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fscrypt: use READ_ONCE() to access ->i_crypt_info

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->i_crypt_info starts out NULL and may later be locklessly set to a
non-NULL value by the cmpxchg() in fscrypt_get_encryption_info().

But ->i_crypt_info is used directly, which technically is incorrect.
It's a data race, and it doesn't include the data dependency barrier
needed to safely dereference the pointer on at least one architecture.

Fix this by using READ_ONCE() instead.  Note: we don't need to use
smp_load_acquire(), since dereferencing the pointer only requires a data
dependency barrier, which is already included in READ_ONCE().  We also
don't need READ_ONCE() in places where ->i_crypt_info is unconditionally
dereferenced, since it must have already been checked.

Also downgrade the cmpxchg() to cmpxchg_release(), since RELEASE
semantics are sufficient on the write side.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Eric Biggers
2019-04-11 14:32:15 -07:00
committed by Theodore Ts'o
parent ff5d3a9707
commit e37a784d8b
5 changed files with 10 additions and 9 deletions
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6
fs/crypto/policy.c
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@@ -194,8 +194,8 @@ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child)
res = fscrypt_get_encryption_info(child);
if (res)
return 0;
parent_ci = parent->i_crypt_info;
child_ci = child->i_crypt_info;
parent_ci = READ_ONCE(parent->i_crypt_info);
child_ci = READ_ONCE(child->i_crypt_info);
if (parent_ci && child_ci) {
return memcmp(parent_ci->ci_master_key_descriptor,
@@ -246,7 +246,7 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child,
if (res < 0)
return res;
ci = parent->i_crypt_info;
ci = READ_ONCE(parent->i_crypt_info);
if (ci == NULL)
return -ENOKEY;