Re: [PATCH] vgacon: fix out of bounds write to the scrollback buffer

Hi Jiri,

On 7/29/20 9:02 AM, Jiri Slaby wrote:

The current vgacon's scroll up implementation uses a circural buffer
in vgacon_scrollback_cur. It always advances tail to prepare it for the
next write and caps it to zero if the next ->vc_size_row bytes won't fit.

But when we change the VT size (e.g. by VT_RESIZE) in the meantime, the new
line might not fit to the end of the scrollback buffer in the next
attempt to scroll. This leads to various crashes as
vgacon_scrollback_update writes out of the buffer:
 BUG: unable to handle page fault for address: ffffc900001752a0
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 RIP: 0010:mutex_unlock+0x13/0x30
...
 Call Trace:
  n_tty_write+0x1a0/0x4d0
  tty_write+0x1a0/0x2e0

Or to KASAN reports:
BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed

So check whether the line fits in the buffer and wrap if needed. Do it
before the loop as console_sem is held and ->vc_size_row cannot change
during the execution of vgacon_scrollback_cur. If it does change, we
need to ensure it does not change elsewhere, not here.

Also, we do not split the write of a line into chunks as that would
break the consumers of the buffer. They expect ->cnt, ->tail and ->size
to be in harmony and advanced by ->vc_size_row.

I found few reports of this in the past, some with patches included,
some even 2 years old:
https://lore.kernel.org/lkml/CAEAjamsJnG-=TSOwgRbbb3B9Z-PA63oWmNPoKYWQ=Z=+X49akg@mail.gmail.com/ (local)

Sorry but I don't work on fixing fbdev/console KASAN/syzbot/etc.
reports (-ENORESOURCES).  This has been made official in the past.

I'm happy to review/apply patches though.

https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@huawei.com/ (local)

This was the first time the patch for issue was submitted.

I tried to apply it to drm-misc but then I have noticed that
it has not been posted to linux-fbdev / dri-devel MLs (so it
was not possible to merge it using dim tool) and thus I've
requested the author to resend it:

https://lore.kernel.org/lkml/62544bd9-e47d-e7f9-92f2-49b8dbb132c1@samsung.com/ (local)

which he did:

https://lore.kernel.org/lkml/20200713105730.550334-1-yangyingliang@huawei.com/ (local)

and the patch is currently under review period (to give people
chance to comment on it) and in my "to apply if no objections"
folder.

I see that your/Yunhai patch addresses the root source of
the issue so I'll be happy to apply/ACK it instead of Yang's
patch once the final version is posted.

Thank you for working on this.

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics