CVE-2024-42240: 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 entrySYSENTERcompat() uses CLEARBRANCHHISTORY and calls the clearbhbloop() before the TF flag is cleared. This causes the #DB handler (excdebugkernel()) to issue a warning because single-step is used outside the entrySYSENTERcompat() function.

To address this issue, entrySYSENTERcompat() should use CLEARBRANCHHISTORY after making sure the TF flag is cleared.

The problem can be reproduced with the following sequence:

$ cat sysenterstep.c int main() { asm("pushf; pop %ax; bts $8,%ax; push %ax; popf; sysenter"); }

$ gcc -o sysenterstep sysenterstep.c

$ ./sysenterstep 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 excdebugkernel+0xd2/0x160 ... RIP: 0010:excdebugkernel+0xd2/0x160 ... Call Trace: <#DB> ? showregs+0x68/0x80 ? warn+0x8c/0x140 ? excdebugkernel+0xd2/0x160 ? reportbug+0x175/0x1a0 ? handlebug+0x44/0x90 ? excinvalidop+0x1c/0x70 ? asmexcinvalidop+0x1f/0x30 ? excdebugkernel+0xd2/0x160 excdebug+0x43/0x50 asmexcdebug+0x1e/0x40 RIP: 0010:clearbhbloop+0x0/0xb0 ... </#DB> <TASK> ? entrySYSENTERcompatafterhwframe+0x6e/0x8d </TASK>

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