KEYS: Fix race between read and revoke (CVE-2015-7550)

debian/changelog

@@ -9,6 +9,7 @@ linux (4.3.3-3) UNRELEASED; urgency=medium
   * [xen] pciback: Fix state validation in MSI control operations
     (CVE-2015-8551, CVE-2015-8852, XSA-157)
   * ptrace: being capable wrt a process requires mapped uids/gids
+  * KEYS: Fix race between read and revoke (CVE-2015-7550)
 
   [ Salvatore Bonaccorso ]
   * ovl: fix permission checking for setattr (CVE-2015-8660)

debian/patches/bugfix/all/keys-fix-race-between-read-and-revoke.patch

@@ -0,0 +1,110 @@
+From: David Howells <dhowells@redhat.com>
+Date: Fri, 18 Dec 2015 01:34:26 +0000
+Subject: KEYS: Fix race between read and revoke
+Origin: https://git.kernel.org/linus/b4a1b4f5047e4f54e194681125c74c0aa64d637d
+
+This fixes CVE-2015-7550.
+
+There's a race between keyctl_read() and keyctl_revoke().  If the revoke
+happens between keyctl_read() checking the validity of a key and the key's
+semaphore being taken, then the key type read method will see a revoked key.
+
+This causes a problem for the user-defined key type because it assumes in
+its read method that there will always be a payload in a non-revoked key
+and doesn't check for a NULL pointer.
+
+Fix this by making keyctl_read() check the validity of a key after taking
+semaphore instead of before.
+
+I think the bug was introduced with the original keyrings code.
+
+This was discovered by a multithreaded test program generated by syzkaller
+(http://github.com/google/syzkaller).  Here's a cleaned up version:
+
+	#include <sys/types.h>
+	#include <keyutils.h>
+	#include <pthread.h>
+	void *thr0(void *arg)
+	{
+		key_serial_t key = (unsigned long)arg;
+		keyctl_revoke(key);
+		return 0;
+	}
+	void *thr1(void *arg)
+	{
+		key_serial_t key = (unsigned long)arg;
+		char buffer[16];
+		keyctl_read(key, buffer, 16);
+		return 0;
+	}
+	int main()
+	{
+		key_serial_t key = add_key("user", "%", "foo", 3, KEY_SPEC_USER_KEYRING);
+		pthread_t th[5];
+		pthread_create(&th[0], 0, thr0, (void *)(unsigned long)key);
+		pthread_create(&th[1], 0, thr1, (void *)(unsigned long)key);
+		pthread_create(&th[2], 0, thr0, (void *)(unsigned long)key);
+		pthread_create(&th[3], 0, thr1, (void *)(unsigned long)key);
+		pthread_join(th[0], 0);
+		pthread_join(th[1], 0);
+		pthread_join(th[2], 0);
+		pthread_join(th[3], 0);
+		return 0;
+	}
+
+Build as:
+
+	cc -o keyctl-race keyctl-race.c -lkeyutils -lpthread
+
+Run as:
+
+	while keyctl-race; do :; done
+
+as it may need several iterations to crash the kernel.  The crash can be
+summarised as:
+
+	BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
+	IP: [<ffffffff81279b08>] user_read+0x56/0xa3
+	...
+	Call Trace:
+	 [<ffffffff81276aa9>] keyctl_read_key+0xb6/0xd7
+	 [<ffffffff81277815>] SyS_keyctl+0x83/0xe0
+	 [<ffffffff815dbb97>] entry_SYSCALL_64_fastpath+0x12/0x6f
+
+Reported-by: Dmitry Vyukov <dvyukov@google.com>
+Signed-off-by: David Howells <dhowells@redhat.com>
+Tested-by: Dmitry Vyukov <dvyukov@google.com>
+Cc: stable@vger.kernel.org
+Signed-off-by: James Morris <james.l.morris@oracle.com>
+---
+ security/keys/keyctl.c | 18 +++++++++---------
+ 1 file changed, 9 insertions(+), 9 deletions(-)
+
+--- a/security/keys/keyctl.c
++++ b/security/keys/keyctl.c
+@@ -757,16 +757,16 @@ long keyctl_read_key(key_serial_t keyid,
+ 
+ 	/* the key is probably readable - now try to read it */
+ can_read_key:
+-	ret = key_validate(key);
+-	if (ret == 0) {
+-		ret = -EOPNOTSUPP;
+-		if (key->type->read) {
+-			/* read the data with the semaphore held (since we
+-			 * might sleep) */
+-			down_read(&key->sem);
++	ret = -EOPNOTSUPP;
++	if (key->type->read) {
++		/* Read the data with the semaphore held (since we might sleep)
++		 * to protect against the key being updated or revoked.
++		 */
++		down_read(&key->sem);
++		ret = key_validate(key);
++		if (ret == 0)
+ 			ret = key->type->read(key, buffer, buflen);
+-			up_read(&key->sem);
+-		}
++		up_read(&key->sem);
+ 	}
+ 
+ error2:

debian/patches/series

@@ -126,3 +126,4 @@ bugfix/all/xen-pciback-do-not-install-an-irq-handler-for-msi-in.patch
 bugfix/all/xen-pciback-for-xen_pci_op_disable_msi-x-only-disabl.patch
 bugfix/all/xen-pciback-don-t-allow-msi-x-ops-if-pci_command_mem.patch
 bugfix/all/ptrace-being-capable-wrt-a-process-requires-mapped-uids-gids.patch
+bugfix/all/keys-fix-race-between-read-and-revoke.patch