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Subject[PATCH] keys: safe concurrent user->{session,uid}_keyring access
From
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: 69664cf16af4 ("keys: don't generate user and user session keyrings unless they're accessed")
Signed-off-by: Jann Horn <jannh@google.com>
---
include/linux/sched/user.h | 7 +++++++
security/keys/process_keys.c | 31 +++++++++++++++++--------------
security/keys/request_key.c | 5 +++--
3 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
index c7b5f86b91a1..468d2565a9fe 100644
--- a/include/linux/sched/user.h
+++ b/include/linux/sched/user.h
@@ -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
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index bd7243cb4c85..f05f7125a7d5 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -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;
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index db72dc4d7639..75d87f9e0f49 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -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:
--
2.21.0.392.gf8f6787159e-goog
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