CVE-2026-98838: vt: skip screen update for DEC alignment test on backgroup consoles

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 fbpadalignedbuffer+0x11f/0x140 Read of size 1 at addr ff1100015fd9f6a4 by task ttyfbconoob/1239 CPU: 7 UID: 0 PID: 1239 Comm: ttyfbconoob 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> ... kasanreport+0xf0/0x120 fbpadalignedbuffer+0x11f/0x140 ccwputcs+0x86c/0xa80 fbconputcs+0x338/0x410 doupdateregion+0x21d/0x450 doconwrite+0x1e0e/0x4880 conwrite+0x13/0x80 nttywrite+0x374/0x1010 filettywrite.isra.0+0x404/0x7a0 ...

reproduce: 1) open /dev/tty0, set a KDFONTOP ioctl with op.op = KDFONTOPSET, op.width = 8 and op.height = 16 (visible VC1) 2) open /dev/tty1, set a KDFONTOP ioctl with op.op = KDFONTOPSET, op.width = 28 and op.height = 24 (invisible VC2) 3) echo 3 > /sys/devices/virtual/graphics/fbcon/rotateall 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 (fbcondosetfont -> ... -> vcdoresize -> updatescreen). This relies on the conshouldupdate() check performed before every updateregion().

However, the ESC # 8 path (docontrol -> doupdateregion) omits the conshouldupdate() check. After changing the font size of an invisible VC, doupdateregion() 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 conshouldupdate() check in the docontrol() like every other call site of doupdateregion() (updateregion(), invertscreen(), ...).