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

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Date:   Mon, 7 Oct 2019 16:44:23 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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


* Ingo Molnar <mingo@...nel.org> wrote:

> > 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");
> 
> Instead of trying to guess the reason of the #GPF (which guess might be 
> wrong), please just state it as the reason if we are sure that the cause 
> is a non-canonical address - and provide a best-guess if it's not but 
> clearly signal that it's a guess.
> 
> I.e. if I understood all the cases correctly we'd have three types of 
> messages generated:
> 
>  !error_code:
> 	"General protection fault in user access, due to non-canonical address."
> 
>  error_code && !is_canonical_addr(fault_addr):
> 	"General protection fault in user access. Non-canonical address?"
> 
>  error_code && is_canonical_addr(fault_addr):
> 	"General protection fault in user access. Segmentation bug?"

Now that I've read the rest of the thread, since fault_addr is always 0 
we can ignore most of this I suspect ...

Thanks,

	Ingo