CVE-2024-42240

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> ? entry_SYSENTER_compat_after_hwframe+0x6e/0x8d

[ bp: Massage commit message. ]

Weakness

The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.

Affected Software

References

• https://git.kernel.org/stable/c/08518d48e5b744620524f0acd7c26c19bda7f513
• https://git.kernel.org/stable/c/a765679defe1dc1b8fa01928a6ad6361e72a1364
• https://git.kernel.org/stable/c/ac8b270b61d48fcc61f052097777e3b5e11591e0
• https://git.kernel.org/stable/c/dae3543db8f0cf8ac1a198c3bb4b6e3c24d576cf
• https://git.kernel.org/stable/c/db56615e96c439e13783d7715330e824b4fd4b84
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html