linux-kernel - Re: [PATCH] x86/mm: determine whether the fault address is canonical

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Date:   Fri, 4 Oct 2019 08:31:15 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Changbin Du <changbin.du@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: determine whether the fault address is canonical

On Fri, Oct 04, 2019 at 07:39:08AM -0700, Dave Hansen wrote:
> On 10/4/19 6:45 AM, Changbin Du wrote:
> > +static inline bool is_canonical_addr(u64 addr)
> > +{
> > +#ifdef CONFIG_X86_64
> > +	int shift = 64 - boot_cpu_data.x86_phys_bits;
> 
> I think you mean to check the virtual bits member, not "phys_bits".
> 
> BTW, I also prefer the IS_ENABLED(CONFIG_) checks to explicit #ifdefs.
> Would one of those work in this case?
> 
> As for the error message:
> 
> >  {
> > -	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
> > +	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault at %s address in user access.",
> > +		  is_canonical_addr(fault_addr) ? "canonical" : "non-canonical");
> 
> I've always read that as "the GP might have been caused by a
> non-canonical access".  The main nit I'd have with the change is that I
> don't think all #GP's during user access functions which are given a
> non-canonical address *necessarily* caused the #GP.
> 
> There are a billion ways you can get a #GP and I bet canonical
> violations aren't the only way you can get one in a user copy function.

All the other reasons would require a fairly egregious kernel bug, hence
the speculation that the #GP is due to a non-canonical address.  Something
like the following would be more precise, though highly unlikely to ever
be exercised, e.g. KVM had a fatal bug related to injecting a non-zero
error code that went unnoticed for years.

	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. %s?\n",
		  (IS_ENABLED(CONFIG_X86_64) && !error_code) ? "Non-canonical address" :
		  					       "Segmentation bug");