[PATCH] Bug in error recovery in fs/buffer.c::__block_prepare_write()

fs/buffer.c::__block_prepare_write() has broken error recovery. It calls
the get_block() callback with "create = 1" and if that succeeds it
immediately clears buffer_new on the just allocated buffer (which has
buffer_new set).

The bug is that if an error occurs and get_block() returns != 0, we break
from this loop and go into recovery code. This code has this comment:

/* Error case: */
/*
* Zero out any newly allocated blocks to avoid exposing stale
* data. If BH_New is set, we know that the block was newly
* allocated in the above loop.
*/

So the intent is obviously good in that it wants to clear just allocated
and hence not zeroed buffers. However the code recognises allocated
buffers by checking for buffer_new being set.

Unfortunately __block_prepare_write() as discussed above already cleared
buffer_new on all allocated buffers thus no buffers will be cleared during
error recovery and old data will be leaked.

The simplest way I can see to fix this is to make the current recovery code
work by _not_ clearing buffer_new after calling get_block() in
__block_prepare_write().

We cannot safely allow buffer_new buffers to "leak out" of
__block_prepare_write(), thus we simply do a quick loop over the buffers
clearing buffer_new on each of them if it is set just before returning
"success" from __block_prepare_write().

Signed-off-by: Anton Altaparmakov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/buffer.c

···
 			if (err)
 				break;
 			if (buffer_new(bh)) {
-				clear_buffer_new(bh);
 				unmap_underlying_metadata(bh->b_bdev,
 							bh->b_blocknr);
 				if (PageUptodate(page)) {
···
 		if (!buffer_uptodate(*wait_bh))
 			err = -EIO;
 	}
-	if (!err)
-		return err;
-
+	if (!err) {
+		bh = head;
+		do {
+			if (buffer_new(bh))
+				clear_buffer_new(bh);
+		} while ((bh = bh->b_this_page) != head);
+		return 0;
+	}
 	/* Error case: */
 	/*
 	 * Zero out any newly allocated blocks to avoid exposing stale