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Commit 7a41c576 authored by Eric Biggers's avatar Eric Biggers Committed by Greg Kroah-Hartman
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UPSTREAM: 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: default avatarEric Biggers <ebiggers@google.com>
(cherry picked from commit 97c6327f7192d7312eabc033b26a41ad6816496c)
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@google.com>
Change-Id: I08747eb4b756d1352d4cd385aff232c2a0a2b405
parent 1091b1b9

fs/crypto/fscrypt_private.h

+9 −6
Original line number Diff line number Diff line
@@ -362,13 +362,16 @@ 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 */
@@ -415,7 +418,7 @@ 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 */

fs/crypto/inline_crypt.c

+4 −2
Original line number Diff line number Diff line
@@ -202,8 +202,10 @@ 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;

fs/crypto/keysetup.c

+4 −2
Original line number Diff line number Diff line
@@ -134,8 +134,10 @@ 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;