aio: fix the "too late munmap()" race

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@zeniv.linux.org.uk>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/aio.c b/fs/aio.c
index f6578cb22d00e1b9fe1b5a9228ae4ef1241b3ba1..b9d64d89a0437aa4db5b26bf07e1ecb870d6fba1 100644 (file)
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -228,12 +228,6 @@ static void __put_ioctx(struct kioctx *ctx)
        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_struct *data)
                        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 *ctx, struct kiocb *req)
         * 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);