# CVE-2021-47191: scsi: scsi_debug: Fix out-of-bound read in resp_readcap16()

> A local attacker with access to the scsi_debug device can trigger an out-of-bounds read, potentially causing a system crash or leaking kernel memory.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Apr 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47191, GHSA-q4cm-jgpj-qg42, UBUNTU-CVE-2021-47191

## What is CVE-2021-47191?

CVE-2021-47191 is a high-severity memory corruption vulnerability in Linux kernel, published on 10 Apr 2024. 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:

scsi: scsi_debug: Fix out-of-bound read in resp_readcap16()

The following warning was observed running syzkaller:

[ 3813.830724] sg_write: data in/out 65466/242 bytes for SCSI command 0x9e-- guessing data in;
[ 3813.830724]    program syz-executor not setting count and/or reply_len properly
[ 3813.836956] ==================================================================
[ 3813.839465] BUG: KASAN: stack-out-of-bounds in sg_copy_buffer+0x157/0x1e0
[ 3813.841773] Read of size 4096 at addr ffff8883cf80f540 by task syz-executor/1549
[ 3813.846612] Call Trace:
[ 3813.846995]  dump_stack+0x108/0x15f
[ 3813.847524]  print_address_description+0xa5/0x372
[ 3813.848243]  kasan_report.cold+0x236/0x2a8
[ 3813.849439]  check_memory_region+0x240/0x270
[ 3813.850094]  memcpy+0x30/0x80
[ 3813.850553]  sg_copy_buffer+0x157/0x1e0
[ 3813.853032]  sg_copy_from_buffer+0x13/0x20
[ 3813.853660]  fill_from_dev_buffer+0x135/0x370
[ 3813.854329]  resp_readcap16+0x1ac/0x280
[ 3813.856917]  schedule_resp+0x41f/0x1630
[ 3813.858203]  scsi_debug_queuecommand+0xb32/0x17e0
[ 3813.862699]  scsi_dispatch_cmd+0x330/0x950
[ 3813.863329]  scsi_request_fn+0xd8e/0x1710
[ 3813.863946]  __blk_run_queue+0x10b/0x230
[ 3813.864544]  blk_execute_rq_nowait+0x1d8/0x400
[ 3813.865220]  sg_common_write.isra.0+0xe61/0x2420
[ 3813.871637]  sg_write+0x6c8/0xef0
[ 3813.878853]  __vfs_write+0xe4/0x800
[ 3813.883487]  vfs_write+0x17b/0x530
[ 3813.884008]  ksys_write+0x103/0x270
[ 3813.886268]  __x64_sys_write+0x77/0xc0
[ 3813.886841]  do_syscall_64+0x106/0x360
[ 3813.887415]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

This issue can be reproduced with the following syzkaller log:

r0 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file0\x00', 0x26e1, 0x0)
r1 = syz_open_procfs(0xffffffffffffffff, &(0x7f0000000000)='fd/3\x00')
open_by_handle_at(r1, &(0x7f00000003c0)=ANY=[@ANYRESHEX], 0x602000)
r2 = syz_open_dev$sg(&(0x7f0000000000), 0x0, 0x40782)
write$binfmt_aout(r2, &(0x7f0000000340)=ANY=[@ANYBLOB="00000000deff000000000000000000000000000000000000000000000000000047f007af9e107a41ec395f1bded7be24277a1501ff6196a83366f4e6362bc0ff2b247f68a972989b094b2da4fb3607fcf611a22dd04310d28c75039d"], 0x126)

In resp_readcap16() we get "int alloc_len" value -1104926854, and then pass
the huge arr_len to fill_from_dev_buffer(), but arr is only 32 bytes. This
leads to OOB in sg_copy_buffer().

To solve this issue, define alloc_len as u32.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2021-47191?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47191?

Apply the vendor's patch.

## Is CVE-2021-47191 being exploited?

There is no known exploitation of CVE-2021-47191 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-47191 and fix it if it is affected.

What it is: CVE-2021-47191 is a high-severity memory corruption vulnerability in Linux kernel, published on 10 Apr 2024. 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-2021-47191 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2021-47191/ (Markdown: https://insights.sbomify.com/CVE-2021-47191/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47191
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47191
- OSV: https://osv.dev/vulnerability/CVE-2021-47191

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47191/
Summaries are generated and may contain mistakes; the official description above is the record.