On Tue, Feb 24, 2015 at 02:47:05AM -0800, Omar Sandoval wrote:
> 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.
Reviewed-by: Liu Bo <bo.li....@oracle.com>
>
> Signed-off-by: Omar Sandoval <osan...@osandov.com>
> ---
> 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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