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authorjohn stultz <johnstul@us.ibm.com>2006-08-13 23:24:24 -0700
committerGreg Kroah-Hartman <gregkh@suse.de>2006-08-14 12:54:29 -0700
commite579dcbf23604cb33c08b5c3c3ac06ca36e7c683 (patch)
tree293581684fc47260a4610e4a77510b459346e2aa
parent6997a6faaa129a1c91775f7344c8d371a05178ea (diff)
[PATCH] futex_handle_fault always fails
We found this issue last week w/ the -RT kernel, but it seems the same issue is in mainline as well. Basically it 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 label. 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 label. 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: http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=4732efbeb997189d9f9b04708dc26bf8613ed721;hp=5b039e681b8c5f30aac9cc04385cc94be45d0823 Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Ingo Molnar <mingo@elte.hu> Acked-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
-rw-r--r--kernel/futex.c16
1 files changed, 10 insertions, 6 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index c2b2e0b83ab..d4633c588f3 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -297,7 +297,7 @@ static int futex_handle_fault(unsigned long address, int attempt)
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 *uaddr, int detect, int trylock,
* 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;
}