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authorEric Biggers <ebiggers@google.com>2020-07-21 15:59:17 -0700
committerEric Biggers <ebiggers@google.com>2020-07-21 16:02:13 -0700
commit97c6327f7192d7312eabc033b26a41ad6816496c (patch)
tree07b36406f882ba6ad91a800c832c8436417f9eb1 /fs/crypto
parentbd0d97b7191e8f3573681fa854fdb04c1a970c1e (diff)
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 <satyat@google.com> Link: https://lore.kernel.org/r/20200721225920.114347-3-ebiggers@kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com>
Diffstat (limited to 'fs/crypto')
-rw-r--r--fs/crypto/fscrypt_private.h15
-rw-r--r--fs/crypto/inline_crypt.c6
-rw-r--r--fs/crypto/keysetup.c6
3 files changed, 17 insertions, 10 deletions
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index bc1a3fcd45ed..8117a61b6f55 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -351,13 +351,16 @@ fscrypt_is_key_prepared(struct fscrypt_prepared_key *prep_key,
const struct fscrypt_info *ci)
{
/*
- * The READ_ONCE() here pairs with the smp_store_release() in
- * fscrypt_prepare_key(). (This only matters for the per-mode keys,
- * which are shared by multiple inodes.)
+ * The two smp_load_acquire()'s here pair with the smp_store_release()'s
+ * in fscrypt_prepare_inline_crypt_key() and fscrypt_prepare_key().
+ * I.e., in some cases (namely, if this prep_key is a per-mode
+ * encryption key) another task can publish blk_key or tfm concurrently,
+ * executing a RELEASE barrier. We need to use smp_load_acquire() here
+ * to safely ACQUIRE the memory the other task published.
*/
if (fscrypt_using_inline_encryption(ci))
- return READ_ONCE(prep_key->blk_key) != NULL;
- return READ_ONCE(prep_key->tfm) != NULL;
+ return smp_load_acquire(&prep_key->blk_key) != NULL;
+ return smp_load_acquire(&prep_key->tfm) != NULL;
}
#else /* CONFIG_FS_ENCRYPTION_INLINE_CRYPT */
@@ -391,7 +394,7 @@ static inline bool
fscrypt_is_key_prepared(struct fscrypt_prepared_key *prep_key,
const struct fscrypt_info *ci)
{
- return READ_ONCE(prep_key->tfm) != NULL;
+ return smp_load_acquire(&prep_key->tfm) != NULL;
}
#endif /* !CONFIG_FS_ENCRYPTION_INLINE_CRYPT */
diff --git a/fs/crypto/inline_crypt.c b/fs/crypto/inline_crypt.c
index d7aecadf33c1..dfb06375099a 100644
--- a/fs/crypto/inline_crypt.c
+++ b/fs/crypto/inline_crypt.c
@@ -176,8 +176,10 @@ int fscrypt_prepare_inline_crypt_key(struct fscrypt_prepared_key *prep_key,
}
}
/*
- * 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 ->blk_key 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->blk_key, blk_key);
return 0;
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;