Re: [PATCH/RFC] Futex mmap_sem deadlock

Olof Johansson
2005-02-22T23:28:00Z

On Tue, Feb 22, 2005 at 03:20:27PM -0800, Andrew Morton wrote:
> olof@austin.ibm.com (Olof Johansson) wrote:
> >
> > + inc_preempt_count();
> > + ret = get_user(curval, (int __user *)uaddr1);
> > + dec_preempt_count();
>
> That _should_ generate a might_sleep() warning, except it looks like we
> forgot to add a check to get_user().
>
> It would be better to use __copy_from_user_inatomic() here, I think.

Thanks for catching that. New rev below.

-----

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@austin.ibm.com>

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 17:20:22.000000000 -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 = __copy_from_user_inatomic(&curval, (int __user *)uaddr1, sizeof(int));
+ 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 = __copy_from_user_inatomic(&curval, (int __user *)uaddr, sizeof(int));
+ 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) {