Re: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a bad area

[Sean Christopherson]

Cc-ing Jann and Andy.

On Tue, Aug 07, 2018 at 10:29:20AM -0700, Sean Christopherson wrote:
> Kernel addresses are always mapped with _PAGE_USER=0, i.e. PKRU
> isn't enforced, and so we should never see X86_PF_PK set on a
> kernel address fault.  WARN once to capture the issue in case we
> somehow don't die, e.g. the access originated in userspace.
> 
> Remove a similar check and its comment from spurious_fault_check().
> The intent of the comment (and later code[1]) was simply to document
> that spurious faults due to protection keys should be impossible, but
> that's irrelevant and a bit of a red herring since we should never
> get a protection keys fault on a kernel address regardless of the
> kernel's TLB flushing behavior.
> 
> [1] http://lists-archives.com/linux-kernel/28407455-x86-pkeys-new-page-fault-error-code-bit-pf_pk.html
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> ---
> There's no indication that this condition has ever been encountered.
> I came across the code in spurious_fault_check() and was confused as
> to why we would unconditionally treat a protection keys fault as
> spurious when the comment explicitly stated that such a case should
> be impossible.
> 
> Dave Hansen suggested adding a WARN_ON_ONCE in spurious_fault_check(),
> but it seemed more appropriate to freak out on any protection keys
> fault on a kernel address since that would imply a hardware issue or
> kernel bug.  I omitted a Suggested-by since this isn't necessarily
> what Dave had in mind.
> 
>  arch/x86/mm/fault.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 2aafa6ab6103..f19a55972136 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1040,12 +1040,6 @@ static int spurious_fault_check(unsigned long error_code, pte_t *pte)
>  
>  	if ((error_code & X86_PF_INSTR) && !pte_exec(*pte))
>  		return 0;
> -	/*
> -	 * Note: We do not do lazy flushing on protection key
> -	 * changes, so no spurious fault will ever set X86_PF_PK.
> -	 */
> -	if ((error_code & X86_PF_PK))
> -		return 1;
>  
>  	return 1;
>  }
> @@ -1241,6 +1235,14 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  	 * protection error (error_code & 9) == 0.
>  	 */
>  	if (unlikely(fault_in_kernel_space(address))) {
> +		/*
> +		 * We should never encounter a protection keys fault on a
> +		 * kernel address as kernel address are always mapped with
> +		 * _PAGE_USER=0, i.e. PKRU isn't enforced.
> +		 */
> +		if (WARN_ON_ONCE(error_code & X86_PF_PK))
> +			goto bad_kernel_address;
> +
>  		if (!(error_code & (X86_PF_RSVD | X86_PF_USER | X86_PF_PROT))) {
>  			if (vmalloc_fault(address) >= 0)
>  				return;
> @@ -1253,6 +1255,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		/* kprobes don't want to hook the spurious faults: */
>  		if (kprobes_fault(regs))
>  			return;
> +
> +bad_kernel_address:
>  		/*
>  		 * Don't take the mm semaphore here. If we fixup a prefetch
>  		 * fault we could otherwise deadlock:
> -- 
> 2.18.0
>