KEYS: fix keyctl_set_reqkey_keyring() to not leak thread keyrings

commit c9f838d104fed6f2f61d68164712e3204bf5271b upstream.

This fixes CVE-2017-7472.

Running the following program as an unprivileged user exhausts kernel
memory by leaking thread keyrings:

	#include <keyutils.h>

	int main()
	{
		for (;;)
			keyctl_set_reqkey_keyring(KEY_REQKEY_DEFL_THREAD_KEYRING);
	}

Fix it by only creating a new thread keyring if there wasn't one before.
To make things more consistent, make install_thread_keyring_to_cred()
and install_process_keyring_to_cred() both return 0 if the corresponding
keyring is already present.

Fixes: d84f4f992cbd ("CRED: Inaugurate COW credentials")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 6b414b7..b9073eb 100644 (file)
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1183,8 +1183,8 @@ error:
  * Read or set the default keyring in which request_key() will cache keys and
  * return the old setting.
  *
- * If a process keyring is specified then this will be created if it doesn't
- * yet exist.  The old setting will be returned if successful.
+ * If a thread or process keyring is specified then it will be created if it
+ * doesn't yet exist.  The old setting will be returned if successful.
  */
 long keyctl_set_reqkey_keyring(int reqkey_defl)
 {
@@ -1209,11 +1209,8 @@ long keyctl_set_reqkey_keyring(int reqkey_defl)
 
        case KEY_REQKEY_DEFL_PROCESS_KEYRING:
                ret = install_process_keyring_to_cred(new);
-               if (ret < 0) {
-                       if (ret != -EEXIST)
-                               goto error;
-                       ret = 0;
-               }
+               if (ret < 0)
+                       goto error;
                goto set;
 
        case KEY_REQKEY_DEFL_DEFAULT:

diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 60d0df7..fe5719f 100644 (file)
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -121,13 +121,18 @@ error:
 }
 
 /*
- * Install a fresh thread keyring directly to new credentials.  This keyring is
- * allowed to overrun the quota.
+ * Install a thread keyring to the given credentials struct if it didn't have
+ * one already.  This is allowed to overrun the quota.
+ *
+ * Return: 0 if a thread keyring is now present; -errno on failure.
  */
 int install_thread_keyring_to_cred(struct cred *new)
 {
        struct key *keyring;
 
+       if (new->thread_keyring)
+               return 0;
+
        keyring = keyring_alloc("_tid", new->uid, new->gid, new,
                                KEY_ALLOC_QUOTA_OVERRUN, NULL);
        if (IS_ERR(keyring))
@@ -138,7 +143,9 @@ int install_thread_keyring_to_cred(struct cred *new)
 }
 
 /*
- * Install a fresh thread keyring, discarding the old one.
+ * Install a thread keyring to the current task if it didn't have one already.
+ *
+ * Return: 0 if a thread keyring is now present; -errno on failure.
  */
 static int install_thread_keyring(void)
 {
@@ -149,8 +156,6 @@ static int install_thread_keyring(void)
        if (!new)
                return -ENOMEM;
 
-       BUG_ON(new->thread_keyring);
-
        ret = install_thread_keyring_to_cred(new);
        if (ret < 0) {
                abort_creds(new);
@@ -161,10 +166,10 @@ static int install_thread_keyring(void)
 }
 
 /*
- * Install a process keyring directly to a credentials struct.
+ * Install a process keyring to the given credentials struct if it didn't have
+ * one already.  This is allowed to overrun the quota.
  *
- * Returns -EEXIST if there was already a process keyring, 0 if one installed,
- * and other value on any other error
+ * Return: 0 if a process keyring is now present; -errno on failure.
  */
 int install_process_keyring_to_cred(struct cred *new)
 {
@@ -172,7 +177,7 @@ int install_process_keyring_to_cred(struct cred *new)
        int ret;
 
        if (new->tgcred->process_keyring)
-               return -EEXIST;
+               return 0;
 
        keyring = keyring_alloc("_pid", new->uid, new->gid,
                                new, KEY_ALLOC_QUOTA_OVERRUN, NULL);
@@ -193,11 +198,9 @@ int install_process_keyring_to_cred(struct cred *new)
 }
 
 /*
- * Make sure a process keyring is installed for the current process.  The
- * existing process keyring is not replaced.
+ * Install a process keyring to the current task if it didn't have one already.
  *
- * Returns 0 if there is a process keyring by the end of this function, some
- * error otherwise.
+ * Return: 0 if a process keyring is now present; -errno on failure.
  */
 static int install_process_keyring(void)
 {
@@ -211,14 +214,18 @@ static int install_process_keyring(void)
        ret = install_process_keyring_to_cred(new);
        if (ret < 0) {
                abort_creds(new);
-               return ret != -EEXIST ? ret : 0;
+               return ret;
        }
 
        return commit_creds(new);
 }
 
 /*
- * Install a session keyring directly to a credentials struct.
+ * Install the given keyring as the session keyring of the given credentials
+ * struct, replacing the existing one if any.  If the given keyring is NULL,
+ * then install a new anonymous session keyring.
+ *
+ * Return: 0 on success; -errno on failure.
  */
 int install_session_keyring_to_cred(struct cred *cred, struct key *keyring)
 {
@@ -258,8 +265,11 @@ int install_session_keyring_to_cred(struct cred *cred, struct key *keyring)
 }
 
 /*
- * Install a session keyring, discarding the old one.  If a keyring is not
- * supplied, an empty one is invented.
+ * Install the given keyring as the session keyring of the current task,
+ * replacing the existing one if any.  If the given keyring is NULL, then
+ * install a new anonymous session keyring.
+ *
+ * Return: 0 on success; -errno on failure.
  */
 static int install_session_keyring(struct key *keyring)
 {