[BUG] futex_handle_fault always fails.

From: john stultz
Date: Mon Aug 07 2006 - 17:45:35 EST

We found this issue last week w/ the -RT kernel, but it seems the same
issue is in mainline as well.

Basically is possible for futex_unlock_pi to return without actually
freeing the lock. This is due to buggy logic in the use of
futex_handle_fault() and its attempt argument in a failure case.

Looking at futex.c the logic is as follows:
1) In futex_unlock_pi() we start w/ ret=0 and we go down to the first
futex_atomic_cmpxchg_inatomic(), where we find uval==-EFAULT. We then
jump to the pi_faulted label.
2) From pi_faulted: We increment attempt, unlock the sem and hit the
retry label.
3) From the retry label, with ret still zero, we again hit EFAULT on the
first futex_atomic_cmpxchg_inatomic(), and again goto the pi_faulted
4) Again from pi_faulted: we increment attempt and enter the
conditional, where we call futex_handle_fault.
5) futex_handle_fault fails, and we goto the out_unlock_release_sem
6) From out_unlock_release_sem we return, and since ret is still zero,
we return without error, while never actually unlocking the lock.

Issue #1: at the first futex_atomic_cmpxchg_inatomic() we should
probably be setting ret=-EFAULT before jumping to pi_faulted: However
in our case this doesn't really affect anything, as the glibc we're
using ignores the error value from futex_unlock_pi().

Issue #2: Look at futex_handle_fault(), its first conditional will
return -EFAULT if attempt is >= 2. However, from the "if(attempt++)
futex_handle_fault(attempt)" logic above, we'll *never* call
futex_handle_fault when attempt is less then two. So we never get a
chance to even try to fault the page in.

The following patch addresses these two issues by 1) Always setting ret
to -EFAULT if futex_handle_fault fails, and 2) Removing the = in
futex_handle_fault's (attempt >= 2) check.

I'm really not sure this is the right fix, but wanted to bring it up so
folks knew the issue is alive and well in the current -git tree. From
looking at the git logs the logic was first introduced (then later
copied to other places) in the following commit almost a year ago:


Thoughts, comments?


diff --git a/kernel/futex.c b/kernel/futex.c
index c2b2e0b..d4633c5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -297,7 +297,7 @@ static int futex_handle_fault(unsigned l
struct vm_area_struct * vma;
struct mm_struct *mm = current->mm;

- if (attempt >= 2 || !(vma = find_vma(mm, address)) ||
+ if (attempt > 2 || !(vma = find_vma(mm, address)) ||
vma->vm_start > address || !(vma->vm_flags & VM_WRITE))
return -EFAULT;

@@ -747,8 +747,10 @@ retry:
if (attempt++) {
if (futex_handle_fault((unsigned long)uaddr2,
- attempt))
+ attempt)) {
+ ret = -EFAULT;
goto out;
+ }
goto retry;

@@ -1322,9 +1324,10 @@ static int do_futex_lock_pi(u32 __user *
* still holding the mmap_sem.
if (attempt++) {
- if (futex_handle_fault((unsigned long)uaddr, attempt))
+ if (futex_handle_fault((unsigned long)uaddr, attempt)) {
+ ret = -EFAULT;
goto out_unlock_release_sem;
+ }
goto retry_locked;

@@ -1506,9 +1509,10 @@ pi_faulted:
* still holding the mmap_sem.
if (attempt++) {
- if (futex_handle_fault((unsigned long)uaddr, attempt))
+ if (futex_handle_fault((unsigned long)uaddr, attempt)) {
+ ret = -EFAULT;
goto out_unlock;
+ }
goto retry_locked;

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