# CVE-2025-40295: fscrypt: fix left shift underflow when inode->i_blkbits > PAGE_SHIFT

> A local attacker can trigger a kernel crash via UBSAN warning when using specific block sizes with fscrypt, requiring a patched kernel.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Dec 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40295, DEBIAN-CVE-2025-40295, GHSA-h3fq-7jwc-95rx, UBUNTU-CVE-2025-40295

## What is CVE-2025-40295?

CVE-2025-40295 is a vulnerability in Linux, published on 8 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

fscrypt: fix left shift underflow when inode->i_blkbits > PAGE_SHIFT

When simulating an nvme device on qemu with both logical_block_size and
physical_block_size set to 8 KiB, an error trace appears during
partition table reading at boot time. The issue is caused by
inode->i_blkbits being larger than PAGE_SHIFT, which leads to a left
shift of -1 and triggering a UBSAN warning.

[    2.697306] ------------[ cut here ]------------
[    2.697309] UBSAN: shift-out-of-bounds in fs/crypto/inline_crypt.c:336:37
[    2.697311] shift exponent -1 is negative
[    2.697315] CPU: 3 UID: 0 PID: 274 Comm: (udev-worker) Not tainted 6.18.0-rc2+ #34 PREEMPT(voluntary)
[    2.697317] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[    2.697320] Call Trace:
[    2.697324]  <TASK>
[    2.697325]  dump_stack_lvl+0x76/0xa0
[    2.697340]  dump_stack+0x10/0x20
[    2.697342]  __ubsan_handle_shift_out_of_bounds+0x1e3/0x390
[    2.697351]  bh_get_inode_and_lblk_num.cold+0x12/0x94
[    2.697359]  fscrypt_set_bio_crypt_ctx_bh+0x44/0x90
[    2.697365]  submit_bh_wbc+0xb6/0x190
[    2.697370]  block_read_full_folio+0x194/0x270
[    2.697371]  ? __pfx_blkdev_get_block+0x10/0x10
[    2.697375]  ? __pfx_blkdev_read_folio+0x10/0x10
[    2.697377]  blkdev_read_folio+0x18/0x30
[    2.697379]  filemap_read_folio+0x40/0xe0
[    2.697382]  filemap_get_pages+0x5ef/0x7a0
[    2.697385]  ? mmap_region+0x63/0xd0
[    2.697389]  filemap_read+0x11d/0x520
[    2.697392]  blkdev_read_iter+0x7c/0x180
[    2.697393]  vfs_read+0x261/0x390
[    2.697397]  ksys_read+0x71/0xf0
[    2.697398]  __x64_sys_read+0x19/0x30
[    2.697399]  x64_sys_call+0x1e88/0x26a0
[    2.697405]  do_syscall_64+0x80/0x670
[    2.697410]  ? __x64_sys_newfstat+0x15/0x20
[    2.697414]  ? x64_sys_call+0x204a/0x26a0
[    2.697415]  ? do_syscall_64+0xb8/0x670
[    2.697417]  ? irqentry_exit_to_user_mode+0x2e/0x2a0
[    2.697420]  ? irqentry_exit+0x43/0x50
[    2.697421]  ? exc_page_fault+0x90/0x1b0
[    2.697422]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[    2.697425] RIP: 0033:0x75054cba4a06
[    2.697426] Code: 5d e8 41 8b 93 08 03 00 00 59 5e 48 83 f8 fc 75 19 83 e2 39 83 fa 08 75 11 e8 26 ff ff ff 66 0f 1f 44 00 00 48 8b 45 10 0f 05 <48> 8b 5d f8 c9 c3 0f 1f 40 00 f3 0f 1e fa 55 48 89 e5 48 83 ec 08
[    2.697427] RSP: 002b:00007fff973723a0 EFLAGS: 00000202 ORIG_RAX: 0000000000000000
[    2.697430] RAX: ffffffffffffffda RBX: 00005ea9a2c02760 RCX: 000075054cba4a06
[    2.697432] RDX: 0000000000002000 RSI: 000075054c190000 RDI: 000000000000001b
[    2.697433] RBP: 00007fff973723c0 R08: 0000000000000000 R09: 0000000000000000
[    2.697434] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
[    2.697434] R13: 00005ea9a2c027c0 R14: 00005ea9a2be5608 R15: 00005ea9a2be55f0
[    2.697436]  </TASK>
[    2.697436] ---[ end trace ]---

This situation can happen for block devices because when
CONFIG_TRANSPARENT_HUGEPAGE is enabled, the maximum logical_block_size
is 64 KiB. set_init_blocksize() then sets the block device
inode->i_blkbits to 13, which is within this limit.

File I/O does not trigger this problem because for filesystems that do
not support the FS_LBS feature, sb_set_blocksize() prevents
sb->s_blocksize_bits from being larger than PAGE_SHIFT. During inode
allocation, alloc_inode()->inode_init_always() assigns inode->i_blkbits
from sb->s_blocksize_bits. Currently, only xfs_fs_type has the FS_LBS
flag, and since xfs I/O paths do not reach submit_bh_wbc(), it does not
hit the left-shift underflow issue.

[EB: use folio_pos() and consolidate the two shifts by i_blkbits]

## Which versions are affected by CVE-2025-40295?


## How do I fix CVE-2025-40295?

Apply the vendor's patch.

## Is CVE-2025-40295 being exploited?

There is no known exploitation of CVE-2025-40295 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40295 and fix it if it is affected.

What it is: CVE-2025-40295 is a vulnerability in Linux, published on 8 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-40295 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-40295/ (Markdown: https://insights.sbomify.com/CVE-2025-40295/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40295
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40295
- OSV: https://osv.dev/vulnerability/CVE-2025-40295

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40295/
Summaries are generated and may contain mistakes; the official description above is the record.