Messages in this thread Patch in this message | | | From | Andy Lutomirski <> | Subject | [PATCH tip:x86/mm] x86/tls: Forcibly set the accessed bit in TLS segments | Date | Sat, 18 Mar 2017 22:17:24 -0700 |
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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@kernel.org> ---
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; + } ++info; ++desc; } -- 2.9.3
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