x86/bhi: Avoid warning in #DB handler due to BHI mitigation

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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