lguest: fix guest kernel stack overflow when TF bit set.

The symptoms are that running gdb on a binary causes the guest to
overflow the kernels stack (after some period of time), resulting in
it finally being killed with a "Bad address" message.

Reported-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 10 insertions, 0 deletions

diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index 28433a155d67..70dfcdc29f1f 100644
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -140,6 +140,16 @@ static void set_guest_interrupt(struct lg_cpu *cpu, u32 lo, u32 hi,
         cpu->regs->eip = idt_address(lo, hi);
 
         /*
+         * Trapping always clears these flags:
+         * TF: Trap flag
+         * VM: Virtual 8086 mode
+         * RF: Resume
+         * NT: Nested task.
+         */
+        cpu->regs->eflags &=
+                ~(X86_EFLAGS_TF|X86_EFLAGS_VM|X86_EFLAGS_RF|X86_EFLAGS_NT);
+
+        /*
          * There are two kinds of interrupt handlers: 0xE is an "interrupt
          * gate" which expects interrupts to be disabled on entry.
          */