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>
<TASK>
? entry_SYSENTER_compat_after_hwframe+0x6e/0x8d
</TASK>
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