lguest: Fix Malicious Guest GDT Host Crash

If a Guest makes hypercall which sets a GDT entry to not present, we
currently set any segment registers using that GDT entry to 0.
Unfortunately, this is not sufficient: there are other ways of
altering GDT entries which will cause a fault.

The correct solution to do what Linux does: let them set any GDT value
they want and handle the #GP when popping causes a fault.  This has
the added benefit of making our Switcher slightly more robust in the
case of any other bugs which cause it to fault.

We kill the Guest if it causes a fault in the Switcher: it's the
Guest's responsibility to make sure it's not using segments when it
changes them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index 49787e9..49aa555 100644
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -195,13 +195,16 @@ static int has_err(unsigned int trap)
 /* deliver_trap() returns true if it could deliver the trap. */
 int deliver_trap(struct lguest *lg, unsigned int num)
 {
-	u32 lo = lg->idt[num].a, hi = lg->idt[num].b;
+	/* Trap numbers are always 8 bit, but we set an impossible trap number
+	 * for traps inside the Switcher, so check that here. */
+	if (num >= ARRAY_SIZE(lg->idt))
+		return 0;
 
 	/* Early on the Guest hasn't set the IDT entries (or maybe it put a
 	 * bogus one in): if we fail here, the Guest will be killed. */
-	if (!idt_present(lo, hi))
+	if (!idt_present(lg->idt[num].a, lg->idt[num].b))
 		return 0;
-	set_guest_interrupt(lg, lo, hi, has_err(num));
+	set_guest_interrupt(lg, lg->idt[num].a, lg->idt[num].b, has_err(num));
 	return 1;
 }