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keys: safe concurrent user->{session,uid}_keyring access 

The current code can perform concurrent updates and reads on
user->session_keyring and user->uid_keyring. Add a comment to
struct user_struct to document the nontrivial locking semantics, and use
READ_ONCE() for unlocked readers and smp_store_release() for writers to
prevent memory ordering issues.

Fixes: 69664cf16a ("keys: don't generate user and user session keyrings unless they're accessed")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: James Morris <james.morris@microsoft.com>
This commit is contained in:
Jann Horn 2019-03-27 16:55:08 +01:00 committed by James Morris
parent 5c7e372caa
commit 0b9dc6c9f0
3 changed files with 27 additions and 16 deletions
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7
include/linux/sched/user.h
View File

@ -31,6 +31,13 @@ struct user_struct {
atomic_long_t pipe_bufs; /* how many pages are allocated in pipe buffers */
#ifdef CONFIG_KEYS
/*
* These pointers can only change from NULL to a non-NULL value once.
* Writes are protected by key_user_keyring_mutex.
* Unlocked readers should use READ_ONCE() unless they know that
* install_user_keyrings() has been called successfully (which sets
* these members to non-NULL values, preventing further modifications).
*/
struct key *uid_keyring; /* UID specific keyring */
struct key *session_keyring; /* UID's default session keyring */
#endif

31
security/keys/process_keys.c
View File

@ -58,7 +58,7 @@ int install_user_keyrings(void)
kenter("%p{%u}", user, uid);
if (user->uid_keyring && user->session_keyring) {
if (READ_ONCE(user->uid_keyring) && READ_ONCE(user->session_keyring)) {
kleave(" = 0 [exist]");
return 0;
}
@ -111,8 +111,10 @@ int install_user_keyrings(void)
}
/* install the keyrings */
user->uid_keyring = uid_keyring;
user->session_keyring = session_keyring;
/* paired with READ_ONCE() */
smp_store_release(&user->uid_keyring, uid_keyring);
/* paired with READ_ONCE() */
smp_store_release(&user->session_keyring, session_keyring);
}
mutex_unlock(&key_user_keyring_mutex);
@ -340,6 +342,7 @@ void key_fsgid_changed(struct task_struct *tsk)
key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
{
key_ref_t key_ref, ret, err;
const struct cred *cred = ctx->cred;
/* we want to return -EAGAIN or -ENOKEY if any of the keyrings were
* searchable, but we failed to find a key or we found a negative key;
@ -353,9 +356,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
err = ERR_PTR(-EAGAIN);
/* search the thread keyring first */
if (ctx->cred->thread_keyring) {
if (cred->thread_keyring) {
key_ref = keyring_search_aux(
make_key_ref(ctx->cred->thread_keyring, 1), ctx);
make_key_ref(cred->thread_keyring, 1), ctx);
if (!IS_ERR(key_ref))
goto found;
@ -371,9 +374,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
}
/* search the process keyring second */
if (ctx->cred->process_keyring) {
if (cred->process_keyring) {
key_ref = keyring_search_aux(
make_key_ref(ctx->cred->process_keyring, 1), ctx);
make_key_ref(cred->process_keyring, 1), ctx);
if (!IS_ERR(key_ref))
goto found;
@ -392,9 +395,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
}
/* search the session keyring */
if (ctx->cred->session_keyring) {
if (cred->session_keyring) {
key_ref = keyring_search_aux(
make_key_ref(ctx->cred->session_keyring, 1), ctx);
make_key_ref(cred->session_keyring, 1), ctx);
if (!IS_ERR(key_ref))
goto found;
@ -413,9 +416,9 @@ key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx)
}
}
/* or search the user-session keyring */
else if (ctx->cred->user->session_keyring) {
else if (READ_ONCE(cred->user->session_keyring)) {
key_ref = keyring_search_aux(
make_key_ref(ctx->cred->user->session_keyring, 1),
make_key_ref(READ_ONCE(cred->user->session_keyring), 1),
ctx);
if (!IS_ERR(key_ref))
goto found;
@ -602,7 +605,7 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
goto error;
goto reget_creds;
} else if (ctx.cred->session_keyring ==
ctx.cred->user->session_keyring &&
READ_ONCE(ctx.cred->user->session_keyring) &&
lflags & KEY_LOOKUP_CREATE) {
ret = join_session_keyring(NULL);
if (ret < 0)
@ -616,7 +619,7 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
break;
case KEY_SPEC_USER_KEYRING:
if (!ctx.cred->user->uid_keyring) {
if (!READ_ONCE(ctx.cred->user->uid_keyring)) {
ret = install_user_keyrings();
if (ret < 0)
goto error;
@ -628,7 +631,7 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
break;
case KEY_SPEC_USER_SESSION_KEYRING:
if (!ctx.cred->user->session_keyring) {
if (!READ_ONCE(ctx.cred->user->session_keyring)) {
ret = install_user_keyrings();
if (ret < 0)
goto error;

5
security/keys/request_key.c
View File

@ -293,11 +293,12 @@ static int construct_get_dest_keyring(struct key **_dest_keyring)
/* fall through */
case KEY_REQKEY_DEFL_USER_SESSION_KEYRING:
dest_keyring =
key_get(cred->user->session_keyring);
key_get(READ_ONCE(cred->user->session_keyring));
break;
case KEY_REQKEY_DEFL_USER_KEYRING:
dest_keyring = key_get(cred->user->uid_keyring);
dest_keyring =
key_get(READ_ONCE(cred->user->uid_keyring));
break;
case KEY_REQKEY_DEFL_GROUP_KEYRING: