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:
parent
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@ -213,7 +213,11 @@ static int allocate_filesystem_keyring(struct super_block *sb)
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if (IS_ERR(keyring))
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return PTR_ERR(keyring);
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/* Pairs with READ_ONCE() in fscrypt_find_master_key() */
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/*
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* Pairs with the smp_load_acquire() in fscrypt_find_master_key().
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* I.e., here we publish ->s_master_keys with a RELEASE barrier so that
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* concurrent tasks can ACQUIRE it.
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*/
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smp_store_release(&sb->s_master_keys, keyring);
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return 0;
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}
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@ -234,8 +238,13 @@ struct key *fscrypt_find_master_key(struct super_block *sb,
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struct key *keyring;
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char description[FSCRYPT_MK_DESCRIPTION_SIZE];
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/* pairs with smp_store_release() in allocate_filesystem_keyring() */
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keyring = READ_ONCE(sb->s_master_keys);
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/*
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* Pairs with the smp_store_release() in allocate_filesystem_keyring().
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* I.e., another task can publish ->s_master_keys concurrently,
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* executing a RELEASE barrier. We need to use smp_load_acquire() here
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* to safely ACQUIRE the memory the other task published.
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*/
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keyring = smp_load_acquire(&sb->s_master_keys);
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if (keyring == NULL)
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return ERR_PTR(-ENOKEY); /* No keyring yet, so no keys yet. */
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