On Fri, May 24, 2024 at 09:04:59AM +0200, Alexandre Chartre wrote:
> 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 flag 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>
>
> Fixes: 7390db8aea0d ("x86/bhi: Add support for clearing branch history at syscall entry")
> Reported-by: Suman Maity <suman.m.maity@xxxxxxxxxx>
> Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
