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btrfs: balance: fix potential bg lookup failure in chunk_usage_filter()

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[BUG]
Running btrfs balance with a usage filter (-dusage=N) can trigger a
null-ptr-deref when metadata corruption causes a chunk to have no
corresponding block group in the in-memory cache:

  KASAN: null-ptr-deref in range [0x0000000000000070-0x0000000000000077]
  RIP: 0010:chunk_usage_filter fs/btrfs/volumes.c:3874 [inline]
  RIP: 0010:should_balance_chunk fs/btrfs/volumes.c:4018 [inline]
  RIP: 0010:__btrfs_balance fs/btrfs/volumes.c:4172 [inline]
  RIP: 0010:btrfs_balance+0x2024/0x42b0 fs/btrfs/volumes.c:4604
  ...
  Call Trace:
    btrfs_ioctl_balance fs/btrfs/ioctl.c:3577 [inline]
    btrfs_ioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313
    vfs_ioctl fs/ioctl.c:51 [inline]
    ...

The bug is reproducible on current development branch.

[CAUSE]
Two separate data structures are involved:

1. The on-disk chunk tree, which records every chunk (logical address
   space region) and is iterated by __btrfs_balance().

2. The in-memory block group cache (fs_info->block_group_cache_tree),
   which is built at mount time by btrfs_read_block_groups() and holds
   a struct btrfs_block_group for each chunk. This cache is what the
   usage filter queries.

On a well-formed filesystem, these two are kept in 1:1 correspondence.
However, btrfs_read_block_groups() builds the cache from block group
items in the extent tree, not directly from the chunk tree. A corrupted
image can therefore contain a chunk item in the chunk tree whose
corresponding block group item is absent from the extent tree; that
chunk's block group is then never inserted into the in-memory cache.

When balance iterates the chunk tree and reaches such an orphaned chunk,
should_balance_chunk() calls chunk_usage_filter(), which queries the block
group cache:

  cache = btrfs_lookup_block_group(fs_info, chunk_offset);
  chunk_used = cache->used;   /* cache may be NULL */

btrfs_lookup_block_group() returns NULL silently when no cached entry
covers chunk_offset. chunk_usage_filter() does not check the return value,
so the immediately following dereference of cache->used triggers the crash.

[FIX]
Add a NULL check after btrfs_lookup_block_group() in chunk_usage_filter().
When the lookup fails, emit a btrfs_err() message identifying the
affected bytenr and return -EUCLEAN to indicate filesystem corruption.

Since chunk_usage_filter() now has an error path, change its return type
from bool to error pointer and 0 if the chunk passes the usage filter,
and 1 if it should be skipped.

Update should_balance_chunk() accordingly to propagate negative errors
from the usage filter.

Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
ZhengYuan Huang 2026-03-25 08:43:36 +08:00 committed by David Sterba
parent c6153ed23e
commit 6dde5221f6
1 changed files with 23 additions and 9 deletions
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32
fs/btrfs/volumes.c
View File

@ -3988,14 +3988,19 @@ static bool chunk_usage_range_filter(struct btrfs_fs_info *fs_info, u64 chunk_of
return ret;
}
static bool chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
struct btrfs_balance_args *bargs)
static int chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
struct btrfs_balance_args *bargs)
{
struct btrfs_block_group *cache;
u64 chunk_used, user_thresh;
bool ret = true;
int ret = 1;
cache = btrfs_lookup_block_group(fs_info, chunk_offset);
if (unlikely(!cache)) {
btrfs_err(fs_info, "balance: chunk at bytenr %llu has no corresponding block group",
chunk_offset);
return -EUCLEAN;
}
chunk_used = cache->used;
if (bargs->usage_min == 0)
@ -4006,7 +4011,7 @@ static bool chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
user_thresh = mult_perc(cache->length, bargs->usage);
if (chunk_used < user_thresh)
ret = false;
ret = 0;
btrfs_put_block_group(cache);
return ret;
@ -4111,8 +4116,8 @@ static bool chunk_soft_convert_filter(u64 chunk_type, struct btrfs_balance_args
return false;
}
static bool should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk *chunk,
u64 chunk_offset)
static int should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk *chunk,
u64 chunk_offset)
{
struct btrfs_fs_info *fs_info = leaf->fs_info;
struct btrfs_balance_control *bctl = fs_info->balance_ctl;
@ -4145,9 +4150,14 @@ static bool should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk
}
/* usage filter */
if ((bargs->flags & BTRFS_BALANCE_ARGS_USAGE) &&
chunk_usage_filter(fs_info, chunk_offset, bargs)) {
return false;
if (bargs->flags & BTRFS_BALANCE_ARGS_USAGE) {
int ret2;
ret2 = chunk_usage_filter(fs_info, chunk_offset, bargs);
if (ret2 < 0)
return ret2;
if (ret2)
return false;
} else if ((bargs->flags & BTRFS_BALANCE_ARGS_USAGE_RANGE) &&
chunk_usage_range_filter(fs_info, chunk_offset, bargs)) {
return false;
@ -4430,6 +4440,10 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
ret = should_balance_chunk(leaf, chunk, found_key.offset);
btrfs_release_path(path);
if (ret < 0) {
mutex_unlock(&fs_info->reclaim_bgs_lock);
goto error;
}
if (!ret) {
mutex_unlock(&fs_info->reclaim_bgs_lock);
goto loop;