linux-kernel - [PATCH] Subject: [PATCH] KEYS: Handle there being no fallback destination keyring for request

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Date:	Thu, 09 Apr 2009 17:14:05 +0100
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...l.org, akpm@...ux-foundation.org
Cc:	dhowells@...hat.com, jlayton@...hat.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH] Subject: [PATCH] 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@...hat.com>
Tested-by: Jeff Layton <jlayton@...hat.com>
---

 security/keys/request_key.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 22a3158..03fe63e 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));

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