HIGH
bcache SuperBlock UAF
CVE-2026-31580
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
KernelScan AI7.8HIGH
01Description
In the Linux kernel, the following vulnerability has been resolved: bcache: fix cached_dev.sb_bio use-after-free and crash In our production environment, we have received multiple crash reports regarding libceph, which have caught our attention: ``` [6888366.280350] Call Trace: [6888366.280452] blk_update_request+0x14e/0x370 [6888366.280561] blk_mq_end_request+0x1a/0x130 [6888366.280671] rbd_img_handle_request+0x1a0/0x1b0 [rbd] [6888366.280792] rbd_obj_handle_request+0x32/0x40 [rbd] [6888366.280903] __complete_request+0x22/0x70 [libceph] [6888366.281032] osd_dispatch+0x15e/0xb40 [libceph] [6888366.281164] ? inet_recvmsg+0x5b/0xd0 [6888366.281272] ? ceph_tcp_recvmsg+0x6f/0xa0 [libceph] [6888366.281405] ceph_con_process_message+0x79/0x140 [libceph] [6888366.281534] ceph_con_v1_try_read+0x5d7/0xf30 [libceph] [6888366.281661] ceph_con_workfn+0x329/0x680 [libceph] ``` After analyzing the coredump file, we found that the address of dc->sb_bio has been freed. We know that cached_dev is only freed when it is stopped. Since sb_bio is a part of struct cached_dev, rather than an alloc every time. If the device is stopped while writing to the superblock, the released address will be accessed at endio. This patch hopes to wait for sb_write to complete in cached_dev_free. It should be noted that we analyzed the cause of the problem, then tell all details to the QWEN and adopted the modifications it made.
02KernelScan AI Analysis
Risk summary
A use-after-free vulnerability in the bcache block layer cache can cause kernel crashes when cached devices are stopped while superblock write operations are in progress. This affects system stability and could potentially be exploited for privilege escalation by local users with device management permissions.
Vulnerability analysis
Root Cause: The bcache cached_dev structure contains an embedded sb_bio field used for superblock I/O operations. When a cached device is stopped and freed via cached_dev_free(), the structure is deallocated immediately without waiting for any pending superblock write operations to complete. If a superblock write is in progress during device shutdown, the I/O completion callback will attempt to access the freed sb_bio field, resulting in a use-after-free condition.
Attack Surface: This vulnerability affects local systems using bcache block layer caching. It requires local access to trigger device shutdown operations while superblock writes are in progress. The vulnerability is triggered through normal bcache device management operations and does not require special privileges beyond those needed to manage block devices.
Fix Mechanism: The patch adds a closure_sync(&dc->sb_write) call in cached_dev_free() before deallocating the cached_dev structure. This ensures that any pending superblock write operations complete before the structure containing the embedded sb_bio is freed, preventing the use-after-free condition.
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 5.10 | 5.10.258 | 81f44ed8c3f5 |
| 5.15 | 5.15.209 | f50e7c325ab1 |
| 6.1 | 6.1.175 | 9467d360be70 |
| 6.12 | 6.12.83 | add4982510f3 |
| 6.18 | 6.18.24 | 2d6965581e16 |
| 6.19 | 6.19.14 | 4f71c8ba2dc0 |
| 6.6 | 6.6.136 | 47fa09fe7f3e |
| 7.0 | 7.0.1 | 383f7fec0de8 |
| mainline | 7.1-rc1 | fec114a98b87 |