Patch "reiserfs: fix race in prealloc discard" has been added to the 4.9-stable tree

Patch "reiserfs: fix race in prealloc discard" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    reiserfs: fix race in prealloc discard

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     reiserfs-fix-race-in-prealloc-discard.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 08db141b5313ac2f64b844fb5725b8d81744b417 Mon Sep 17 00:00:00 2001
From: Jeff Mahoney <jeffm@suse.com>
Date: Thu, 22 Jun 2017 16:47:34 -0400
Subject: reiserfs: fix race in prealloc discard

From: Jeff Mahoney <jeffm@suse.com>

commit 08db141b5313ac2f64b844fb5725b8d81744b417 upstream.

The main loop in __discard_prealloc is protected by the reiserfs write lock
which is dropped across schedules like the BKL it replaced.  The problem is
that it checks the value, calls a routine that schedules, and then adjusts
the state.  As a result, two threads that are calling
reiserfs_prealloc_discard at the same time can race when one calls
reiserfs_free_prealloc_block, the lock is dropped, and the other calls
reiserfs_free_prealloc_block with the same block number.  In the right
circumstances, it can cause the prealloc count to go negative.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/reiserfs/bitmap.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/fs/reiserfs/bitmap.c
+++ b/fs/reiserfs/bitmap.c
@@ -513,9 +513,17 @@ static void __discard_prealloc(struct re
 			       "inode has negative prealloc blocks count.");
 #endif
 	while (ei->i_prealloc_count > 0) {
-		reiserfs_free_prealloc_block(th, inode, ei->i_prealloc_block);
-		ei->i_prealloc_block++;
+		b_blocknr_t block_to_free;
+
+		/*
+		 * reiserfs_free_prealloc_block can drop the write lock,
+		 * which could allow another caller to free the same block.
+		 * We can protect against it by modifying the prealloc
+		 * state before calling it.
+		 */
+		block_to_free = ei->i_prealloc_block++;
 		ei->i_prealloc_count--;
+		reiserfs_free_prealloc_block(th, inode, block_to_free);
 		dirty = 1;
 	}
 	if (dirty)


Patches currently in stable-queue which might be from jeffm@suse.com are

queue-4.9/reiserfs-don-t-preallocate-blocks-for-extended-attributes.patch
queue-4.9/reiserfs-fix-race-in-prealloc-discard.patch