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>

diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index 8035145f043b4f5ffbb9b8ffe26572b390184fc9..62087028a9ce1e079d10d791117069a6a065093f 100644 (file)
--- 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);
        }