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@xxxxxxxxxxxxxx/ https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang@xxxxxxxxxx/ This fixes CVE-2020-14331. Big thanks to guys mentioned in the Reported-and-debugged-by lines below who actually found the root cause. Signed-off-by: Jiri Slaby <jslaby@xxxxxxx> Reported-and-debugged-by: 张云海 <zhangyunhai@xxxxxxxxxxx> Reported-and-debugged-by: Yang Yingliang <yangyingliang@xxxxxxxxxx> Reported-by: Kyungtae Kim <kt0755@xxxxxxxxx> Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Greg KH <greg@xxxxxxxxx> Cc: Solar Designer <solar@xxxxxxxxxxxx> Cc: "Srivatsa S. Bhat" <srivatsa@xxxxxxxxxxxxx> Cc: Anthony Liguori <aliguori@xxxxxxxxxx> Cc: Security Officers <security@xxxxxxxxxx> Cc: linux-distros@xxxxxxxxxxxxxxx Cc: Yang Yingliang <yangyingliang@xxxxxxxxxx> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx Cc: linux-fbdev@xxxxxxxxxxxxxxx --- drivers/video/console/vgacon.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c index f0f3d573f848..13194bb246f8 100644 --- a/drivers/video/console/vgacon.c +++ b/drivers/video/console/vgacon.c @@ -250,6 +250,11 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count) p = (void *) (c->vc_origin + t * c->vc_size_row); + /* vc_size_row might have changed by VT_RESIZE in the meantime */ + if ((vgacon_scrollback_cur->tail + c->vc_size_row) >= + vgacon_scrollback_cur->size) + vgacon_scrollback_cur->tail = 0; + while (count--) { scr_memcpyw(vgacon_scrollback_cur->data + vgacon_scrollback_cur->tail, -- 2.28.0 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
