x86/vdso: Make the PER_CPU segment start out accessed
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@amacapital.net>
Link: http://lkml.kernel.org/r/ac63814ca4c637a08ec2fd0360d67ca67560a9ee.1411494540.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/vdso/vma.c b/arch/x86/vdso/vma.c
index 261b134..0c799746 100644
--- a/arch/x86/vdso/vma.c
+++ b/arch/x86/vdso/vma.c
@@ -264,7 +264,7 @@
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 */