uml: fix irqstack crash

This patch fixes a crash caused by an interrupt coming in when an IRQ stack
is being torn down.  When this happens, handle_signal will loop, setting up
the IRQ stack again because the tearing down had finished, and handling
whatever signals had come in.

However, to_irq_stack returns a mask of pending signals to be handled, plus
bit zero is set if the IRQ stack was already active, and thus shouldn't be
torn down.  This causes a problem because when handle_signal goes around
the loop, sig will be zero, and to_irq_stack will duly set bit zero in the
returned mask, faking handle_signal into believing that it shouldn't tear
down the IRQ stack and return thread_info pointers back to their original
values.

This will eventually cause a crash, as the IRQ stack thread_info will
continue pointing to the original task_struct and an interrupt will look
into it after it has been freed.

The fix is to stop passing a signal number into to_irq_stack.  Rather, the
pending signals mask is initialized beforehand with the bit for sig already
set.  References to sig in to_irq_stack can be replaced with references to
the mask.

[akpm@linux-foundation.org: use UL]
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 7 insertions, 6 deletions

diff --git a/arch/um/include/kern_util.h b/arch/um/include/kern_util.h
index 8d7f7c1cb9c6..6c2be26f1d7d 100644
--- a/arch/um/include/kern_util.h
+++ b/arch/um/include/kern_util.h
@@ -117,7 +117,7 @@ extern void sigio_handler(int sig, union uml_pt_regs *regs);
 
 extern void copy_sc(union uml_pt_regs *regs, void *from);
 
-unsigned long to_irq_stack(int sig, unsigned long *mask_out);
+extern unsigned long to_irq_stack(unsigned long *mask_out);
 unsigned long from_irq_stack(int nested);
 
 #endif
diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
index 9870febdbead..cf0dd9cf8c43 100644
--- a/arch/um/kernel/irq.c
+++ b/arch/um/kernel/irq.c
@@ -518,13 +518,13 @@ int init_aio_irq(int irq, char *name, irq_handler_t handler)
 
 static unsigned long pending_mask;
 
-unsigned long to_irq_stack(int sig, unsigned long *mask_out)
+unsigned long to_irq_stack(unsigned long *mask_out)
 {
         struct thread_info *ti;
         unsigned long mask, old;
         int nested;
 
-        mask = xchg(&pending_mask, 1 << sig);
+        mask = xchg(&pending_mask, *mask_out);
         if(mask != 0){
                 /* If any interrupts come in at this point, we want to
                  * make sure that their bits aren't lost by our
@@ -534,7 +534,7 @@ unsigned long to_irq_stack(int sig, unsigned long *mask_out)
                  * and pending_mask contains a bit for each interrupt
                  * that came in.
                  */
-                old = 1 << sig;
+                old = *mask_out;
                 do {
                         old |= mask;
                         mask = xchg(&pending_mask, old);
@@ -550,6 +550,7 @@ unsigned long to_irq_stack(int sig, unsigned long *mask_out)
 
                 task = cpu_tasks[ti->cpu].task;
                 tti = task_thread_info(task);
+
                 *ti = *tti;
                 ti->real_thread = tti;
                 task->stack = ti;
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 18e5c8b67eb8..b98f7ea2d2f6 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -119,7 +119,7 @@ void (*handlers[_NSIG])(int sig, struct sigcontext *sc);
 
 void handle_signal(int sig, struct sigcontext *sc)
 {
-        unsigned long pending = 0;
+        unsigned long pending = 1UL << sig;
 
         do {
                 int nested, bail;
@@ -134,7 +134,7 @@ void handle_signal(int sig, struct sigcontext *sc)
                  * have to return, and the upper handler will deal
                  * with this interrupt.
                  */
-                bail = to_irq_stack(sig, &pending);
+                bail = to_irq_stack(&pending);
                 if(bail)
                         return;