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Subject[PATCH 4/6] keys: Provide request_key_rcu() [ver #2]
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Provide a request_key_rcu() function that can be used to request a key
under RCU conditions. It can only search and check permissions; it cannot
allocate a new key, upcall or wait for an upcall to complete. It may
return a partially constructed key.

Signed-off-by: David Howells <dhowells@redhat.com>
---

Documentation/security/keys/core.rst | 10 ++++++
Documentation/security/keys/request-key.rst | 9 ++++++
include/linux/key.h | 3 ++
security/keys/request_key.c | 44 +++++++++++++++++++++++++++
4 files changed, 66 insertions(+)

diff --git a/Documentation/security/keys/core.rst b/Documentation/security/keys/core.rst
index 82dd457ff78d..003f1452a5b7 100644
--- a/Documentation/security/keys/core.rst
+++ b/Documentation/security/keys/core.rst
@@ -1147,6 +1147,16 @@ payload contents" for more information.
case error ERESTARTSYS will be returned.


+ * To search for a key under RCU conditions, call::
+
+ struct key *request_key_rcu(const struct key_type *type,
+ const char *description);
+
+ which is similar to request_key() except that it does not check for keys
+ that are under construction and it will not call out to userspace to
+ construct a key if it can't find a match.
+
+
* When it is no longer required, the key should be released using::

void key_put(struct key *key);
diff --git a/Documentation/security/keys/request-key.rst b/Documentation/security/keys/request-key.rst
index 07af991463b5..7caedc4d29f1 100644
--- a/Documentation/security/keys/request-key.rst
+++ b/Documentation/security/keys/request-key.rst
@@ -36,6 +36,11 @@ or::
size_t callout_len,
void *aux);

+or::
+
+ struct key *request_key_rcu(const struct key_type *type,
+ const char *description);
+
Or by userspace invoking the request_key system call::

key_serial_t request_key(const char *type,
@@ -57,6 +62,10 @@ The two async in-kernel calls may return keys that are still in the process of
being constructed. The two non-async ones will wait for construction to
complete first.

+The request_key_rcu() call is like the in-kernel request_key() call, except
+that it doesn't check for keys that are under construction and doesn't attempt
+to construct missing keys.
+
The userspace interface links the key to a keyring associated with the process
to prevent the key from going away, and returns the serial number of the key to
the caller.
diff --git a/include/linux/key.h b/include/linux/key.h
index 612e1cf84049..3604a554df99 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -274,6 +274,9 @@ extern struct key *request_key(struct key_type *type,
const char *description,
const char *callout_info);

+extern struct key *request_key_rcu(struct key_type *type,
+ const char *description);
+
extern struct key *request_key_with_auxdata(struct key_type *type,
const char *description,
const void *callout_info,
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index bf1d223ec21c..b4b3677657d6 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -756,3 +756,47 @@ struct key *request_key_async_with_auxdata(struct key_type *type,
callout_len, aux, NULL, KEY_ALLOC_IN_QUOTA);
}
EXPORT_SYMBOL(request_key_async_with_auxdata);
+
+/**
+ * request_key_rcu - Request key from RCU-read-locked context
+ * @type: The type of key we want.
+ * @description: The name of the key we want.
+ *
+ * Request a key from a context that we may not sleep in (such as RCU-mode
+ * pathwalk). Keys under construction are ignored.
+ *
+ * Return a pointer to the found key if successful, -ENOKEY if we couldn't find
+ * a key or some other error if the key found was unsuitable or inaccessible.
+ */
+struct key *request_key_rcu(struct key_type *type, const char *description)
+{
+ struct keyring_search_context ctx = {
+ .index_key.type = type,
+ .index_key.description = description,
+ .index_key.desc_len = strlen(description),
+ .cred = current_cred(),
+ .match_data.cmp = key_default_cmp,
+ .match_data.raw_data = description,
+ .match_data.lookup_type = KEYRING_SEARCH_LOOKUP_DIRECT,
+ .flags = (KEYRING_SEARCH_DO_STATE_CHECK |
+ KEYRING_SEARCH_SKIP_EXPIRED),
+ };
+ struct key *key;
+ key_ref_t key_ref;
+
+ kenter("%s,%s", type->name, description);
+
+ /* search all the process keyrings for a key */
+ key_ref = search_process_keyrings_rcu(&ctx);
+ if (IS_ERR(key_ref)) {
+ key = ERR_CAST(key_ref);
+ if (PTR_ERR(key_ref) == -EAGAIN)
+ key = ERR_PTR(-ENOKEY);
+ } else {
+ key = key_ref_to_ptr(key_ref);
+ }
+
+ kleave(" = %p", key);
+ return key;
+}
+EXPORT_SYMBOL(request_key_rcu);
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