CVE-2024-42240 x86/bhi: Avoid warning in #DB handler due to BHI mitigation
A local attacker can trigger a kernel warning and potential crash by exploiting a flaw in the BHI mitigation's single-step handling during SYSENTER execution.
- 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-2024-42240?
CVE-2024-42240 is a medium-severity denial of service vulnerability in Linux kernel, published on 7 Aug 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:
x86/bhi: Avoid warning in #DB handler due to BHI mitigation
When BHI mitigation is enabled, if SYSENTER is invoked with the TF flag set then entry_SYSENTER_compat() uses CLEAR_BRANCH_HISTORY and calls the clear_bhb_loop() before the TF flag is cleared. This causes the #DB handler (exc_debug_kernel()) to issue a warning because single-step is used outside the entry_SYSENTER_compat() function.
To address this issue, entry_SYSENTER_compat() should use CLEAR_BRANCH_HISTORY after making sure the TF flag is cleared.
The problem can be reproduced with the following sequence:
$ cat sysenter_step.c int main() { asm("pushf; pop %ax; bts $8,%ax; push %ax; popf; sysenter"); }
$ gcc -o sysenter_step sysenter_step.c
$ ./sysenter_step Segmentation fault (core dumped)
The program is expected to crash, and the #DB handler will issue a warning.
Kernel log:
WARNING: CPU: 27 PID: 7000 at arch/x86/kernel/traps.c:1009 exc_debug_kernel+0xd2/0x160 ... RIP: 0010:exc_debug_kernel+0xd2/0x160 ... Call Trace: <#DB> ? show_regs+0x68/0x80 ? __warn+0x8c/0x140 ? exc_debug_kernel+0xd2/0x160 ? report_bug+0x175/0x1a0 ? handle_bug+0x44/0x90 ? exc_invalid_op+0x1c/0x70 ? asm_exc_invalid_op+0x1f/0x30 ? exc_debug_kernel+0xd2/0x160 exc_debug+0x43/0x50 asm_exc_debug+0x1e/0x40 RIP: 0010:clear_bhb_loop+0x0/0xb0 ... </#DB> <TASK> ? entry_SYSENTER_compat_after_hwframe+0x6e/0x8d </TASK>
[ bp: Massage commit message. ]
Weakness: CWE-835
Which versions are affected by CVE-2024-42240?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-42240?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2024-42240 being exploited?
There is no known exploitation of CVE-2024-42240 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-47528, AZL-47791, BELL-CVE-2024-42240, DEBIAN-CVE-2024-42240, GHSA-jxv8-jmp2-87p8, UBUNTU-CVE-2024-42240.