uml: Fix unsafe pid reference to foreground process group

Although the tty core maintains a pid reference for the foreground
process group, if the foreground process group is changed that
pid reference is dropped. Thus, the pid reference used for signalling
could become stale.

Safely obtain a pid reference to the foreground process group and
release the reference after signalling is complete.

cc: Jeff Dike <jdike@addtoit.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index 8035145f043b..62087028a9ce 100644
--- a/arch/um/drivers/line.c
+++ b/arch/um/drivers/line.c
@@ -632,6 +632,7 @@ static irqreturn_t winch_interrupt(int irq, void *data)
         int fd = winch->fd;
         int err;
         char c;
+        struct pid *pgrp;
 
         if (fd != -1) {
                 err = generic_read(fd, &c, NULL);
@@ -657,7 +658,10 @@ static irqreturn_t winch_interrupt(int irq, void *data)
                 if (line != NULL) {
                         chan_window_size(line, &tty->winsize.ws_row,
                                          &tty->winsize.ws_col);
-                        kill_pgrp(tty->pgrp, SIGWINCH, 1);
+                        pgrp = tty_get_pgrp(tty);
+                        if (pgrp)
+                                kill_pgrp(pgrp, SIGWINCH, 1);
+                        put_pid(pgrp);
                 }
                 tty_kref_put(tty);
         }