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x86/vdso: Make the PER_CPU segment start out accessed

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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>
This commit is contained in:
Andy Lutomirski 2014-09-23 10:50:54 -07:00 committed by Ingo Molnar
parent 258801563b
commit 9c0080ef93
1 changed files with 1 additions and 1 deletions
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2
arch/x86/vdso/vma.c
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@ -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 */