From: Eric Biggers <ebiggers@xxxxxxxxxx> ->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@xxxxxxxxxx> --- This supersedes my earlier patch: "fscrypt: use proper memory barriers for ->i_crypt_info" https://patchwork.kernel.org/patch/10858269/ fs/crypto/crypto.c | 2 +- fs/crypto/fname.c | 4 ++-- fs/crypto/keyinfo.c | 4 ++-- fs/crypto/policy.c | 6 +++--- include/linux/fscrypt.h | 3 ++- 5 files changed, 10 insertions(+), 9 deletions(-) diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c index 5efc494a4e38f..0988077882574 100644 --- a/fs/crypto/crypto.c +++ b/fs/crypto/crypto.c @@ -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); /* diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c index 7ff40a73dbece..050384c79f40e 100644 --- a/fs/crypto/fname.c +++ b/fs/crypto/fname.c @@ -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)) diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c index 34c4682f23bee..82989098b2fc9 100644 --- a/fs/crypto/keyinfo.c +++ b/fs/crypto/keyinfo.c @@ -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) diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c index bd7eaf9b3f003..d536889ac31bf 100644 --- a/fs/crypto/policy.c +++ b/fs/crypto/policy.c @@ -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; diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h index 02c3e320f5b3f..48145beedaa7f 100644 --- a/include/linux/fscrypt.h +++ b/include/linux/fscrypt.h @@ -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) -- 2.21.0.392.gf8f6787159e-goog
