From: Eric Biggers <ebiggers@xxxxxxxxxx>
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.
super_block::s_master_keys is a keyring, which is internal to and is
allocated by the keyrings subsystem. By using READ_ONCE() for it, we're
relying on internal implementation details of the keyrings subsystem.
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: 22d94f493bfb ("fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
fs/crypto/keyring.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
index 7f8ac61a20d6..71d56f8e2870 100644
--- a/fs/crypto/keyring.c
+++ b/fs/crypto/keyring.c
@@ -213,7 +213,11 @@ static int allocate_filesystem_keyring(struct super_block *sb)
if (IS_ERR(keyring))
return PTR_ERR(keyring);
- /* Pairs with READ_ONCE() in fscrypt_find_master_key() */
+ /*
+ * Pairs with the smp_load_acquire() in fscrypt_find_master_key().
+ * I.e., here we publish ->s_master_keys with a RELEASE barrier so that
+ * concurrent tasks can ACQUIRE it.
+ */
smp_store_release(&sb->s_master_keys, keyring);
return 0;
}
@@ -234,8 +238,13 @@ struct key *fscrypt_find_master_key(struct super_block *sb,
struct key *keyring;
char description[FSCRYPT_MK_DESCRIPTION_SIZE];
- /* pairs with smp_store_release() in allocate_filesystem_keyring() */
- keyring = READ_ONCE(sb->s_master_keys);
+ /*
+ * Pairs with the smp_store_release() in allocate_filesystem_keyring().
+ * I.e., another task can publish ->s_master_keys concurrently,
+ * executing a RELEASE barrier. We need to use smp_load_acquire() here
+ * to safely ACQUIRE the memory the other task published.
+ */
+ keyring = smp_load_acquire(&sb->s_master_keys);
if (keyring == NULL)
return ERR_PTR(-ENOKEY); /* No keyring yet, so no keys yet. */
--
2.27.0
