keys: change keyctl_session_to_parent() to use task_work_add()

Change keyctl_session_to_parent() to use task_work_add() and move
key_replace_session_keyring() logic into task_work->func().

Note that we do task_work_cancel() before task_work_add() to ensure that
only one work can be pending at any time.  This is important, we must not
allow user-space to abuse the parent's ->task_works list.

The callback, replace_session_keyring(), checks PF_EXITING.  I guess this
is not really needed but looks better.

As a side effect, this fixes the (unlikely) race.  The callers of
key_replace_session_keyring() and keyctl_session_to_parent() lack the
necessary barriers, the parent can miss the request.

Now we can remove task_struct->replacement_session_keyring and related
code.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: David Smith <dsmith@redhat.com>
Cc: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

3 files changed, 44 insertions, 41 deletions

diff --git a/security/keys/internal.h b/security/keys/internal.h
index f711b094ed41..3dcbf86b0d31 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -14,6 +14,7 @@
 
 #include <linux/sched.h>
 #include <linux/key-type.h>
+#include <linux/task_work.h>
 
 #ifdef __KDEBUG
 #define kenter(FMT, ...) \
@@ -148,6 +149,7 @@ extern key_ref_t lookup_user_key(key_serial_t id, unsigned long flags,
 #define KEY_LOOKUP_FOR_UNLINK   0x04
 
 extern long join_session_keyring(const char *name);
+extern void key_change_session_keyring(struct task_work *twork);
 
 extern struct work_struct key_gc_work;
 extern unsigned key_gc_delay;
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 534a634283a4..2f28126215a2 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -1456,47 +1456,55 @@ long keyctl_session_to_parent(void)
 {
         struct task_struct *me, *parent;
         const struct cred *mycred, *pcred;
-        struct cred *cred, *oldcred;
+        struct task_work *newwork, *oldwork;
         key_ref_t keyring_r;
+        struct cred *cred;
         int ret;
 
         keyring_r = lookup_user_key(KEY_SPEC_SESSION_KEYRING, 0, KEY_LINK);
         if (IS_ERR(keyring_r))
                 return PTR_ERR(keyring_r);
 
+        ret = -ENOMEM;
+        newwork = kmalloc(sizeof(struct task_work), GFP_KERNEL);
+        if (!newwork)
+                goto error_keyring;
+
         /* our parent is going to need a new cred struct, a new tgcred struct
          * and new security data, so we allocate them here to prevent ENOMEM in
          * our parent */
-        ret = -ENOMEM;
         cred = cred_alloc_blank();
         if (!cred)
-                goto error_keyring;
+                goto error_newwork;
 
         cred->tgcred->session_keyring = key_ref_to_ptr(keyring_r);
-        keyring_r = NULL;
+        init_task_work(newwork, key_change_session_keyring, cred);
 
         me = current;
         rcu_read_lock();
         write_lock_irq(&tasklist_lock);
 
-        parent = me->real_parent;
         ret = -EPERM;
+        oldwork = NULL;
+        parent = me->real_parent;
 
         /* the parent mustn't be init and mustn't be a kernel thread */
         if (parent->pid <= 1 || !parent->mm)
-                goto not_permitted;
+                goto unlock;
 
         /* the parent must be single threaded */
         if (!thread_group_empty(parent))
-                goto not_permitted;
+                goto unlock;
 
         /* the parent and the child must have different session keyrings or
          * there's no point */
         mycred = current_cred();
         pcred = __task_cred(parent);
         if (mycred == pcred ||
-            mycred->tgcred->session_keyring == pcred->tgcred->session_keyring)
-                goto already_same;
+            mycred->tgcred->session_keyring == pcred->tgcred->session_keyring) {
+                ret = 0;
+                goto unlock;
+        }
 
         /* the parent must have the same effective ownership and mustn't be
          * SUID/SGID */
@@ -1506,38 +1514,37 @@ long keyctl_session_to_parent(void)
             pcred->gid  != mycred->egid ||
             pcred->egid != mycred->egid ||
             pcred->sgid != mycred->egid)
-                goto not_permitted;
+                goto unlock;
 
         /* the keyrings must have the same UID */
         if ((pcred->tgcred->session_keyring &&
              pcred->tgcred->session_keyring->uid != mycred->euid) ||
             mycred->tgcred->session_keyring->uid != mycred->euid)
-                goto not_permitted;
+                goto unlock;
 
-        /* if there's an already pending keyring replacement, then we replace
-         * that */
-        oldcred = parent->replacement_session_keyring;
+        /* cancel an already pending keyring replacement */
+        oldwork = task_work_cancel(parent, key_change_session_keyring);
 
         /* the replacement session keyring is applied just prior to userspace
          * restarting */
-        parent->replacement_session_keyring = cred;
-        cred = NULL;
-        set_ti_thread_flag(task_thread_info(parent), TIF_NOTIFY_RESUME);
-
-        write_unlock_irq(&tasklist_lock);
-        rcu_read_unlock();
-        if (oldcred)
-                put_cred(oldcred);
-        return 0;
-
-already_same:
-        ret = 0;
-not_permitted:
+        ret = task_work_add(parent, newwork, true);
+        if (!ret)
+                newwork = NULL;
+unlock:
         write_unlock_irq(&tasklist_lock);
         rcu_read_unlock();
-        put_cred(cred);
+        if (oldwork) {
+                put_cred(oldwork->data);
+                kfree(oldwork);
+        }
+        if (newwork) {
+                put_cred(newwork->data);
+                kfree(newwork);
+        }
         return ret;
 
+error_newwork:
+        kfree(newwork);
 error_keyring:
         key_ref_put(keyring_r);
         return ret;
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index d71056db7b67..4ad54eea1ea4 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -834,23 +834,17 @@ error:
  * Replace a process's session keyring on behalf of one of its children when
  * the target  process is about to resume userspace execution.
  */
-void key_replace_session_keyring(void)
+void key_change_session_keyring(struct task_work *twork)
 {
-        const struct cred *old;
-        struct cred *new;
-
-        if (!current->replacement_session_keyring)
-                return;
+        const struct cred *old = current_cred();
+        struct cred *new = twork->data;
 
-        write_lock_irq(&tasklist_lock);
-        new = current->replacement_session_keyring;
-        current->replacement_session_keyring = NULL;
-        write_unlock_irq(&tasklist_lock);
-
-        if (!new)
+        kfree(twork);
+        if (unlikely(current->flags & PF_EXITING)) {
+                put_cred(new);
                 return;
+        }
 
-        old = current_cred();
         new->  uid      = old->  uid;
         new-> euid      = old-> euid;
         new-> suid      = old-> suid;