[PATCH 4/8] x86,vdso: Make the PER_CPU segment start out accessed

[Andy Lutomirski]

The first userspace attempt to read or write the PER_CPU segment
will write the accessed bit to the GDT.  This is visible to
userspace using the LAR instruction, and it also pointlessly
dirties a cache line.

Set the segment's accessed bit at boot to prevent userspace
access to segments from having side effects.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/vdso/vma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/vdso/vma.c b/arch/x86/vdso/vma.c
index 261b1349acc9..0c7997467be0 100644
--- a/arch/x86/vdso/vma.c
+++ b/arch/x86/vdso/vma.c
@@ -264,7 +264,7 @@ static void vsyscall_set_cpu(int cpu)
 	d = (struct desc_struct) {
 		.limit0 = cpu | ((node & 0xf) << 12),
 		.limit = node >> 4,
-		.type = 4,		/* RO data, expand down */
+		.type = 5,		/* RO data, expand down, accessed */
 		.dpl = 3,		/* Visible to user code */
 		.s = 1,			/* Not a system segment */
 		.p = 1,			/* Present */
-- 
1.9.3

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