Commit-ID: 5b781c7e317fcf9f74475dc82bfce2e359dfca13
Gitweb: http://git.kernel.org/tip/5b781c7e317fcf9f74475dc82bfce2e359dfca13
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Sat, 18 Mar 2017 22:17:24 -0700
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Sun, 19 Mar 2017 12:14:35 +0100
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>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@xxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
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 6c89344..dcd699b 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;
}
--
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