Commit e37a784d authored by Eric Biggers's avatar Eric Biggers Committed by Theodore Ts'o
Browse files

fscrypt: use READ_ONCE() to access ->i_crypt_info 



->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: default avatarEric Biggers <ebiggers@google.com>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent ff5d3a97

fs/crypto/crypto.c

+1 −1
Original line number Diff line number Diff line
@@ -328,7 +328,7 @@ static int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags) 
	spin_lock(&dentry->d_lock);

	cached_with_key = dentry->d_flags & DCACHE_ENCRYPTED_WITH_KEY;

	spin_unlock(&dentry->d_lock);

	dir_has_key = (d_inode(dir)->i_crypt_info != NULL);

	dir_has_key = fscrypt_has_encryption_key(d_inode(dir));

	dput(dir);



	/*

fs/crypto/fname.c

+2 −2
Original line number Diff line number Diff line
@@ -269,7 +269,7 @@ int fscrypt_fname_disk_to_usr(struct inode *inode, 
	if (iname->len < FS_CRYPTO_BLOCK_SIZE)

		return -EUCLEAN;



	if (inode->i_crypt_info)

	if (fscrypt_has_encryption_key(inode))

		return fname_decrypt(inode, iname, oname);



	if (iname->len <= FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE) {
@@ -336,7 +336,7 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname, 
	if (ret)

		return ret;



	if (dir->i_crypt_info) {

	if (fscrypt_has_encryption_key(dir)) {

		if (!fscrypt_fname_encrypted_size(dir, iname->len,

						  dir->i_sb->s_cop->max_namelen,

						  &fname->crypto_buf.len))

fs/crypto/keyinfo.c

+2 −2
Original line number Diff line number Diff line
@@ -509,7 +509,7 @@ int fscrypt_get_encryption_info(struct inode *inode) 
	u8 *raw_key = NULL;

	int res;



	if (inode->i_crypt_info)

	if (fscrypt_has_encryption_key(inode))

		return 0;



	res = fscrypt_initialize(inode->i_sb->s_cop->flags);
@@ -573,7 +573,7 @@ int fscrypt_get_encryption_info(struct inode *inode) 
	if (res)

		goto out;



	if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)

	if (cmpxchg_release(&inode->i_crypt_info, NULL, crypt_info) == NULL)

		crypt_info = NULL;

out:

	if (res == -ENOKEY)

fs/crypto/policy.c

+3 −3
Original line number Diff line number Diff line
@@ -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;

include/linux/fscrypt.h

+2 −1
Original line number Diff line number Diff line
@@ -79,7 +79,8 @@ struct fscrypt_ctx { 


static inline bool fscrypt_has_encryption_key(const struct inode *inode)

{

	return (inode->i_crypt_info != NULL);

	/* pairs with cmpxchg_release() in fscrypt_get_encryption_info() */

	return READ_ONCE(inode->i_crypt_info) != NULL;

}



static inline bool fscrypt_dummy_context_enabled(struct inode *inode)

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