Re: [PATCH] unserialized task->files changing (v2)

From: Pavel V. Emelianov
Date: Tue Aug 08 2006 - 09:15:32 EST

Eric Dumazet wrote:
On Tuesday 08 August 2006 13:31, Kirill Korotaev wrote:
Fixed race on put_files_struct on exec with proc.
Restoring files on current on error path may lead
to proc having a pointer to already kfree-d files_struct.

->files changing at exit.c and khtread.c are safe as
exit_files() makes all things under lock.

v2 patch changes:
- introduced reset_files_struct() as Christoph Hellwig suggested

Found during OpenVZ stress testing.

Sorry but there is something I dont understand. You ignored my point.

+void reset_files_struct(struct task_struct *tsk, struct files_struct *files)
+ struct files_struct *old;
+ old = tsk->files;
+ task_lock(tsk);
+ tsk->files = files;
+ task_unlock(tsk);
+ put_files_struct(old);

Its seems very strange to protect tsk->files = files with a
task_lock()/task_unlock(). What is it supposed to guard against ???

If this patch corrects the 'bug', then a simpler fix would be to use a memory
barrier between "tsk->files = files" and "put_files_struct(old);"

No need to perform 2 atomics ops on the task lock.

old = tsk->files;
tsk->files = files;

No. The race being discussed is:

proc code: resetting code:
files = tsk->files;
old = tsk->files;
tsk->files = files;
put_files_struct(old); /* dec to 0 */
`- kmem_cache_free(files);
get_files_struct(file); /* already free */

So having smp_mb() before put_files_struct() does not fix the problem.

That would be enough to guard against proc code (because this code only needs
to read tsk->files of course)

The same remark can be said for __exit_files() from kernel/exit.c

If this task_lock()/task_unlock() patch is really needed, then a comment in
the source would be very fair.


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