linux-kernel - [PATCH] check smap and !cr0.wp

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Date:	Thu,  7 May 2015 16:44:02 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	pbonzini@...hat.com
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Subject: [PATCH] check smap and !cr0.wp

This test case is used to produce the bug that:

KVM may turn a user page to a kernel page when kernel writes a readonly
user page if CR0.WP = 1. This shadow page entry will be reused after
SMAP is enabled so that kernel is allowed to access this user page

Signed-off-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
---
 x86/smap.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/x86/smap.c b/x86/smap.c
index 042c5aa..66f97b8 100644
--- a/x86/smap.c
+++ b/x86/smap.c
@@ -48,6 +48,7 @@ asm ("pf_tss:\n"
 
 #define USER_BASE	(1 << 24)
 #define USER_VAR(v)	(*((__typeof__(&(v))) (((unsigned long)&v) + USER_BASE)))
+#define USER_ADDR(v)   ((void *)((unsigned long)(&v) + USER_BASE))
 
 static void init_test(int i)
 {
@@ -58,6 +59,29 @@ static void init_test(int i)
 	}
 }
 
+static void check_smap_nowp(void)
+{
+	test = 0x99;
+
+	*get_pte(phys_to_virt(read_cr3()), USER_ADDR(test)) &= ~PTE_WRITE;
+
+	write_cr4(read_cr4() & ~X86_CR4_SMAP);
+	write_cr0(read_cr0() & ~X86_CR0_WP);
+	clac();
+	write_cr3(read_cr3());
+
+	init_test(0);
+	USER_VAR(test) = 0x99;
+	report("write from user page with SMAP=0, AC=0, WP=0, PTE.U=1 && PTE.W=0", pf_count == 0);
+
+	write_cr4(read_cr4() | X86_CR4_SMAP);
+	write_cr3(read_cr3());
+
+	init_test(0);
+	(void)USER_VAR(test);
+	report("read from user page with SMAP=1, AC=0, WP=0, PTE.U=1 && PTE.W=0", pf_count == 1 && save == 0x99);
+}
+
 int main(int ac, char **av)
 {
 	unsigned long i;
@@ -150,6 +174,8 @@ int main(int ac, char **av)
 		report("executing on user page with AC=0", pf_count == 0);
 	}
 
+	check_smap_nowp();
+
 	// TODO: implicit kernel access from ring 3 (e.g. int)
 
 	return report_summary();
-- 
2.1.0

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