linux-kernel - Re: BUG: KASAN: Use-after-free

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Date:   Tue, 24 Jan 2017 01:52:09 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Matias Bjørling <m@...rling.me>
Cc:     Christoph Hellwig <hch@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: BUG: KASAN: Use-after-free

On Tue, Jan 24, 2017 at 10:32:11AM +0100, Matias Bjørling wrote:
> *(gdb) list *blkdev_direct_IO+0x50c
> 0xffffffff8142ab8c is in blkdev_direct_IO (fs/block_dev.c:401).
> 396			submit_bio(bio);
> 397			bio = bio_alloc(GFP_KERNEL, nr_pages);
> 398		}
> 399		blk_finish_plug(&plug);
> 400	
> 401		if (!dio->is_sync)
> 402			return -EIOCBQUEUED;
> 
> It looks like the qc = submit_bio() completes the I/O before the
> blkdev_direct_IO completes, which then leads to the use after free case,
> when the private dio struct is accessed.

Ok, the is_sync check here is clearly a bug, we need a local variable
for is_sync as well for the aio case:

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 5db5d13..3c47614 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -331,7 +331,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
 	struct blk_plug plug;
 	struct blkdev_dio *dio;
 	struct bio *bio;
-	bool is_read = (iov_iter_rw(iter) == READ);
+	bool is_read = (iov_iter_rw(iter) == READ), is_sync;
 	loff_t pos = iocb->ki_pos;
 	blk_qc_t qc = BLK_QC_T_NONE;
 	int ret;
@@ -344,7 +344,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
 	bio_get(bio); /* extra ref for the completion handler */
 
 	dio = container_of(bio, struct blkdev_dio, bio);
-	dio->is_sync = is_sync_kiocb(iocb);
+	dio->is_sync = is_sync = is_sync_kiocb(iocb);
 	if (dio->is_sync)
 		dio->waiter = current;
 	else
@@ -398,7 +398,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
 	}
 	blk_finish_plug(&plug);
 
-	if (!dio->is_sync)
+	if (!is_sync)
 		return -EIOCBQUEUED;
 
 	for (;;) {