Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace

From: Dave Hansen <dave.hansen@linux.intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	moritz.lipp@iaik.tugraz.at, daniel.gruss@iaik.tugraz.at,
	michael.schwarz@iaik.tugraz.at,
	richard.fellner@student.tugraz.at, luto@kernel.org,
	torvalds@linux-foundation.org, keescook@google.com,
	hughd@google.com, x86@kernel.org
Subject: Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace
Date: Tue, 21 Nov 2017 14:09:03 -0800	[thread overview]
Message-ID: <7e458284-b334-bb70-a374-c65cc4ef9f02@linux.intel.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1711202057581.2348@nanos>

On 11/20/2017 12:12 PM, Thomas Gleixner wrote:
>> +			 */
>> +			native_get_shadow_pgd(pgdp)->pgd = pgd.pgd;
>> +			/*
>> +			 * For the copy of the pgd that the kernel
>> +			 * uses, make it unusable to userspace.  This
>> +			 * ensures if we get out to userspace with the
>> +			 * wrong CR3 value, userspace will crash
>> +			 * instead of running.
>> +			 */
>> +			pgd.pgd |= _PAGE_NX;
>> +		}
>> +	} else if (!pgd.pgd) {
>> +		/*
>> +		 * We are clearing the PGD and can not check  _PAGE_USER
>> +		 * in the zero'd PGD.
> 
> Just the argument cannot be checked because it's clearing the entry. The
> pgd entry itself is not yet modified, so it could be checked.

So, I guess we could enforce that only PGDs with _PAGE_USER set can ever
be cleared.  That has a nice symmetry to it because we set the shadow
when we see _PAGE_USER and we would then clear the shadow when we see
_PAGE_USER.

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