Re: [PATCH 4.17 16/46] vt: prevent leaking uninitialized data to userspace via /dev/vcs*

From: syzbot <syzbot+17a8efdf800000@syzkaller.appspotmail.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: glider@google.com, gregkh@linuxfoundation.org,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH 4.17 16/46] vt: prevent leaking uninitialized data to userspace via /dev/vcs*
Date: Thu, 05 Jul 2018 22:49:19 -0700	[thread overview]
Message-ID: <0000000000009d986105704e3979@google.com> (raw)
In-Reply-To: <20180706054525.376507928@linuxfoundation.org>

> 4.17-stable review patch.  If anyone has any objections, please let me  
> know.

> ------------------

> From: Alexander Potapenko <glider@google.com>

> commit 21eff69aaaa0e766ca0ce445b477698dc6a9f55a upstream.

> 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>

> ---
>   drivers/tty/vt/vt.c |    4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -784,7 +784,7 @@ int vc_allocate(unsigned int currcons)	/
>   	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_struc

>   	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;

Can't find the corresponding bug.