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Subject[PATCH] Fix potential OOPS in generic_setlease() (v2)
This code is run under lock_kernel(), which is dropped during
sleeping operations, so the following race is possible:

CPU1: CPU2:
vfs_setlease(); vfs_setlease();
lock_kernel();
lock_kernel(); /* spin */
generic_setlease():
...
for (before = ...)
/* here we found some lease after
* which we will insert the new one
*/
fl = locks_alloc_lock();
/* go to sleep in this allocation and
* drop the BKL
*/
generic_setlease():
...
for (before = ...)
/* here we find the "before" pointing
* at the one we found on CPU1
*/
->fl_change(my_before, arg);
lease_modify();
locks_free_lock();
/* and we freed it */
...
unlock_kernel();
locks_insert_lock(before, fl);
/* OOPS! We have just tried to add the lease
* at the tail of already removed one
*/

The similar races are already handled in other code - all the
allocations are performed before any checks/updates.

Fixed the problem, spotted by J. Bruce Fields, about the fl
variable reuse.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---

diff --git a/fs/locks.c b/fs/locks.c
index a1c1c01..d5b9653 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1354,6 +1354,7 @@ int fcntl_getlease(struct file *filp)
int generic_setlease(struct file *filp, long arg, struct file_lock **flp)
{
struct file_lock *fl, **before, **my_before = NULL, *lease;
+ struct file_lock *new_fl = NULL;
struct dentry *dentry = filp->f_path.dentry;
struct inode *inode = dentry->d_inode;
int error, rdlease_count = 0, wrlease_count = 0;
@@ -1380,6 +1381,11 @@ int generic_setlease(struct file *filp,
|| (atomic_read(&inode->i_count) > 1)))
goto out;

+ error = -ENOMEM;
+ new_fl = locks_alloc_lock();
+ if (new_fl == NULL)
+ goto out;
+
/*
* At this point, we know that if there is an exclusive
* lease on this file, then we hold it on this filp
@@ -1422,18 +1428,15 @@ int generic_setlease(struct file *filp,
if (!leases_enable)
goto out;

- error = -ENOMEM;
- fl = locks_alloc_lock();
- if (fl == NULL)
- goto out;
-
- locks_copy_lock(fl, lease);
-
- locks_insert_lock(before, fl);
+ locks_copy_lock(new_fl, lease);
+ locks_insert_lock(before, new_fl);

*flp = fl;
- error = 0;
+ return 0;
+
out:
+ if (new_fl != NULL)
+ locks_free_lock(new_fl);
return error;
}
EXPORT_SYMBOL(generic_setlease);
-
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