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Date:   Tue, 15 Nov 2022 12:47:22 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
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        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v3 15/37] x86/mm: Check Shadow Stack page fault errors
Message-ID: <Y3N8Sn65TzyD6jwL@hirez.programming.kicks-ass.net>
References: <20221104223604.29615-1-rick.p.edgecombe@intel.com>
 <20221104223604.29615-16-rick.p.edgecombe@intel.com>
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On Fri, Nov 04, 2022 at 03:35:42PM -0700, Rick Edgecombe wrote:
> @@ -1331,6 +1345,18 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  
> +	/*
> +	 * To service shadow stack read faults, unlike normal read faults, the
> +	 * fault handler needs to create a type of memory that will also be
> +	 * writable (with instructions that generate shadow stack writes).
> +	 * In the case of COW memory, the COW needs to take place even with
> +	 * a shadow stack read. Otherwise the shared page will be left (shadow
> +	 * stack) writable in userspace. So to trigger the appropriate behavior
> +	 * by setting FAULT_FLAG_WRITE for shadow stack accesses, even if the
> +	 * access was a shadow stack read.
> +	 */

Clear as mud... So SS pages are 'Write=0,Dirty=1', which, per
construction, lack a RW bit. And these pages are writable (WRUSS).

pte_wrprotect() seems to do: _PAGE_DIRTY->_PAGE_COW (which is really
weird in this situation), resulting in: 'Write=0,Dirty=0,Cow=1'.

That's regular RO memory and won't raise read-faults.

But I'm thinking RET will trip #PF here when it tries to read the SS
because the SSP is not a proper shadow stack page?

And in that case you want to tickle pte_mkwrite() to undo the
pte_wrprotect() above?

So while the #PF is a 'read' fault due to RET not actually writing to
the shadow stack, you want to force a write fault so it will re-instate
the SS page.

Did I get that right?

> +	if (error_code & X86_PF_SHSTK)
> +		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_WRITE)
>  		flags |= FAULT_FLAG_WRITE;
>  	if (error_code & X86_PF_INSTR)
> -- 
> 2.17.1
>