futex-prevent-requeue-pi-on-same-futex.patch futex: Forbid uaddr == uaddr2 in futex_requeue(…, requeue_pi=1) [Linux 3.15]

This Linux kernel change "futex-prevent-requeue-pi-on-same-futex.patch futex: Forbid uaddr == uaddr2 in futex_requeue(…, requeue_pi=1)" is included in the Linux 3.15 release. This change is authored by Thomas Gleixner <tglx [at] linutronix.de> on Tue Jun 3 12:27:06 2014 +0000. The commit for this change in Linux stable tree is e9c243a (patch).

futex-prevent-requeue-pi-on-same-futex.patch futex: Forbid uaddr == uaddr2 in futex_requeue(..., requeue_pi=1)

If uaddr == uaddr2, then we have broken the rule of only requeueing from
a non-pi futex to a pi futex with this call.  If we attempt this, then
dangling pointers may be left for rt_waiter resulting in an exploitable
condition.

This change brings futex_requeue() in line with futex_wait_requeue_pi()
which performs the same check as per commit 6f7b0a2a5c0f ("futex: Forbid
uaddr == uaddr2 in futex_wait_requeue_pi()")

[ tglx: Compare the resulting keys as well, as uaddrs might be
    different depending on the mapping ]

Fixes CVE-2014-3153.

Reported-by: Pinkie Pie
Signed-off-by: Will Drewry <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Cc: [email protected]
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Darren Hart <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

There are 25 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/futex.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 81dbe77..663ea2b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1442,6 +1442,13 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,

    if (requeue_pi) {
        /*
+        * Requeue PI only works on two distinct uaddrs. This
+        * check is only valid for private futexes. See below.
+        */
+       if (uaddr1 == uaddr2)
+           return -EINVAL;
+
+       /*
         * requeue_pi requires a pi_state, try to allocate it now
         * without any locks in case it fails.
         */
@@ -1479,6 +1486,15 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
    if (unlikely(ret != 0))
        goto out_put_key1;

+   /*
+    * The check above which compares uaddrs is not sufficient for
+    * shared futexes. We need to compare the keys:
+    */
+   if (requeue_pi && match_futex(&key1, &key2)) {
+       ret = -EINVAL;
+       goto out_put_keys;
+   }
+
    hb1 = hash_futex(&key1);
    hb2 = hash_futex(&key2);

@@ -2525,6 +2541,15 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
    if (ret)
        goto out_key2;

+   /*
+    * The check above which compares uaddrs is not sufficient for
+    * shared futexes. We need to compare the keys:
+    */
+   if (match_futex(&q.key, &key2)) {
+       ret = -EINVAL;
+       goto out_put_keys;
+   }
+
    /* Queue the futex_q, drop the hb lock, wait for wakeup. */
    futex_wait_queue_me(hb, &q, to);

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