linux-kernel - [ 04/41] aio: fix the "too late munmap()" race

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Date:	Fri, 16 Mar 2012 16:38:14 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Al Viro <viro@...iv.linux.org.uk>,
	Jeff Moyer <jmoyer@...hat.com>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: [ 04/41] aio: fix the "too late munmap()" race

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@...IV.linux.org.uk>

commit c7b285550544c22bc005ec20978472c9ac7138c6 upstream.

Current code has put_ioctx() called asynchronously from aio_fput_routine();
that's done *after* we have killed the request that used to pin ioctx,
so there's nothing to stop io_destroy() waiting in wait_for_all_aios()
from progressing.  As the result, we can end up with async call of
put_ioctx() being the last one and possibly happening during exit_mmap()
or elf_core_dump(), neither of which expects stray munmap() being done
to them...

We do need to prevent _freeing_ ioctx until aio_fput_routine() is done
with that, but that's all we care about - neither io_destroy() nor
exit_aio() will progress past wait_for_all_aios() until aio_fput_routine()
does really_put_req(), so the ioctx teardown won't be done until then
and we don't care about the contents of ioctx past that point.

Since actual freeing of these suckers is RCU-delayed, we don't need to
bump ioctx refcount when request goes into list for async removal.
All we need is rcu_read_lock held just over the ->ctx_lock-protected
area in aio_fput_routine().

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
Acked-by: Benjamin LaHaise <bcrl@...ck.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/aio.c |   14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

--- a/fs/aio.c
+++ b/fs/aio.c
@@ -228,12 +228,6 @@ static void __put_ioctx(struct kioctx *c
 	call_rcu(&ctx->rcu_head, ctx_rcu_free);
 }

-static inline void get_ioctx(struct kioctx *kioctx)
-{
-	BUG_ON(atomic_read(&kioctx->users) <= 0);
-	atomic_inc(&kioctx->users);
-}
-
 static inline int try_get_ioctx(struct kioctx *kioctx)
 {
 	return atomic_inc_not_zero(&kioctx->users);
@@ -609,11 +603,16 @@ static void aio_fput_routine(struct work
 			fput(req->ki_filp);

 		/* Link the iocb into the context's free list */
+		rcu_read_lock();
 		spin_lock_irq(&ctx->ctx_lock);
 		really_put_req(ctx, req);
+		/*
+		 * at that point ctx might've been killed, but actual
+		 * freeing is RCU'd
+		 */
 		spin_unlock_irq(&ctx->ctx_lock);
+		rcu_read_unlock();

-		put_ioctx(ctx);
 		spin_lock_irq(&fput_lock);
 	}
 	spin_unlock_irq(&fput_lock);
@@ -644,7 +643,6 @@ static int __aio_put_req(struct kioctx *
 	 * this function will be executed w/out any aio kthread wakeup.
 	 */
 	if (unlikely(!fput_atomic(req->ki_filp))) {
-		get_ioctx(ctx);
 		spin_lock(&fput_lock);
 		list_add(&req->ki_list, &fput_head);
 		spin_unlock(&fput_lock);

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