[tip:x86/vdso] x86/vdso: Make the PER_CPU segment start out accessed

[tip-bot for Andy Lutomirski]

Commit-ID:  9c0080ef93bce34db130db6dc3bd946348e6a203
Gitweb:     http://git.kernel.org/tip/9c0080ef93bce34db130db6dc3bd946348e6a203
Author:     Andy Lutomirski <luto@xxxxxxxxxxxxxx>
AuthorDate: Tue, 23 Sep 2014 10:50:54 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 28 Oct 2014 11:22:11 +0100

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@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/ac63814ca4c637a08ec2fd0360d67ca67560a9ee.1411494540.git.luto@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 261b134..0c799746 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 */
--
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