[PATCH v2 2/3] 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.

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/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 28d0ad6..ae80ef6 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;		/* Not a system segment */
 	d.p = 1;		/* Present */
-- 
1.9.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/