CVE-2021-47089 kfence: fix memory leak when cat kfence objects

Low 3.3 Published 4 Mar 2024 Updated 11 May 2026

A memory leak in the Linux kernel's kfence debug interface allows local attackers to exhaust system memory by repeatedly reading object data, potentially causing denial of service.

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

What is CVE-2021-47089?

CVE-2021-47089 is a low-severity memory corruption vulnerability in Linux kernel, published on 4 Mar 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:

kfence: fix memory leak when cat kfence objects

Hulk robot reported a kmemleak problem:

unreferenced object 0xffff93d1d8cc02e8 (size 248): comm "cat", pid 23327, jiffies 4624670141 (age 495992.217s) hex dump (first 32 bytes): 00 40 85 19 d4 93 ff ff 00 10 00 00 00 00 00 00 .@.............. 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ backtrace: seq_open+0x2a/0x80 full_proxy_open+0x167/0x1e0 do_dentry_open+0x1e1/0x3a0 path_openat+0x961/0xa20 do_filp_open+0xae/0x120 do_sys_openat2+0x216/0x2f0 do_sys_open+0x57/0x80 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 unreferenced object 0xffff93d419854000 (size 4096): comm "cat", pid 23327, jiffies 4624670141 (age 495992.217s) hex dump (first 32 bytes): 6b 66 65 6e 63 65 2d 23 32 35 30 3a 20 30 78 30 kfence-#250: 0x0 30 30 30 30 30 30 30 37 35 34 62 64 61 31 32 2d 0000000754bda12- backtrace: seq_read_iter+0x313/0x440 seq_read+0x14b/0x1a0 full_proxy_read+0x56/0x80 vfs_read+0xa5/0x1b0 ksys_read+0xa0/0xf0 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9

I find that we can easily reproduce this problem with the following commands:

cat /sys/kernel/debug/kfence/objects echo scan > /sys/kernel/debug/kmemleak cat /sys/kernel/debug/kmemleak

The leaked memory is allocated in the stack below:

do_syscall_64 do_sys_open do_dentry_open full_proxy_open seq_open ---> alloc seq_file vfs_read full_proxy_read seq_read seq_read_iter traverse ---> alloc seq_buf

And it should have been released in the following process:

do_syscall_64 syscall_exit_to_user_mode exit_to_user_mode_prepare task_work_run ____fput __fput full_proxy_release ---> free here

However, the release function corresponding to file_operations is not implemented in kfence. As a result, a memory leak occurs. Therefore, the solution to this problem is to implement the corresponding release function.

Weakness: CWE-401

Which versions are affected by CVE-2021-47089?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2021-47089?

Apply the vendor's patch.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2021-47089 being exploited?

There is no known exploitation of CVE-2021-47089 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as DEBIAN-CVE-2021-47089, GHSA-97gj-q9fx-vc88, UBUNTU-CVE-2021-47089.