From ab673b987488c4fab7a0bc4824a48211f9d910e3 Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Tue, 21 Jul 2020 15:59:19 -0700 Subject: 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: e37a784d8b6a ("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 --- fs/crypto/keysetup.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) (limited to 'fs/crypto/keysetup.c') diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c index 7f85fc645602..fea6226afc2b 100644 --- a/fs/crypto/keysetup.c +++ b/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; /* -- cgit