GHSA-798G-XWJP-798R

Vulnerability from github – Published: 2026-10-18 18:32 – Updated: 2026-10-30 15:34
VLAI
Details

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: ... 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(), ...).

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{
  "affected": [],
  "aliases": [
    "CVE-2026-98838"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-10-18T16:17:38Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nvt: skip screen update for DEC alignment test on backgroup consoles\n\n[BUG]\nRecently, we encountered a KASAN warning as follows:\n\nBUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer+0x11f/0x140\nRead of size 1 at addr ff1100015fd9f6a4 by task tty_fbcon_oob/1239\nCPU: 7 UID: 0 PID: 1239 Comm: tty_fbcon_oob Not tainted 7.3.0-rc1 #100 PREEMPT(full)\nHardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014\nCall Trace:\n \u003cTASK\u003e\n ...\n kasan_report+0xf0/0x120\n fb_pad_aligned_buffer+0x11f/0x140\n ccw_putcs+0x86c/0xa80\n fbcon_putcs+0x338/0x410\n do_update_region+0x21d/0x450\n do_con_write+0x1e0e/0x4880\n con_write+0x13/0x80\n n_tty_write+0x374/0x1010\n file_tty_write.isra.0+0x404/0x7a0\n ...\n\nreproduce:\n1) open /dev/tty0, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET,\nop.width = 8 and op.height = 16 (visible VC1)\n2) open /dev/tty1, set a KDFONTOP ioctl with op.op = KD_FONT_OP_SET,\nop.width = 28 and op.height = 24 (invisible VC2)\n3) echo 3 \u003e /sys/devices/virtual/graphics/fbcon/rotate_all\n4) write EShash8(esc hash8) to tty1\n\n[CAUSE]\nAll VCs render to the framebuffer. setfont only modifies the target VC\u0027s\nfont without resizing the framebuffer backing buffer (par-\u003erotated.buf);\nthe buffer is only resized for the visible VC\n(fbcon_do_set_font -\u003e ... -\u003e vc_do_resize -\u003e update_screen). This relies on\nthe con_should_update() check performed before every update_region().\n\nHowever, the ESC # 8 path (do_con_trol -\u003e do_update_region) omits the\ncon_should_update() check. After changing the font size of an invisible VC,\ndo_update_region() fills using the new font size against a buffer that was\nnever resized, causing an out-of-bounds access.\n\n[FIX]\nOnly push the update when the console is visible and not blanked, add\nthe con_should_update() check in the do_con_trol() like every other call\nsite of do_update_region() (update_region(), invert_screen(), ...).",
  "id": "GHSA-798g-xwjp-798r",
  "modified": "2026-10-30T15:34:04Z",
  "published": "2026-10-18T18:32:05Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98838"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/0928ed9bd7946baee911efa401088697836e3b1e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/0ee53081425a1a31ce15089012cd96b8353eb44e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/2fc4e71aefab57c80ccc9775aad2af503fc90810"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/663cf98baced6941a0f3f00e1be61024e57d4429"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/82561eee12a342994daf3a70590f5d07c35da743"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/985c000b046ca4ee11ee34ef85db500c569ef6e8"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ad06f7312d3f615dd466483ce81a9b9619166686"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/d388658505bf707ff0cd7142c37c868febf8c2a5"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}



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