From 5b781c7e317fcf9f74475dc82bfce2e359dfca13 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Sat, 18 Mar 2017 22:17:24 -0700 Subject: [PATCH] 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 Cc: Linus Torvalds Cc: Borislav Petkov Cc: Thomas Garnier Link: http://lkml.kernel.org/r/62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org Signed-off-by: Thomas Gleixner --- 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; + } ++info; ++desc; } -- 2.20.1