keys: Handle there being no fallback destination keyring for request_key()

When request_key() is called, without there being any standard process
keyrings on which to fall back if a destination keyring is not specified, an
oops is liable to occur when construct_alloc_key() calls down_write() on
dest_keyring's semaphore.

Due to function inlining this may be seen as an oops in down_write() as called
from request_key_and_link().

This situation crops up during boot, where request_key() is called from within
the kernel (such as in CIFS mounts) where nobody is actually logged in, and so
PAM has not had a chance to create a session keyring and user keyrings to act
as the fallback.

To fix this, make construct_alloc_key() not attempt to cache a key if there is
no fallback key if no destination keyring is given specifically.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 22a31582bfaa..03fe63ed55bd 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -311,7 +311,8 @@ static int construct_alloc_key(struct key_type *type,
 
         set_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags);
 
-        down_write(&dest_keyring->sem);
+        if (dest_keyring)
+                down_write(&dest_keyring->sem);
 
         /* attach the key to the destination keyring under lock, but we do need
          * to do another check just in case someone beat us to it whilst we
@@ -322,10 +323,12 @@ static int construct_alloc_key(struct key_type *type,
         if (!IS_ERR(key_ref))
                 goto key_already_present;
 
-        __key_link(dest_keyring, key);
+        if (dest_keyring)
+                __key_link(dest_keyring, key);
 
         mutex_unlock(&key_construction_mutex);
-        up_write(&dest_keyring->sem);
+        if (dest_keyring)
+                up_write(&dest_keyring->sem);
         mutex_unlock(&user->cons_lock);
         *_key = key;
         kleave(" = 0 [%d]", key_serial(key));