[kvm-unit-tests PATCH v4 02/18] x86: cet: Remove unnecessary memory zeroing for shadow stack

[Sean Christopherson <seanjc@google.com>]

From: Chao Gao <chao.gao@intel.com>

Skip mapping the shadow stack as a writable page and the redundant memory
zeroing.

Currently, the shadow stack is allocated using alloc_page(), then mapped as
a writable page, zeroed, and finally mapped as a shadow stack page. The
memory zeroing is redundant as alloc_page() already does that.

This also eliminates the need for invlpg, as the shadow stack is no
longer mapped writable.

Signed-off-by: Chao Gao <chao.gao@intel.com>
[mks: drop invlpg() as it's no longer needed, adapted changelog accordingly]
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
[sean: add a comment to explain the magic shadow stack protections]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 x86/cet.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/x86/cet.c b/x86/cet.c
index 51a54a50..e2681886 100644
--- a/x86/cet.c
+++ b/x86/cet.c
@@ -67,7 +67,6 @@ int main(int ac, char **av)
 {
 	char *shstk_virt;
 	unsigned long shstk_phys;
-	unsigned long *ptep;
 	pteval_t pte = 0;
 	bool rvc;
 
@@ -89,18 +88,14 @@ int main(int ac, char **av)
 	shstk_virt = alloc_vpage();
 	shstk_phys = (unsigned long)virt_to_phys(alloc_page());
 
-	/* Install the new page. */
-	pte = shstk_phys | PT_PRESENT_MASK | PT_WRITABLE_MASK | PT_USER_MASK;
+	/*
+	 * Install a mapping for the shadow stack page.  Shadow stack pages are
+	 * denoted by an "impossible" combination of a !WRITABLE, DIRTY PTE
+	 * (writes from CPU for shadow stack operations are allowed, but writes
+	 * from software are not).
+	 */
+	pte = shstk_phys | PT_PRESENT_MASK | PT_USER_MASK | PT_DIRTY_MASK;
 	install_pte(current_page_table(), 1, shstk_virt, pte, 0);
-	memset(shstk_virt, 0x0, PAGE_SIZE);
-
-	/* Mark it as shadow-stack page. */
-	ptep = get_pte_level(current_page_table(), shstk_virt, 1);
-	*ptep &= ~PT_WRITABLE_MASK;
-	*ptep |= PT_DIRTY_MASK;
-
-	/* Flush the paging cache. */
-	invlpg((void *)shstk_virt);
 
 	/* Enable shadow-stack protection */
 	wrmsr(MSR_IA32_U_CET, ENABLE_SHSTK_BIT);
-- 
2.52.0.rc1.455.g30608eb744-goog