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

[Liu Bo <bo.li.liu@oracle.com>]

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.liu@oracle.com>

> 
> Signed-off-by: Omar Sandoval <osandov@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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