Vulnerability CVE-2024-26904
Published: 2024-04-17

Description:
In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix data race at btrfs_use_block_rsv() when accessing block reserve

At btrfs_use_block_rsv() we read the size of a block reserve without
locking its spinlock, which makes KCSAN complain because the size of a
block reserve is always updated while holding its spinlock. The report
from KCSAN is the following:

[653.313148] BUG: KCSAN: data-race in btrfs_update_delayed_refs_rsv [btrfs] / btrfs_use_block_rsv [btrfs]

[653.314755] read to 0x000000017f5871b8 of 8 bytes by task 7519 on cpu 0:
[653.314779] btrfs_use_block_rsv+0xe4/0x2f8 [btrfs]
[653.315606] btrfs_alloc_tree_block+0xdc/0x998 [btrfs]
[653.316421] btrfs_force_cow_block+0x220/0xe38 [btrfs]
[653.317242] btrfs_cow_block+0x1ac/0x568 [btrfs]
[653.318060] btrfs_search_slot+0xda2/0x19b8 [btrfs]
[653.318879] btrfs_del_csums+0x1dc/0x798 [btrfs]
[653.319702] __btrfs_free_extent.isra.0+0xc24/0x2028 [btrfs]
[653.320538] __btrfs_run_delayed_refs+0xd3c/0x2390 [btrfs]
[653.321340] btrfs_run_delayed_refs+0xae/0x290 [btrfs]
[653.322140] flush_space+0x5e4/0x718 [btrfs]
[653.322958] btrfs_preempt_reclaim_metadata_space+0x102/0x2f8 [btrfs]
[653.323781] process_one_work+0x3b6/0x838
[653.323800] worker_thread+0x75e/0xb10
[653.323817] kthread+0x21a/0x230
[653.323836] __ret_from_fork+0x6c/0xb8
[653.323855] ret_from_fork+0xa/0x30

[653.323887] write to 0x000000017f5871b8 of 8 bytes by task 576 on cpu 3:
[653.323906] btrfs_update_delayed_refs_rsv+0x1a4/0x250 [btrfs]
[653.324699] btrfs_add_delayed_data_ref+0x468/0x6d8 [btrfs]
[653.325494] btrfs_free_extent+0x76/0x120 [btrfs]
[653.326280] __btrfs_mod_ref+0x6a8/0x6b8 [btrfs]
[653.327064] btrfs_dec_ref+0x50/0x70 [btrfs]
[653.327849] walk_up_proc+0x236/0xa50 [btrfs]
[653.328633] walk_up_tree+0x21c/0x448 [btrfs]
[653.329418] btrfs_drop_snapshot+0x802/0x1328 [btrfs]
[653.330205] btrfs_clean_one_deleted_snapshot+0x184/0x238 [btrfs]
[653.330995] cleaner_kthread+0x2b0/0x2f0 [btrfs]
[653.331781] kthread+0x21a/0x230
[653.331800] __ret_from_fork+0x6c/0xb8
[653.331818] ret_from_fork+0xa/0x30

So add a helper to get the size of a block reserve while holding the lock.
Reading the field while holding the lock instead of using the data_race()
annotation is used in order to prevent load tearing.

 References:
https://git.kernel.org/stable/c/2daa2a8e895e6dc2395f8628c011bcf1e019040d
https://git.kernel.org/stable/c/ab1be3f1aa7799f99155488c28eacaef65eb68fb
https://git.kernel.org/stable/c/f6d4d29a12655b42a13cec038c2902bb7efc50ed
https://git.kernel.org/stable/c/7e9422d35d574b646269ca46010a835ca074b310
https://git.kernel.org/stable/c/c7bb26b847e5b97814f522686068c5628e2b3646
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