Re: [PATCH/RFC] Futex mmap_sem deadlock

[Olof Johansson]

On Tue, Feb 22, 2005 at 10:34:57PM +0000, Jamie Lokier wrote:
> There is one small but important error: the "return ret" mustn't just
> return.  It must call unqueue_me(&q) just like the code at out_unqueue,
> _including_ the conditional "ret = 0", but _excluding_ the up_read().

Not only that, but someone might already have dequeued us, right? It's
probably a pathological case though, i.e. someone did a wake on the same
(bad) address.

How's this patch? It's closer to Linus' pseudo-code than Andrew's, to
avoid the extra get_user() at function entry and keep the common case
path short.

It also includes the feedback from Andrew on the sys_get_mempolicy(),
making the patch even simpler there.

----


Some futex functions do get_user calls while holding mmap_sem for
reading. If get_user() faults, and another thread happens to be in mmap
(or somewhere else holding waiting on down_write for the same semaphore),
then do_page_fault will deadlock. Most architectures seem to be exposed
to this.

To avoid it, make sure the page is available. If not, release the
semaphore, fault it in and retry.

I also found another exposure by inspection, moving some of the code
around avoids the possible deadlock there.

Signed-off-by: Olof Johansson <olof@xxxxxxxxxxxxxx>


Index: linux-2.5/kernel/futex.c
===================================================================
--- linux-2.5.orig/kernel/futex.c	2005-02-21 16:09:38.000000000 -0600
+++ linux-2.5/kernel/futex.c	2005-02-22 16:38:24.000000000 -0600
@@ -329,6 +329,7 @@
 	int ret, drop_count = 0;
 	unsigned int nqueued;
 
+ retry:
 	down_read(&current->mm->mmap_sem);
 
 	ret = get_futex_key(uaddr1, &key1);
@@ -355,9 +356,19 @@
 		   before *uaddr1.  */
 		smp_mb();
 
-		if (get_user(curval, (int __user *)uaddr1) != 0) {
-			ret = -EFAULT;
-			goto out;
+		inc_preempt_count();
+		ret = get_user(curval, (int __user *)uaddr1);
+		dec_preempt_count();
+
+		if (unlikely(ret)) {
+			up_read(&current->mm->mmap_sem);
+			/* Re-do the access outside the lock */
+			ret = get_user(curval, (int __user *)uaddr1);
+
+			if (!ret)
+				goto retry;
+
+			return ret;
 		}
 		if (curval != *valp) {
 			ret = -EAGAIN;
@@ -480,6 +491,7 @@
 	int ret, curval;
 	struct futex_q q;
 
+ retry:
 	down_read(&current->mm->mmap_sem);
 
 	ret = get_futex_key(uaddr, &q.key);
@@ -508,9 +520,21 @@
 	 * We hold the mmap semaphore, so the mapping cannot have changed
 	 * since we looked it up in get_futex_key.
 	 */
-	if (get_user(curval, (int __user *)uaddr) != 0) {
-		ret = -EFAULT;
-		goto out_unqueue;
+	inc_preempt_count();
+	ret = get_user(curval, (int __user *)uaddr);
+	dec_preempt_count();
+	if (unlikely(ret)) {
+		up_read(&current->mm->mmap_sem);
+
+		if (!unqueue_me(&q)) /* There's a chance we got woken already */
+			return 0;
+
+		/* Re-do the access outside the lock */
+		ret = get_user(curval, (int __user *)uaddr);
+
+		if (!ret)
+			goto retry;
+		return ret;
 	}
 	if (curval != val) {
 		ret = -EWOULDBLOCK;
Index: linux-2.5/mm/mempolicy.c
===================================================================
--- linux-2.5.orig/mm/mempolicy.c	2005-02-04 00:27:40.000000000 -0600
+++ linux-2.5/mm/mempolicy.c	2005-02-22 14:34:19.000000000 -0600
@@ -524,9 +524,13 @@
 	} else
 		pval = pol->policy;
 
-	err = -EFAULT;
+	if (vma) {
+		up_read(&current->mm->mmap_sem);
+		vma = NULL;
+	}
+
 	if (policy && put_user(pval, policy))
-		goto out;
+		return -EFAULT;
 
 	err = 0;
 	if (nmask) {
-
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