CVE-2015-7872: Improper Input Validation (CWE-20)
value: The key_gc_unused_keys function in security/keys/gc.c in the Linux kernel through 4.2.6 allows local users to cause a denial of service (OOPS) via crafted keyctl commands.
CWE description: The product receives input or data, but it does
        not validate or incorrectly validates that the input has the
        properties that are required to process the data safely and
        correctly.
Published: 2015-11-16T11:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: security/keys/gc.c
Commit date: 2015-10-15 17:21:37 +0100
Commit message: KEYS: Fix crash when attempt to garbage collect an uninstantiated keyring

The following sequence of commands:

    i=`keyctl add user a a @s`
    keyctl request2 keyring foo bar @t
    keyctl unlink $i @s

tries to invoke an upcall to instantiate a keyring if one doesn't already
exist by that name within the user's keyring set.  However, if the upcall
fails, the code sets keyring->type_data.reject_error to -ENOKEY or some
other error code.  When the key is garbage collected, the key destroy
function is called unconditionally and keyring_destroy() uses list_empty()
on keyring->type_data.link - which is in a union with reject_error.
Subsequently, the kernel tries to unlink the keyring from the keyring names
list - which oopses like this:

	BUG: unable to handle kernel paging request at 00000000ffffff8a
	IP: [<ffffffff8126e051>] keyring_destroy+0x3d/0x88
	...
	Workqueue: events key_garbage_collector
	...
	RIP: 0010:[<ffffffff8126e051>] keyring_destroy+0x3d/0x88
	RSP: 0018:ffff88003e2f3d30  EFLAGS: 00010203
	RAX: 00000000ffffff82 RBX: ffff88003bf1a900 RCX: 0000000000000000
	RDX: 0000000000000000 RSI: 000000003bfc6901 RDI: ffffffff81a73a40
	RBP: ffff88003e2f3d38 R08: 0000000000000152 R09: 0000000000000000
	R10: ffff88003e2f3c18 R11: 000000000000865b R12: ffff88003bf1a900
	R13: 0000000000000000 R14: ffff88003bf1a908 R15: ffff88003e2f4000
	...
	CR2: 00000000ffffff8a CR3: 000000003e3ec000 CR4: 00000000000006f0
	...
	Call Trace:
	 [<ffffffff8126c756>] key_gc_unused_keys.constprop.1+0x5d/0x10f
	 [<ffffffff8126ca71>] key_garbage_collector+0x1fa/0x351
	 [<ffffffff8105ec9b>] process_one_work+0x28e/0x547
	 [<ffffffff8105fd17>] worker_thread+0x26e/0x361
	 [<ffffffff8105faa9>] ? rescuer_thread+0x2a8/0x2a8
	 [<ffffffff810648ad>] kthread+0xf3/0xfb
	 [<ffffffff810647ba>] ? kthread_create_on_node+0x1c2/0x1c2
	 [<ffffffff815f2ccf>] ret_from_fork+0x3f/0x70
	 [<ffffffff810647ba>] ? kthread_create_on_node+0x1c2/0x1c2

Note the value in RAX.  This is a 32-bit representation of -ENOKEY.

The solution is to only call ->destroy() if the key was successfully
instantiated.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>

Diff: diff --git a/security/keys/gc.c b/security/keys/gc.c
index 39eac1fd5706c6..addf060399e095 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -132,12 +132,14 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		list_del(&key->graveyard_link);
 
 		kdebug("- %u", key->serial);
 		key_check(key);
 
-		/* Throw away the key data */
-		if (key->type->destroy)
+		/* Throw away the key data if the key is instantiated */
+		if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
+		    !test_bit(KEY_FLAG_NEGATIVE, &key->flags) &&
+		    key->type->destroy)
 			key->type->destroy(key);
 
 		security_key_free(key);
 
 		/* deal with the user's key tracking and quota */

Vulnerable code: 		/* Throw away the key data */
		if (key->type->destroy)
Fixed code: 		/* Throw away the key data if the key is instantiated */
		if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
		    !test_bit(KEY_FLAG_NEGATIVE, &key->flags) &&
		    key->type->destroy)