt_needs_contents_encryption(const struct inode *inode)
@@ -619,6 +624,20 @@ static inline bool fscrypt_inode_uses_fs_layer_crypto(const struct inode *inode)
 	       !__fscrypt_inode_uses_inline_crypto(inode);
 }
 
+/**
+ * fscrypt_has_encryption_key() - check whether an inode has had its key set up
+ * @inode: the inode to check
+ *
+ * Return: %true if the inode has had its encryption key set up, else %false.
+ *
+ * Usually this should be preceded by fscrypt_get_encryption_info() to try to
+ * set up the key first.
+ */
+static inline bool fscrypt_has_encryption_key(const struct inode *inode)
+{
+	return fscrypt_get_info(inode) != NULL;
+}
+
 /**
  * fscrypt_require_key() - require an inode's encryption key
  * @inode: the inode we need the key for