x86/bhi: Avoid warning in #DB handler due to BHI mitigation
[ Upstream commit ac8b270b ] 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. ] Fixes: 7390db8a ("x86/bhi: Add support for clearing branch history at syscall entry") Reported-by:Suman Maity <suman.m.maity@oracle.com> Signed-off-by:
Alexandre Chartre <alexandre.chartre@oracle.com> Signed-off-by:
Borislav Petkov (AMD) <bp@alien8.de> Reviewed-by:
Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by:
Pawan Gupta <pawan.kumar.gupta@linux.intel.com> Reviewed-by:
Josh Poimboeuf <jpoimboe@kernel.org> Link: https://lore.kernel.org/r/20240524070459.3674025-1-alexandre.chartre@oracle.com Signed-off-by:
Sasha Levin <sashal@kernel.org>
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