KASAN: slab-out-of-bounds Write in vgacon_scroll

[Dmitry Vyukov]

On Wed, Jan 29, 2020 at 3:59 PM Dmitry Vyukov <dvyukov at google.com> wrote:
>
> On Wed, Jan 29, 2020 at 3:40 PM Sergey Senozhatsky
> <sergey.senozhatsky at gmail.com> wrote:
> >
> > Cc-ing Dmitry and Tetsuo
> >
> > Original Message-id: CAA=061EoW8AmjUrBLsJy5nTDz-1jeArLeB+z6HJuyZud0zZXug at mail.gmail.com
> >
> > On (20/01/29 23:17), Sergey Senozhatsky wrote:
> > > > Hmm. There is something strange about it. I use vga console quite
> > > > often, and scrolling happens all the time, yet I can't get the same
> > > > out-of-bounds report (nor have I ever seen it in the past), even with
> > > > the reproducer. Is it supposed to be executed as it is, or are there
> > > > any preconditions? Any chance that something that runs prior to that
> > > > reproducer somehow impacts the system? Just asking.
> > >
> > > These questions were addressed to anon anon (742991625abc at gmail.com),
> > > not to Bartlomiej.
> >
> > Could this be GCC_PLUGIN related?
>
> syzkaller repros are meant to be self-contained, but they don't
> capture the image and VM setup (or actual hardware). I suspect it may
> have something to do with these bugs.
> syzbot has reported a bunch of similar bugs in one of our internal kernels:
>
> KASAN: slab-out-of-bounds Read in vgacon_scroll
> KASAN: slab-out-of-bounds Read in vgacon_invert_region
> KASAN: use-after-free Write in vgacon_scroll
> KASAN: use-after-free Read in vgacon_scroll
> KASAN: use-after-free Read in vgacon_invert_region
> BUG: unable to handle kernel paging request in vgacon_scroll
>
> But none on upstream kernels. That may be some difference in config?
> I actually don't know what affects these things. When I tried to get
> at least some coverage of that code in syzkaller I just understood
> that relations between all these
> tty/pty/ptmx/vt/pt/ldisc/vcs/vcsu/fb/con/dri/drm/etc are complex to
> say the least...


It would also be good to figure out how we can cover this on syzbot/upstream.

Our upstream config is:

$ grep VGA upstream-kasan.config
CONFIG_VGA_ARB=y
CONFIG_VGA_ARB_MAX_GPUS=16
# CONFIG_VGA_SWITCHEROO is not set
CONFIG_FB_VGA16=y
CONFIG_VGASTATE=y
CONFIG_VGA_CONSOLE=y
CONFIG_VGACON_SOFT_SCROLLBACK=y
CONFIG_VGACON_SOFT_SCROLLBACK_SIZE=64
# CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT is not set
CONFIG_LOGO_LINUX_VGA16=y
# CONFIG_USB_SISUSBVGA is not set
# CONFIG_VFIO_PCI_VGA is not set

where anon's is:
CONFIG_VGA_ARB=y
CONFIG_VGA_ARB_MAX_GPUS=16
# CONFIG_VGA_SWITCHEROO is not set
# CONFIG_FB_VGA16 is not set
CONFIG_VGA_CONSOLE=y
CONFIG_VGACON_SOFT_SCROLLBACK=y
CONFIG_VGACON_SOFT_SCROLLBACK_SIZE=64
# CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT_ENABLE_BY_DEFAULT is not set
# CONFIG_LOGO_LINUX_VGA16 is not set
# CONFIG_USB_SISUSBVGA is not set

And the one on which are catching the bugs in vgacon on internal kernel is:
CONFIG_VGA_ARB=y
CONFIG_VGA_ARB_MAX_GPUS=16
# CONFIG_VGA_SWITCHEROO is not set
# CONFIG_VGASTATE is not set
CONFIG_VGA_CONSOLE=y
# CONFIG_VGACON_SOFT_SCROLLBACK is not set
# CONFIG_USB_SISUSBVGA is not set
# CONFIG_VFIO_PCI_VGA is not set


May it be related to CONFIG_VGASTATE?