vt: skip screen update for DEC alignment test on backgroup consoles

Overview

In the Linux kernel, the following vulnerability has been resolved:

vt: skip screen update for DEC alignment test on backgroup consoles

[BUG] Recently, we encountered a KASAN warning as follows:

BUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer+0x11f/0x140 Read of size 1 at addr ff1100015fd9f6a4 by task tty_fbcon_oob/1239 CPU: 7 UID: 0 PID: 1239 Comm: tty_fbcon_oob Not tainted 7.3.0-rc1 #100 PREEMPT(full) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 Call Trace: <TASK> ... kasan_report+0xf0/0x120 fb_pad_aligned_buffer+0x11f/0x140 ccw_putcs+0x86c/0xa80 fbcon_putcs+0x338/0x410 do_update_region+0x21d/0x450 do_con_write+0x1e0e/0x4880 con_write+0x13/0x80 n_tty_write+0x374/0x1010 file_tty_write.isra.0+0x404/0x7a0 ...

reproduce:

1. open /dev/tty0, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET, op.width = 8 and op.height = 16 (visible VC1)
2. open /dev/tty1, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET, op.width = 28 and op.height = 24 (invisible VC2)
3. echo 3 > /sys/devices/virtual/graphics/fbcon/rotate_all
4. write EShash8(esc hash8) to tty1

[CAUSE] All VCs render to the framebuffer. setfont only modifies the target VC's font without resizing the framebuffer backing buffer (par->rotated.buf); the buffer is only resized for the visible VC (fbcon_do_set_font -> ... -> vc_do_resize -> update_screen). This relies on the con_should_update() check performed before every update_region().

However, the ESC # 8 path (do_con_trol -> do_update_region) omits the con_should_update() check. After changing the font size of an invisible VC, do_update_region() fills using the new font size against a buffer that was never resized, causing an out-of-bounds access.

[FIX] Only push the update when the console is visible and not blanked, add the con_should_update() check in the do_con_trol() like every other call site of do_update_region() (update_region(), invert_screen(), ...).

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 985c000b046ca4ee11ee34ef85db500c569ef6e8
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < ad06f7312d3f615dd466483ce81a9b9619166686
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 0ee53081425a1a31ce15089012cd96b8353eb44e
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 0928ed9bd7946baee911efa401088697836e3b1e
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.