In 2023, security researchers uncovered a subtle but serious flaw in how the Linux kernel implemented protections against Spectre variant 2 (Branch Target Injection or BTI) attacks. This vulnerability, tracked as CVE-2023-1998, reveals that even when system administrators or virtual machine users tried their best to lock down their processes using recommended interfaces like prctl() or seccomp, they were left exposed under certain scenarios. This post explains the problem in plain language, shows where it comes from in the code, and demonstrates how exploitation might work—even in environments where you’d expect to be safe.

The Trusted Tools: prctl() and seccomp

Since the disclosure of Spectre and Meltdown type vulnerabilities, the Linux kernel added ways for applications to ask the kernel to turn on security mitigations. For Spectre-BTI, this is done using prctl():

prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, );

Or simply

prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, , );

And sometimes via seccomp

prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);


Cloud providers and security tooling rely on these.

IBRS (Indirect Branch Restricted Speculation): Stops certain branch-target attacks.

- STIBP (Single Thread Indirect Branch Predictors): Prevents one thread affecting another via branch predictors on Hyper-Threaded CPUs.

Modern CPUs have “Enhanced IBRS”, which keeps protection active between kernel and user. Older CPUs (or some configs) have "Legacy IBRS" where, for performance, the kernel clears IBRS on returning to userspace.

STIBP is NOT enabled when IBRS is active, according to kernel logic

Result: Userspace threads are left exposed to cross-thread branch target injection (Spectre-v2) attacks from sibling threads, even though you asked for them to stop using prctl().

- On bare-metal, you can force legacy IBRS mode with a kernel boot argument

  spectre_v2=ibrs

If (IBRS is active) → Don’t enable STIBP

- But if IBRS is legacy and turned off in userspace → Userspace loses implicit cross-thread protection!

Real World Consequence

Suppose you have a process (a secret application, or a container) and you call prctl() to disable branch speculation. As far as you can tell, you’re protected. But an attacker on a sibling hyperthread could still perform branch target injection attacks because STIBP is not set, and IBRS is no longer enabled for userspace.

Code Dive: Where It Went Wrong

From the Linux kernel’s source (see e.g. [arch/x86/kernel/cpu/bugs.c)]:

...
/*
 * If IBRS is in use, don't enable STIBP -- IBRS makes it redundant.
 */
if (spectre_v2_enabled == SPECTRE_V2_IBRS) {
    ...
    if (use_enhanced_ibrs)
        pr_info("Spectre V2 mitigation: Enhanced IBRS\n");
    else
        pr_info("Spectre V2 mitigation: IBRS\n");
    ...
    prefer_stibp = false;
}
...

But in legacy mode, after a ret to user, the kernel disables IBRS

static void switch_user(void)
{
    /* if not using enhanced ibrs, clear IBRS for userspace */
    if (!use_enhanced_ibrs)
        wrmsrl(MSR_IA32_SPEC_CTRL, );
}

The kernel thinks IBRS means STIBP isn’t needed

- But once in userspace, IBRS is off! So, no STIBP and no IBRS in userspace—attackers can exploit this window.

Demo: Exploiting the Window

To exploit, an attacker creates a sibling thread (or VM core) on the same CPU, running at the same time as your protected process. The attacker leverages branch predictor attacks to poison the branch prediction for your process—even though you explicitly asked the kernel with prctl or seccomp to shut this down.

Here’s a highly simplified PoC to check your exposure

#include <stdio.h>
#include <sys/prctl.h>
#include <linux/prctl.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    int ret;
    // Try to ask for Spectre v2 protection
    ret = prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, , );
    if (ret != ) perror("prctl");

    // Print contents of SPEC_CTRL msr
    int fd = open("/dev/cpu//msr", O_RDONLY);
    if (fd < ) return 1;
    unsigned long val;
    pread(fd, &val, sizeof(val), x48);   // MSR_IA32_SPEC_CTRL is x48
    printf("MSR_IA32_SPEC_CTRL: x%lx\n", val);

    close(fd);
    return ;
}


Run this when the kernel is in legacy IBRS mode and see if the right bits are set when in userspace—you may find they're not!

The Fix: What Needs to Happen

After CVE-2023-1998 was disclosed, kernel maintainers updated the logic to ensure STIBP is enabled when IBRS is turned off when returning to userspace, or provided warnings that mitigation wasn’t active.

Patches are available and backported for all major long-term Linux kernel series

- RedHat security advisory
- Ubuntu security note

References

- CVE-2023-1998 at NIST NVD
- Linux Kernel Spectre V2 code
- RedHat CVE-2023-1998 advisory
- Git commit fixing the bug

Conclusion

CVE-2023-1998 is a perfect example of how minor differences in hardware support and kernel logic can leave big security holes, even after you think you’ve done everything right. If your threat model includes cross-thread attacks (in cloud, container, or multi-user situations), make sure your kernel is patched, and watch out for default settings and boot arguments. Ask your provider if you’re unsure.

Stay safe and always keep your systems updated—corner cases like these often go unnoticed until a deep security audit or a real-world exploit brings them to light.


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Timeline

Published on: 04/21/2023 15:15:00 UTC
Last modified on: 05/03/2023 15:16:00 UTC