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[PATCH tip:x86/mm] x86/tls: Forcibly set the accessed bit in TLS segments

For mysterious historical reasons, struct user_desc doesn't indicate
whether segments are accessed.  set_thread_area() has always
programmed segments as non-accessed, so the first write will set the
accessed bit.  This will fault if the GDT is read-only.

Fix it by making TLS segments start out accessed.

If this ends up breaking something, we could, in principle, leave
TLS segments non-accessed and fix them up when we get the page
fault.  I'd be surprised, though -- AFAIK all the nasty legacy
segmented programs (DOSEMU, Wine, things that run on DOSEMU and
Wine, etc.) do their nasty segmented things using the LDT and not
the GDT.  I assume this is mainly because old OSes (Linux and
otherwise) didn't historically provide APIs to do nasty things in
the GDT.

Fixes: 45fc8757d1d2 ("x86: Make the GDT remapping read-only on 64-bit")
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>

Normally this would come with a test case update, but the relevant
testcase (ldt_gdt_32) currently has some issues.  I'm working on it,
but I don't want to delay this bugfix.

 arch/x86/kernel/tls.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 6c8934406dc9..dcd699baea1b 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -92,10 +92,17 @@ static void set_tls_desc(struct task_struct *p, int idx,
 	cpu = get_cpu();
 	while (n-- > 0) {
-		if (LDT_empty(info) || LDT_zero(info))
+		if (LDT_empty(info) || LDT_zero(info)) {
 			desc->a = desc->b = 0;
-		else
+		} else {
 			fill_ldt(desc, info);
+			/*
+			 * Always set the accessed bit so that the CPU
+			 * doesn't try to write to the (read-only) GDT.
+			 */
+			desc->type |= 1;
+		}