CVE-2015-8539: Improper Privilege Management (CWE-269)
value: The KEYS subsystem in the Linux kernel before 4.4 allows local users to gain privileges or cause a denial of service (BUG) via crafted keyctl commands that negatively instantiate a key, related to security/keys/encrypted-keys/encrypted.c, security/keys/trusted.c, and security/keys/user_defined.c.
CWE description: The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor.
CVSS v3 base score: 7.8
Published: 2016-02-08T03:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: security/keys/encrypted-keys/encrypted.c, security/keys/trusted.c, security/keys/user_defined.c
Commit date: 2015-11-25 14:19:47 +1100
Commit message: KEYS: Fix handling of stored error in a negatively instantiated user key

If a user key gets negatively instantiated, an error code is cached in the
payload area.  A negatively instantiated key may be then be positively
instantiated by updating it with valid data.  However, the ->update key
type method must be aware that the error code may be there.

The following may be used to trigger the bug in the user key type:

    keyctl request2 user user "" @u
    keyctl add user user "a" @u

which manifests itself as:

	BUG: unable to handle kernel paging request at 00000000ffffff8a
	IP: [<ffffffff810a376f>] __call_rcu.constprop.76+0x1f/0x280 kernel/rcu/tree.c:3046
	PGD 7cc30067 PUD 0
	Oops: 0002 [#1] SMP
	Modules linked in:
	CPU: 3 PID: 2644 Comm: a.out Not tainted 4.3.0+ #49
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
	task: ffff88003ddea700 ti: ffff88003dd88000 task.ti: ffff88003dd88000
	RIP: 0010:[<ffffffff810a376f>]  [<ffffffff810a376f>] __call_rcu.constprop.76+0x1f/0x280
	 [<ffffffff810a376f>] __call_rcu.constprop.76+0x1f/0x280 kernel/rcu/tree.c:3046
	RSP: 0018:ffff88003dd8bdb0  EFLAGS: 00010246
	RAX: 00000000ffffff82 RBX: 0000000000000000 RCX: 0000000000000001
	RDX: ffffffff81e3fe40 RSI: 0000000000000000 RDI: 00000000ffffff82
	RBP: ffff88003dd8bde0 R08: ffff88007d2d2da0 R09: 0000000000000000
	R10: 0000000000000000 R11: ffff88003e8073c0 R12: 00000000ffffff82
	R13: ffff88003dd8be68 R14: ffff88007d027600 R15: ffff88003ddea700
	FS:  0000000000b92880(0063) GS:ffff88007fd00000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
	CR2: 00000000ffffff8a CR3: 000000007cc5f000 CR4: 00000000000006e0
	Stack:
	 ffff88003dd8bdf0 ffffffff81160a8a 0000000000000000 00000000ffffff82
	 ffff88003dd8be68 ffff88007d027600 ffff88003dd8bdf0 ffffffff810a39e5
	 ffff88003dd8be20 ffffffff812a31ab ffff88007d027600 ffff88007d027620
	Call Trace:
	 [<ffffffff810a39e5>] kfree_call_rcu+0x15/0x20 kernel/rcu/tree.c:3136
	 [<ffffffff812a31ab>] user_update+0x8b/0xb0 security/keys/user_defined.c:129
	 [<     inline     >] __key_update security/keys/key.c:730
	 [<ffffffff8129e5c1>] key_create_or_update+0x291/0x440 security/keys/key.c:908
	 [<     inline     >] SYSC_add_key security/keys/keyctl.c:125
	 [<ffffffff8129fc21>] SyS_add_key+0x101/0x1e0 security/keys/keyctl.c:60
	 [<ffffffff8185f617>] entry_SYSCALL_64_fastpath+0x12/0x6a arch/x86/entry/entry_64.S:185

Note the error code (-ENOKEY) in EDX.

A similar bug can be tripped by:

    keyctl request2 trusted user "" @u
    keyctl add trusted user "a" @u

This should also affect encrypted keys - but that has to be correctly
parameterised or it will fail with EINVAL before getting to the bit that
will crashes.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

Diff: diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index 927db9f35ad6f1..696ccfa08d103c 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -843,10 +843,12 @@ static int encrypted_update(struct key *key, struct key_preparsed_payload *prep)
 	char *new_master_desc = NULL;
 	const char *format = NULL;
 	size_t datalen = prep->datalen;
 	int ret = 0;
 
+	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+		return -ENOKEY;
 	if (datalen <= 0 || datalen > 32767 || !prep->data)
 		return -EINVAL;
 
 	buf = kmalloc(datalen + 1, GFP_KERNEL);
 	if (!buf)
diff --git a/security/keys/trusted.c b/security/keys/trusted.c
index 903dace648a173..16dec53184b663 100644
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1005,17 +1005,20 @@ static void trusted_rcu_free(struct rcu_head *rcu)
 /*
  * trusted_update - reseal an existing key with new PCR values
  */
 static int trusted_update(struct key *key, struct key_preparsed_payload *prep)
 {
-	struct trusted_key_payload *p = key->payload.data[0];
+	struct trusted_key_payload *p;
 	struct trusted_key_payload *new_p;
 	struct trusted_key_options *new_o;
 	size_t datalen = prep->datalen;
 	char *datablob;
 	int ret = 0;
 
+	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+		return -ENOKEY;
+	p = key->payload.data[0];
 	if (!p->migratable)
 		return -EPERM;
 	if (datalen <= 0 || datalen > 32767 || !prep->data)
 		return -EINVAL;
 
diff --git a/security/keys/user_defined.c b/security/keys/user_defined.c
index 28cb30f8025691..8705d79b2c6f28 100644
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -118,11 +118,14 @@ int user_update(struct key *key, struct key_preparsed_payload *prep)
 
 	ret = key_payload_reserve(key, datalen);
 
 	if (ret == 0) {
 		/* attach the new data, displacing the old */
-		zap = key->payload.data[0];
+		if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+			zap = key->payload.data[0];
+		else
+			zap = NULL;
 		rcu_assign_keypointer(key, upayload);
 		key->expiry = 0;
 	}
 
 	if (zap)

Vulnerable code: 	struct trusted_key_payload *p = key->payload.data[0];
		zap = key->payload.data[0];
Fixed code: 	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
		return -ENOKEY;
	struct trusted_key_payload *p;
	if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
		return -ENOKEY;
	p = key->payload.data[0];
		if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
			zap = key->payload.data[0];
		else
			zap = NULL;