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[PATCH] xfs: fix buffer use after free on IO error

To: xfs-oss <xfs@xxxxxxxxxxx>
Subject: [PATCH] xfs: fix buffer use after free on IO error
From: Eric Sandeen <sandeen@xxxxxxxxxx>
Date: Fri, 21 Mar 2014 21:48:50 -0500
Delivered-to: xfs@xxxxxxxxxxx
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.4.0
When testing exhaustion of dm snapshots, the following appeared
with CONFIG_DEBUG_OBJECTS_FREE enabled:

ODEBUG: free active (active state 0) object type: work_struct hint: 
xfs_buf_iodone_work+0x0/0x1d0 [xfs]

indicating that we'd freed a buffer which still had a pending reference,
down this path:

[  190.867975]  [<ffffffff8133e6fb>] debug_check_no_obj_freed+0x22b/0x270
[  190.880820]  [<ffffffff811da1d0>] kmem_cache_free+0xd0/0x370
[  190.892615]  [<ffffffffa02c5924>] xfs_buf_free+0xe4/0x210 [xfs]
[  190.905629]  [<ffffffffa02c6167>] xfs_buf_rele+0xe7/0x270 [xfs]
[  190.911770]  [<ffffffffa034c826>] xfs_trans_read_buf_map+0x7b6/0xac0 [xfs]

At issue is the fact that if IO fails in xfs_buf_iorequest,
we'll queue completion unconditionally, and then call
xfs_buf_rele; but if IO failed, there are no IOs remaining,
and xfs_buf_rele will free the bp while work is still queued.

Fix this by not scheduling completion if the buffer has
an error on it; run it immediately.  The rest is only comment
changes.

Thanks to dchinner for spotting the root cause.

Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---

p.s. or maybe this could be moved into _xfs_buf_ioend ...
I think I see some nice cleanups for xfs_buf_ioend vs.
_xfs_buf_ioend w.r.t. when "schedule" is true, so maybe
I can clean it up then.


diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 9c061ef..45eb5ef 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -1361,21 +1361,29 @@ xfs_buf_iorequest(
                xfs_buf_wait_unpin(bp);
        xfs_buf_hold(bp);
 
-       /* Set the count to 1 initially, this will stop an I/O
+       /*
+        * Set the count to 1 initially, this will stop an I/O
         * completion callout which happens before we have started
         * all the I/O from calling xfs_buf_ioend too early.
         */
        atomic_set(&bp->b_io_remaining, 1);
        _xfs_buf_ioapply(bp);
-       _xfs_buf_ioend(bp, 1);
+       /*
+        * If _xfs_buf_ioapply failed, we'll get back here with
+        * only the reference we took above.  _xfs_buf_ioend will
+        * drop it to zero, so we'd better not queue it for later,
+        * or we'll free it before it's done.
+        */
+       _xfs_buf_ioend(bp, bp->b_error ? 0 : 1);
 
        xfs_buf_rele(bp);
 }
 
 /*
  * Waits for I/O to complete on the buffer supplied.  It returns immediately if
- * no I/O is pending or there is already a pending error on the buffer.  It
- * returns the I/O error code, if any, or 0 if there was no error.
+ * no I/O is pending or there is already a pending error on the buffer, in 
which
+ * case nothing will ever complete.  It returns the I/O error code, if any, or
+ * 0 if there was no error.
  */
 int
 xfs_buf_iowait(

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