# CVE-2021-47508: linux: In the Linux kernel, the following vulnerability has been resolved:  btrfs: free exchange changeset on failures  Fste...

CVE-2021-47508 is fixed in linux on 1 platform, first by DEBIAN-CVE-2021-47508, released May 24, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2021-47508
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0021
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 24, 2024

## Description

In the Linux kernel, the following vulnerability has been resolved:  btrfs: free exchange changeset on failures  Fstests runs on my VMs have show several kmemleak reports like the following.    unreferenced object 0xffff88811ae59080 (size 64):     comm "xfs_io", pid 12124, jiffies 4294987392 (age 6.368s)     hex dump (first 32 bytes):       00 c0 1c 00 00 00 00 00 ff cf 1c 00 00 00 00 00  ................       90 97 e5 1a 81 88 ff ff 90 97 e5 1a 81 88 ff ff  ................     backtrace:       [<00000000ac0176d2>] ulist_add_merge+0x60/0x150 [btrfs]       [<0000000076e9f312>] set_state_bits+0x86/0xc0 [btrfs]       [<0000000014fe73d6>] set_extent_bit+0x270/0x690 [btrfs]       [<000000004f675208>] set_record_extent_bits+0x19/0x20 [btrfs]       [<00000000b96137b1>] qgroup_reserve_data+0x274/0x310 [btrfs]       [<0000000057e9dcbb>] btrfs_check_data_free_space+0x5c/0xa0 [btrfs]       [<0000000019c4511d>] btrfs_delalloc_reserve_space+0x1b/0xa0 [btrfs]       [<000000006d37e007>] btrfs_dio_iomap_begin+0x415/0x970 [btrfs]       [<00000000fb8a74b8>] iomap_iter+0x161/0x1e0       [<0000000071dff6ff>] __iomap_dio_rw+0x1df/0x700       [<000000002567ba53>] iomap_dio_rw+0x5/0x20       [<0000000072e555f8>] btrfs_file_write_iter+0x290/0x530 [btrfs]       [<000000005eb3d845>] new_sync_write+0x106/0x180       [<000000003fb505bf>] vfs_write+0x24d/0x2f0       [<000000009bb57d37>] __x64_sys_pwrite64+0x69/0xa0       [<000000003eba3fdf>] do_syscall_64+0x43/0x90  In case brtfs_qgroup_reserve_data() or btrfs_delalloc_reserve_metadata() fail the allocated extent_changeset will not be freed.  So in btrfs_check_data_free_space() and btrfs_delalloc_reserve_space() free the allocated extent_changeset to get rid of the allocated memory.  The issue currently only happens in the direct IO write path, but only after 65b3c08606e5 ("btrfs: fix ENOSPC failure when attempting direct IO write into NOCOW range"), and also at defrag_one_locked_target(). Every other place is always calling extent_changeset_free() even if its call to btrfs_delalloc_reserve_space() or btrfs_check_data_free_space() has failed.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.15.15-1 | [DEBIAN-CVE-2021-47508](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47508.md) | May 24, 2024 |
| Linux | linux Debian 13.5 (trixie) | 5.15.15-1 | [DEBIAN-CVE-2021-47508](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47508.md) | May 24, 2024 |
| Linux | linux Debian 14 | 5.15.15-1 | [DEBIAN-CVE-2021-47508](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47508.md) | May 24, 2024 |

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Data as of 2026-11-08T13:06:02Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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