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Subject[PATCH] security: don't use RCU accessors for cred->session_keyring
From
sparse complains that a bunch of places in kernel/cred.c access
cred->session_keyring without the RCU helpers required by the __rcu
annotation.

cred->session_keyring is written in the following places:

- prepare_kernel_cred() [in a new cred struct]
- keyctl_session_to_parent() [in a new cred struct]
- prepare_creds [in a new cred struct, via memcpy]
- install_session_keyring_to_cred()
- from install_session_keyring() on new creds
- from join_session_keyring() on new creds [twice]
- from umh_keys_init()
- from call_usermodehelper_exec_async() on new creds

All of these writes are before the creds are committed; therefore,
cred->session_keyring doesn't need RCU protection.

Remove the __rcu annotation and fix up all existing users that use __rcu.

Signed-off-by: Jann Horn <jannh@google.com>
---
include/linux/cred.h | 2 +-
security/keys/process_keys.c | 12 ++++--------
security/keys/request_key.c | 9 ++-------
3 files changed, 7 insertions(+), 16 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index ddd45bb74887..efb6edf32de7 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -138,7 +138,7 @@ struct cred {
#ifdef CONFIG_KEYS
unsigned char jit_keyring; /* default keyring to attach requested
* keys to */
- struct key __rcu *session_keyring; /* keyring inherited over fork */
+ struct key *session_keyring; /* keyring inherited over fork */
struct key *process_keyring; /* keyring private to this process */
struct key *thread_keyring; /* keyring private to this thread */
struct key *request_key_auth; /* assumed request_key authority */
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 9320424c4a46..bd7243cb4c85 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -227,6 +227,7 @@ static int install_process_keyring(void)
* Install the given keyring as the session keyring of the given credentials
* struct, replacing the existing one if any. If the given keyring is NULL,
* then install a new anonymous session keyring.
+ * @cred can not be in use by any task yet.
*
* Return: 0 on success; -errno on failure.
*/
@@ -254,7 +255,7 @@ int install_session_keyring_to_cred(struct cred *cred, struct key *keyring)

/* install the keyring */
old = cred->session_keyring;
- rcu_assign_pointer(cred->session_keyring, keyring);
+ cred->session_keyring = keyring;

if (old)
key_put(old);
@@ -392,11 +393,8 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)

/* search the session keyring */
if (ctx->cred->session_keyring) {
- rcu_read_lock();
key_ref = keyring_search_aux(
- make_key_ref(rcu_dereference(ctx->cred->session_keyring), 1),
- ctx);
- rcu_read_unlock();
+ make_key_ref(ctx->cred->session_keyring, 1), ctx);

if (!IS_ERR(key_ref))
goto found;
@@ -612,10 +610,8 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
goto reget_creds;
}

- rcu_read_lock();
- key = rcu_dereference(ctx.cred->session_keyring);
+ key = ctx.cred->session_keyring;
__key_get(key);
- rcu_read_unlock();
key_ref = make_key_ref(key, 1);
break;

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 2f17d84d46f1..db72dc4d7639 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -142,12 +142,10 @@ static int call_sbin_request_key(struct key *authkey, void *aux)
prkey = cred->process_keyring->serial;
sprintf(keyring_str[1], "%d", prkey);

- rcu_read_lock();
- session = rcu_dereference(cred->session_keyring);
+ session = cred->session_keyring;
if (!session)
session = cred->user->session_keyring;
sskey = session->serial;
- rcu_read_unlock();

sprintf(keyring_str[2], "%d", sskey);

@@ -287,10 +285,7 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)

/* fall through */
case KEY_REQKEY_DEFL_SESSION_KEYRING:
- rcu_read_lock();
- dest_keyring = key_get(
- rcu_dereference(cred->session_keyring));
- rcu_read_unlock();
+ dest_keyring = key_get(cred->session_keyring);

if (dest_keyring)
break;
--
2.21.0.392.gf8f6787159e-goog
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