linux-kernel - Re: [PATCH v6 1/6] x86/tdx: Fix "in-kernel MMIO" check

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Message-ID: <ZuGITwFiv5X3wg0y@example.org>
Date: Wed, 11 Sep 2024 14:08:47 +0200
From: Alexey Gladkov <legion@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yuan Yao <yuan.yao@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Yuntao Wang <ytcoode@...il.com>, Kai Huang <kai.huang@...el.com>,
	Baoquan He <bhe@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	cho@...rosoft.com, decui@...rosoft.com, John.Starks@...rosoft.com,
	stable@...r.kernel.org
Subject: Re: [PATCH v6 1/6] x86/tdx: Fix "in-kernel MMIO" check

On Tue, Sep 10, 2024 at 12:54:19PM -0700, Dave Hansen wrote:
> On 9/6/24 04:49, Alexey Gladkov wrote:
> > +static inline bool is_kernel_addr(unsigned long addr)
> > +{
> > +	return (long)addr < 0;
> > +}
> > +
> >  static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> >  {
> >  	unsigned long *reg, val, vaddr;
> > @@ -434,6 +439,11 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> >  			return -EINVAL;
> >  	}
> >  
> > +	if (!user_mode(regs) && !is_kernel_addr(ve->gla)) {
> > +		WARN_ONCE(1, "Access to userspace address is not supported");
> > +		return -EINVAL;
> > +	}
> 
> Should we really be open-coding a "is_kernel_addr" check?  I mean,
> TASK_SIZE_MAX is there for a reason.  While I doubt we'd ever change the
> positive vs. negative address space convention on 64-bit, I don't see a
> good reason to write a 64-bit x86-specific is_kernel_addr() when a more
> generic, portable and conventional idiom would do.

I took arch/x86/events/perf_event.h:1262 as an example. There is no
special reason in its own function.

> So, please use either a:
> 
> 	addr < TASK_SIZE_MAX
> 
> check, or use fault_in_kernel_space() directly.

I'll use fault_in_kernel_space() since SEV uses it. Thanks.

-- 
Rgrds, legion