fscrypt_find_mode_key() walks an RCU list and hands out a per-mode key without an acquire: setup task ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ setup_per_mode_enc_key() ├─ fscrypt_prepare_key() │ └─ fscrypt_prepare_inline_crypt_key() │ └─ blk_crypto_start_using_key() │ └─ blk_crypto_fallback_start_using_mode() │ ├─ init profile, keyslots, tfms[] │ └─ smp_store_release(tfms_inited[mode]) # init order └─ list_add_tail_rcu(&new_node->link, ...) # release -> finder finding task (later submits the I/O itself) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ fscrypt_find_mode_key() ├─ list_for_each_entry_rcu(node, ...) └─ return &node->key # no acquire, chain breaks ... blk_crypto_fallback_bio_prep() # WRITE path ├─ tfms_inited[mode] # addr-dep, reads true ├─ blk_crypto_fallback_profile # no dependency ├─ blk_crypto_keyslots # no dependency └─ slotp->tfms[mode] # no dependency RCU publishes the node and the key reachable from it, so tfms_inited[mode] reads true through the address dependency. The independent fallback globals are not covered by it and can be read as uninitialized. On a weakly ordered machine, a WRITE bio can crash in blk_crypto_get_keyslot() or crypto_sync_skcipher_setkey(). Other tasks reach the key through fscrypt_inode_info's cmpxchg_release()/smp_load_acquire(), which is already ordered after the list walk; only the finding task's own later I/O was not. Fix this by upgrading the control dependency to an acquire with smp_acquire__after_ctrl_dep() once a matching entry has been found. Fixes: dd015b566d50 ("fscrypt: Fix key setup in edge case with multiple data unit sizes") Cc: stable@vger.kernel.org Suggested-by: Eric Biggers Link: https://lore.kernel.org/r/20261014161153.GC322435@quark Signed-off-by: Baokun Li --- Changes since v3: * Update commit message and comments as suggested by Eric. v3: https://patch.msgid.link/20261023160747.1382973-1-libaokun@linux.alibaba.com fs/crypto/keysetup.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c index 892044ebcaca..a77b889d7578 100644 --- a/fs/crypto/keysetup.c +++ b/fs/crypto/keysetup.c @@ -205,8 +205,14 @@ fscrypt_find_mode_key(struct fscrypt_master_key *mk, u8 hkdf_context, if (node->hkdf_context == hkdf_context && node->mode_num == mode_num && node->data_unit_bits == ci->ci_data_unit_bits && - fscrypt_is_key_prepared(&node->key, ci)) + fscrypt_is_key_prepared(&node->key, ci)) { + /* + * Upgrade the RCU dereference to a full ACQUIRE, so + * that blk-crypto global state is seen as initialized. + */ + smp_acquire__after_ctrl_dep(); return &node->key; + } } return NULL; } base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b -- 2.43.7