From: Peilin Ye <yepeilin.cs@gmail.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: syzbot <syzbot+b308f5fd049fbbc6e74f@syzkaller.appspotmail.com>,
linux-fbdev@vger.kernel.org, b.zolnierkie@samsung.com,
daniel.vetter@ffwll.ch, deller@gmx.de,
syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org,
dri-devel@lists.freedesktop.org, gregkh@linuxfoundation.org,
jirislaby@kernel.org, yepeilin.cs@gmail.com
Subject: Re: KASAN: use-after-free Read in bit_putcs
Date: Sat, 26 Sep 2020 19:39:57 +0000 [thread overview]
Message-ID: <20200926193957.GA1033221@PWN> (raw)
In-Reply-To: <bbcef674-4ac6-c933-b55d-8961ada97f4c@i-love.sakura.ne.jp>
On Sun, Sep 27, 2020 at 01:25:17AM +0900, Tetsuo Handa wrote:
> A simplified reproducer and debug printk() patch shown below reported that
> vc_font.height is increased to 9 via ioctl(VT_RESIZEX) after it was once
> decreased from 16 to 2 via ioctl(PIO_FONT).
>
>
>
> Since vc_resize() with v.v_rows = 0 preserves current vc->vc_rows value,
> this reproducer is bypassing
>
> if (v.v_clin) {
> int rows = v.v_vlin / v.v_clin;
> if (v.v_rows != rows) {
> if (v.v_rows) /* Parameters don't add up */
> return -EINVAL;
> v.v_rows = rows;
> }
> }
>
> check by setting v.v_vlin = 1 and v.v_clin = 9.
>
> If v.v_vcol > 0 and v.v_vcol != vc->vc_cols (though this reproducer is passing
> v.v_vcol = 0), tty_do_resize() from vc_do_resize() from vc_resize() can make
> "struct tty_struct"->winsize.ws_ypixel = 1 despite
> "struct tty_struct"->winsize.vc->vc_rows = vc->vc_rows (which is usually larger
> than 1). Does such winsize (a row has 1 / vc->vc_rows pixel) make sense?
>
>
>
> Since I don't know the meaning of "struct vt_consize"->v_clin (which is commented
> with "/* number of pixel rows per character */" but does it mean font size ?),
> I don't know why we can assign that value to vcp->vc_font.height via
>
> if (v.v_clin)
> vcp->vc_font.height = v.v_clin;
>
> in vt_resizex(). While ioctl(PIO_FONT) needs to pass vc->vc_sw->con_font_set()
> check in con_font_set(), ioctl(VT_RESIZEX) does not pass it in vt_resizex()...
>
> Since this problem does not happen if I remove
>
> if (v.v_clin)
> vcp->vc_font.height = v.v_clin;
Hi Tetsuo!
> from vt_resizex(), I guess that some variables are getting confused by change
> of vc->vc_font.height ...
Yes, see bit_putcs():
(drivers/video/fbdev/core/bitblit.c)
static void bit_putcs(struct vc_data *vc, struct fb_info *info,
const unsigned short *s, int count, int yy, int xx,
int fg, int bg)
{
struct fb_image image;
u32 width = DIV_ROUND_UP(vc->vc_font.width, 8);
u32 cellsize = width * vc->vc_font.height;
^^^^^^^^ ^^^^^^^^^^^^^^
`cellsize` is now too large. Later, in bit_putcs_aligned():
while (cnt--) {
src = vc->vc_font.data + (scr_readw(s++)&
charmask)*cellsize;
^^^^^^^^
`src` goes out of bounds of the data buffer. At first glance I guess
this is an out-of-bound read reported as a use-after-free read? The
crashlog says:
[ 149.732103][ T6693] Allocated by task 6667:
[ 149.732115][ T6693] kasan_save_stack (mm/kasan/common.c:48)
[ 149.732121][ T6693] __kasan_kmalloc.constprop.0 (mm/kasan/common.c:56 mm/kasan/common.c:461)
[ 149.732126][ T6693] __kmalloc (mm/slab.c:3656 mm/slab.c:3664)
[ 149.732133][ T6693] alloc_pipe_info (fs/pipe.c:810)
[ 149.732139][ T6693] create_pipe_files (fs/pipe.c:883 fs/pipe.c:914)
[ 149.732145][ T6693] do_pipe2 (fs/pipe.c:965 fs/pipe.c:1012)
I'm not sure, but I don't think a buffer allocated in fs/pipe.c is
related here. Maybe they just live near each other on the heap?
To resolve this out-of-bound issue for now, I think the easiest way
is to add a range check in bit_putcs(), or bit_putcs_aligned().
...but yeah, that `VT_RESIZEX` ioctl looks really buggy, and is already
causing more issues:
KASAN: global-out-of-bounds Read in fbcon_get_font
Link: https://syzkaller.appspot.com/bug?id\bb8be45afea11888776f897895aef9ad1c3ecfd
This was also caused by `VT_RESIZEX`...
Thank you,
Peilin Ye
next prev parent reply other threads:[~2020-09-26 19:39 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-23 17:30 KASAN: use-after-free Read in bit_putcs syzbot
2020-09-26 2:03 ` syzbot
2020-09-26 16:25 ` Tetsuo Handa
2020-09-26 19:39 ` Peilin Ye [this message]
2020-09-27 0:25 ` Tetsuo Handa
2020-09-27 8:28 ` Tetsuo Handa
2020-09-27 9:27 ` Peilin Ye
2020-09-27 11:46 ` [PATCH] vt_ioctl: make VT_RESIZEX behave like VT_RESIZE Tetsuo Handa
2020-09-27 12:06 ` Greg KH
2020-09-28 17:59 ` Martin Hostettler
2020-09-29 1:12 ` Tetsuo Handa
2020-09-29 10:52 ` Martin Hostettler
2020-09-29 16:56 ` Daniel Vetter
2020-09-29 17:10 ` Greg KH
2021-04-11 21:43 ` Maciej W. Rozycki
2021-04-11 22:15 ` Linus Torvalds
2021-04-12 7:01 ` Daniel Vetter
2021-04-12 13:30 ` Maciej W. Rozycki
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