From 97c6327f7192d7312eabc033b26a41ad6816496c Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Tue, 21 Jul 2020 15:59:17 -0700 Subject: fscrypt: use smp_load_acquire() for fscrypt_prepared_key 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_prepared_key includes a pointer to a crypto_skcipher object, which is internal to and is allocated by the crypto subsystem. By using READ_ONCE() for it, we're relying on internal implementation details of the crypto 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: 5fee36095cda ("fscrypt: add inline encryption support") Cc: Satya Tangirala Link: https://lore.kernel.org/r/20200721225920.114347-3-ebiggers@kernel.org Signed-off-by: Eric Biggers --- fs/crypto/keysetup.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'fs/crypto/keysetup.c') diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c index 22a94b18fe70..7f85fc645602 100644 --- a/fs/crypto/keysetup.c +++ b/fs/crypto/keysetup.c @@ -129,8 +129,10 @@ int fscrypt_prepare_key(struct fscrypt_prepared_key *prep_key, if (IS_ERR(tfm)) return PTR_ERR(tfm); /* - * Pairs with READ_ONCE() in fscrypt_is_key_prepared(). (Only matters - * for the per-mode keys, which are shared by multiple inodes.) + * Pairs with the smp_load_acquire() in fscrypt_is_key_prepared(). + * I.e., here we publish ->tfm with a RELEASE barrier so that + * concurrent tasks can ACQUIRE it. Note that this concurrency is only + * possible for per-mode keys, not for per-file keys. */ smp_store_release(&prep_key->tfm, tfm); return 0; -- cgit