linux-kernel - Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write

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Date:   Mon, 3 Feb 2020 12:54:26 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...capital.net>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write

On Mon, Feb 03, 2020 at 11:16:05PM +0800, Xiaoyao Li wrote:
> If split lock detect is enabled (warn/fatal), #AC handler calls die()
> when split lock happens in kernel.
> 
> A sane guest should never tigger emulation on a split-lock access, but
> it cannot prevent malicous guest from doing this. So just emulating the
> access as a write if it's a split-lock access to avoid malicous guest
> polluting the kernel log.
> 
> More detail analysis can be found:
> https://lkml.kernel.org/r/20200131200134.GD18946@linux.intel.com
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>  arch/x86/kvm/x86.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2d3be7f3ad67..821b7404c0fd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5847,6 +5847,13 @@ static int emulator_write_emulated(struct x86_emulate_ctxt *ctxt,
>  	(cmpxchg64((u64 *)(ptr), *(u64 *)(old), *(u64 *)(new)) == *(u64 *)(old))
>  #endif
>  
> +static inline bool across_cache_line_access(gpa_t gpa, unsigned int bytes)

s/across/split so as not to introduce another name.

> +{
> +	unsigned int cache_line_size = cache_line_size();
> +
> +	return (gpa & (cache_line_size - 1)) + bytes > cache_line_size;

I'd prefer to use the same logic as the page-split to avoid having to
reason about the correctness of two different algorithms.

> +}
> +
>  static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
>  				     unsigned long addr,
>  				     const void *old,
> @@ -5873,6 +5880,10 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
>  	if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
>  		goto emul_write;
>  
> +	if (get_split_lock_detect_state() != sld_off &&
> +	    across_cache_line_access(gpa, bytes))
> +		goto emul_write;

As an alternative to the above, the page/line splits can be handled in a
single check, e.g.

	page_line_mask = PAGE_MASK;
	if (is_split_lock_detect_enabled())
		page_line_mask = cache_line_size() - 1;
	if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
		goto emul_write;

> +
>  	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))
>  		goto emul_write;
>  
> -- 
> 2.23.0
>