[PATCH 2/3] x86,vdso: Make the PER_CPU segment start out accessed

Andy Lutomirski
2014-07-26T01:13:25Z

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.

Set the segment's accessed bit at boot to keep all userspace GDT
access idempotent.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
arch/x86/kernel/vsyscall_64.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 158cdff..0e2c229 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -305,7 +305,7 @@ static void vsyscall_set_cpu(int cpu)
memset(&d, 0, sizeof(d));
d.limit0 = cpu | ((node & 0xf) << 12);
d.limit = node >> 4;
- d.type = 4; /* RO data, expand down */
+ d.type = 5; /* RO data, expand down, accessed */
d.dpl = 3; /* Visible to user code */
d.s = 1; /* Non a system segment */
d.p = 1; /* Present */
--
1.9.3