linux-kernel - Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

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Date: Tue, 21 May 2024 06:35:49 -0700
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>
Cc: linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
 cho@...rosoft.com, decui@...rosoft.com, John.Starks@...rosoft.com
Subject: Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO


On 5/21/24 12:35 AM, Kirill A. Shutemov wrote:
> Currently attempt to do MMIO from userspace in TDX guest leads to
> warning about unexpect #VE and SIGSEGV being delivered to the process.
>
> Enlightened userspace might choose to deal with MMIO on their own if
> kernel doesn't emulate it.

Any specific use cases ? Like who is using it?

> Handle EPT_VIOLATION exit reason for userspace and deliver SIGBUS
> instead of SIGSEV. SIGBUS is more appropriate for MMIO situation.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---

Code looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>

>  arch/x86/coco/tdx/tdx.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index c1cb90369915..d2aa93cebf5a 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -7,6 +7,7 @@
>  #include <linux/cpufeature.h>
>  #include <linux/export.h>
>  #include <linux/io.h>
> +#include <linux/sched/signal.h>
>  #include <asm/coco.h>
>  #include <asm/tdx.h>
>  #include <asm/vmx.h>
> @@ -630,6 +631,11 @@ void tdx_get_ve_info(struct ve_info *ve)
>  	ve->instr_info  = upper_32_bits(args.r10);
>  }
>  
> +static inline bool is_private_gpa(u64 gpa)
> +{
> +	return gpa == cc_mkenc(gpa);
> +}
> +
>  /*
>   * Handle the user initiated #VE.
>   *
> @@ -641,17 +647,20 @@ static int virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
>  	switch (ve->exit_reason) {
>  	case EXIT_REASON_CPUID:
>  		return handle_cpuid(regs, ve);
> +	case EXIT_REASON_EPT_VIOLATION:
> +		if (is_private_gpa(ve->gpa))
> +			panic("Unexpected EPT-violation on private memory.");
> +
> +		force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)ve->gla);
> +
> +		/* Return 0 to avoid incrementing RIP */
> +		return 0;
>  	default:
>  		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
>  		return -EIO;
>  	}
>  }
>  
> -static inline bool is_private_gpa(u64 gpa)
> -{
> -	return gpa == cc_mkenc(gpa);
> -}
> -
>  /*
>   * Handle the kernel #VE.
>   *

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer