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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: 22d94f493b ("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>
This commit is contained in:
Eric Biggers 2020-07-21 15:59:18 -07:00
parent 97c6327f71
commit 777afe4e68
1 changed files with 12 additions and 3 deletions
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15
fs/crypto/keyring.c
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@ -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. */