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

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Message-ID: <ijxp5xk6kq7gk4myfaijnq5vunrq6tcoqu3eoqoy6n7qfvvqjg@5ca372qmmy2o>
Date: Wed, 11 Sep 2024 16:03:33 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Alexey Gladkov <legion@...nel.org>
Cc: Dave Hansen <dave.hansen@...el.com>, 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>, 
	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 Wed, Sep 11, 2024 at 02:08:47PM +0200, Alexey Gladkov wrote:
> 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.

Also user_mode() check is redundant until later in the patchset. Move it
to the patch that allows userspace MMIO.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov