Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB handler

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, May 23, 2024 at 02:33:22PM +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@oracle.com>
> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> ---
>  arch/x86/entry/entry_64_compat.S | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index 11c9b8efdc4c..7fa04edc87e9 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -91,7 +91,6 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>  
>  	IBRS_ENTER
>  	UNTRAIN_RET
> -	CLEAR_BRANCH_HISTORY
>  
>  	/*
>  	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
> @@ -116,6 +115,12 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>  	jnz	.Lsysenter_fix_flags
>  .Lsysenter_flags_fixed:
>  
> +	/*
> +	 * CLEAR_BRANCH_HISTORY can call other functions. It should be invoked
> +	 * after making sure TF is cleared because single-step is ignored only
> +	 * for instructions inside the entry_SYSENTER_compat function.
> +	 */
> +	CLEAR_BRANCH_HISTORY
>  	movq	%rsp, %rdi
>  	call	do_SYSENTER_32
>  	jmp	sysret32_from_system_call
> -- 
> 2.39.3
> 

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
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