vt: prevent leaking uninitialized data to userspace via /dev/vcs*

KMSAN reported an infoleak when reading from /dev/vcs*:

  BUG: KMSAN: kernel-infoleak in vcs_read+0x18ba/0x1cc0
  Call Trace:
  ...
   kmsan_copy_to_user+0x7a/0x160 mm/kmsan/kmsan.c:1253
   copy_to_user ./include/linux/uaccess.h:184
   vcs_read+0x18ba/0x1cc0 drivers/tty/vt/vc_screen.c:352
   __vfs_read+0x1b2/0x9d0 fs/read_write.c:416
   vfs_read+0x36c/0x6b0 fs/read_write.c:452
  ...
  Uninit was created at:
   kmsan_save_stack_with_flags mm/kmsan/kmsan.c:279
   kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:189
   kmsan_kmalloc+0x94/0x100 mm/kmsan/kmsan.c:315
   __kmalloc+0x13a/0x350 mm/slub.c:3818
   kmalloc ./include/linux/slab.h:517
   vc_allocate+0x438/0x800 drivers/tty/vt/vt.c:787
   con_install+0x8c/0x640 drivers/tty/vt/vt.c:2880
   tty_driver_install_tty drivers/tty/tty_io.c:1224
   tty_init_dev+0x1b5/0x1020 drivers/tty/tty_io.c:1324
   tty_open_by_driver drivers/tty/tty_io.c:1959
   tty_open+0x17b4/0x2ed0 drivers/tty/tty_io.c:2007
   chrdev_open+0xc25/0xd90 fs/char_dev.c:417
   do_dentry_open+0xccc/0x1440 fs/open.c:794
   vfs_open+0x1b6/0x2f0 fs/open.c:908
  ...
  Bytes 0-79 of 240 are uninitialized

Consistently allocating |vc_screenbuf| with kzalloc() fixes the problem

Reported-by: syzbot+17a8efdf800000@syzkaller.appspotmail.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 1eb1a376a041..15eb6c829d39 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -784,7 +784,7 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
         if (!*vc->vc_uni_pagedir_loc)
                 con_set_default_unimap(vc);
 
-        vc->vc_screenbuf = kmalloc(vc->vc_screenbuf_size, GFP_KERNEL);
+        vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_KERNEL);
         if (!vc->vc_screenbuf)
                 goto err_free;
 
@@ -871,7 +871,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
 
         if (new_screen_size > (4 << 20))
                 return -EINVAL;
-        newscreen = kmalloc(new_screen_size, GFP_USER);
+        newscreen = kzalloc(new_screen_size, GFP_USER);
         if (!newscreen)
                 return -ENOMEM;