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

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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
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@ -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