Re: [PATCH] btrfs: balance: fix null-ptr-deref in btrfs_may_alloc_data_chunk

[Qu Wenruo <wqu@suse.com>]

在 2026/3/17 00:16, ZhengYuan Huang 写道:
> [BUG]
> Running btrfs balance can trigger a null-ptr-deref before relocating a
> data chunk when metadata corruption leaves a chunk in the chunk tree
> without a corresponding block group in the in-memory cache:
> 
>    KASAN: null-ptr-deref in range [0x0000000000000088-0x000000000000008f]
>    RIP: 0010:btrfs_may_alloc_data_chunk+0x40/0x1c0 fs/btrfs/volumes.c:3601
>    Call Trace:
>      __btrfs_balance fs/btrfs/volumes.c:4217 [inline]
>      btrfs_balance+0x2516/0x42b0 fs/btrfs/volumes.c:4604
>      btrfs_ioctl_balance fs/btrfs/ioctl.c:3577 [inline]
>      btrfs_ioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313
>      ...
> 
> [CAUSE]
> __btrfs_balance() iterates the on-disk chunk tree and passes the chunk
> logical bytenr to btrfs_may_alloc_data_chunk() before relocating a data
> chunk. That helper then queries the in-memory block group cache:
> 
>    cache = btrfs_lookup_block_group(fs_info, chunk_offset);
>    chunk_type = cache->flags;   /* cache may be NULL */
> 
> On a corrupt image can contain a chunk item whose matching block group
> item is missing, so no block group is ever inserted into the cache. In
> that case btrfs_lookup_block_group() returns NULL.

I'd say adding a proper chunk/bg mapping check is the root fix.

Or you'll need to adhoc a lot of null pointer checks.

> 
> The code only guards this with ASSERT(cache), which becomes a no-op when
> CONFIG_BTRFS_ASSERT is disabled. The subsequent dereference of
> cache->flags therefore crashes the kernel.
> 
> [FIX]
> Add a NULL check after btrfs_lookup_block_group() in
> btrfs_may_alloc_data_chunk(). If the lookup fails, emit a btrfs_err()
> message identifying the affected bytenr and return -EUCLEAN to report
> filesystem corruption instead of dereferencing NULL.
> 
> The caller already treats negative returns from
> btrfs_may_alloc_data_chunk() as fatal errors, so balance aborts cleanly
> and reports the corruption to userspace.
> 
> Fixes: a6f93c71d412 ("Btrfs: avoid losing data raid profile when deleting a device")
> Cc: stable@vger.kernel.org
> Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
> ---
>   fs/btrfs/volumes.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 4958e074d420..4657b826b48b 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3597,7 +3597,12 @@ static int btrfs_may_alloc_data_chunk(struct btrfs_fs_info *fs_info,
>   	u64 bytes_used;
>   	u64 chunk_type;
>   
>   	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
> -	ASSERT(cache);
> +	if (!cache) {
> +		btrfs_err(fs_info,
> +			  "balance: chunk at bytenr %llu has no corresponding block group",
> +			  chunk_offset);
> +		return -EUCLEAN;
> +	}
>   	chunk_type = cache->flags;
>   	btrfs_put_block_group(cache);
>