[PATCH v2 2/3] btrfs: fix race on ENOMEM in alloc_extent_buffer

[Omar Sandoval]

Consider the following interleaving of overlapping calls to
alloc_extent_buffer:

Call 1:

- Successfully allocates a few pages with find_or_create_page
- find_or_create_page fails, goto free_eb
- Unlocks the allocated pages

Call 2:
- Calls find_or_create_page and gets a page in call 1's extent_buffer
- Finds that the page is already associated with an extent_buffer
- Grabs a reference to the half-written extent_buffer and calls
  mark_extent_buffer_accessed on it

mark_extent_buffer_accessed will then try to call mark_page_accessed on
a null page and panic.

The fix is to decrement the reference count on the half-written
extent_buffer before unlocking the pages so call 2 won't use it. We
should also set exists = NULL in the case that we don't use exists to
avoid accidentally returning a freed extent_buffer in an error case.

Signed-off-by: Omar Sandoval <osandov@xxxxxxxxxxx>
---
 fs/btrfs/extent_io.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index c7233ff..7ecfae0 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4867,6 +4867,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 				mark_extent_buffer_accessed(exists, p);
 				goto free_eb;
 			}
+			exists = NULL;
 
 			/*
 			 * Do this so attach doesn't complain and we need to
@@ -4930,12 +4931,12 @@ again:
 	return eb;
 
 free_eb:
+	WARN_ON(!atomic_dec_and_test(&eb->refs));
 	for (i = 0; i < num_pages; i++) {
 		if (eb->pages[i])
 			unlock_page(eb->pages[i]);
 	}
 
-	WARN_ON(!atomic_dec_and_test(&eb->refs));
 	btrfs_release_extent_buffer(eb);
 	return exists;
 }
-- 
2.3.0

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