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| author | Eric Biggers <ebiggers@google.com> | 2020-07-21 15:59:18 -0700 |
|---|---|---|
| committer | Eric Biggers <ebiggers@google.com> | 2020-07-21 16:02:13 -0700 |
| commit | 777afe4e68d7ac37711d36098bea65650ec305a0 (patch) | |
| tree | a213425e6dcd1cacf1f37b086b186d0743ddee5b | |
| parent | 97c6327f7192d7312eabc033b26a41ad6816496c (diff) | |
| download | linux-stable-777afe4e68d7ac37711d36098bea65650ec305a0.tar.gz linux-stable-777afe4e68d7ac37711d36098bea65650ec305a0.tar.bz2 linux-stable-777afe4e68d7ac37711d36098bea65650ec305a0.zip | |
fscrypt: use smp_load_acquire() for ->s_master_keys
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")
Link: https://lore.kernel.org/r/20200721225920.114347-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
| -rw-r--r-- | fs/crypto/keyring.c | 15 |
1 files 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. */ |