linux-kernel - [PATCH 1/4] fs: btrfs: fix a data race in btrfs_block_group

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Date:   Sat,  9 May 2020 13:20:01 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     clm@...com, josef@...icpanda.com, dsterba@...e.com
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jia-Ju Bai <baijiaju1990@...il.com>
Subject: [PATCH 1/4] fs: btrfs: fix a data race in btrfs_block_group_done()

The functions btrfs_block_group_done() and caching_thread() are 
concurrently executed at runtime in the following call contexts:

Thread 1:
  btrfs_sync_file()
    start_ordered_ops()
      btrfs_fdatawrite_range()
        btrfs_writepages() [via function pointer]
          extent_writepages()
            extent_write_cache_pages()
              __extent_writepage()
                writepage_delalloc()
                  btrfs_run_delalloc_range()
                    cow_file_range()
                      btrfs_reserve_extent()
                        find_free_extent()
                          btrfs_block_group_done()

Thread 2:
  caching_thread()

In btrfs_block_group_done():
  smp_mb();
  return cache->cached == BTRFS_CACHE_FINISHED ||
      cache->cached == BTRFS_CACHE_ERROR;

In caching_thread():
  spin_lock(&block_group->lock);
  block_group->caching_ctl = NULL;
  block_group->cached = ret ? BTRFS_CACHE_ERROR : BTRFS_CACHE_FINISHED;
  spin_unlock(&block_group->lock);

The values cache->cached and block_group->cached access the same memory, 
and thus a data race can occur.
This data race was found and actually reproduced by our concurrency 
fuzzer.

To fix this race, the spinlock cache->lock is used to protect the 
access to cache->cached in btrfs_block_group_done().

Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
---
 fs/btrfs/block-group.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h
index 107bb557ca8d..fb5f12acea40 100644
--- a/fs/btrfs/block-group.h
+++ b/fs/btrfs/block-group.h
@@ -278,9 +278,13 @@ static inline u64 btrfs_system_alloc_profile(struct btrfs_fs_info *fs_info)
 
 static inline int btrfs_block_group_done(struct btrfs_block_group *cache)
 {
+	int flag;
 	smp_mb();
-	return cache->cached == BTRFS_CACHE_FINISHED ||
-		cache->cached == BTRFS_CACHE_ERROR;
+	spin_lock(&cache->lock);
+	flag = (cache->cached == BTRFS_CACHE_FINISHED ||
+		cache->cached == BTRFS_CACHE_ERROR);
+	spin_unlock(&cache->lock);
+	return flag;
 }
 
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
-- 
2.17.1