Commit 5ca64f45 authored Feb 24, 2015 by Omar Sandoval Committed by Chris Mason Apr 26, 2015

btrfs: fix race on ENOMEM in alloc_extent_buffer



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@osandov.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

parent 67b7859e

fs/btrfs/extent_io.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -4870,6 +4870,7 @@ 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
		@@ -4933,12 +4934,12 @@ 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;
		}

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